41410 Sps 51 90 Unbrako Engineering Guide

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Engineering Guide

Socket Products

A comprehensive catalog of
UNBRAKO® socket screws and related products
In this catalog you will find complete information about UNBRAKO socket screws and such related
products as shoulder screws, dowel pins, pressure plugs and hex keys. Everything you need to select,
specify and order these precision products is at your finger tips except actual prices. Furthermore, all
data has been organized to let you find the facts you want with the greatest speed and the least effort.
Wherever possible, all data for a particular product is presented in a two-page spread for your
convenience.
Included in this catalog are:

 UNBRAKO fastener product descriptions


 Features and technical data about each product


 Technical discussions for application and use
For prices of stock items, see current UNBRAKO fastener price lists
or call your local UNBRAKO fastener distributor.
For non-stock items, consult your UNBRAKO fastener distributor, or contact the UNBRAKO
Engineered Fastener Group by phone at 216-581-3000 or by fax on 800-225-5777,or Internet at
http://www.spstech.com.
Commercial and Government Entity (CAGE) Code 71838

IMPORTANT
Referenced consensus standards can change over time. UNBRAKO products are manufactured in
accordance with revisions valid at time of manufacture.
This guide refers to products and sizes that may not be manufactured to stock. Please consult an
UNBRAKO distributor or UNBRAKO to determine stock status.
The technical discussions represent typical applications only.
The use of the information is at the sole discretion of the reader. Because applications vary
enormously, UNBRAKO does not warrant the scenarios described are appropriate for any specific
application. The reader must consider all variables prior to using this information.
Products modified other than by UNBRAKO are not guaranteed and not subject to return.

LIMITED WARRANTY AND EXCLUSIVE REMEDY
SPS Technologies, Inc., through its Unbrako Division warrants that these product conform to industry standards specified
herein and will be free from defects in materials and workmanship. THIS WARRANTY IS EXPRESSLY GIVEN IN LIEU OF
ANY AND ALL OTHER EXPRESS OR IMPLIED WARRANTIES, INCLUDING ANY IMPLIED WARRANTY OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE, AND IN LIEU OF ANY OTHER OBLIGATION ON THE PART
OF SPS TECHNOLOGIES, INC. SPS Technologies, Inc. will, at its option, repair or replace free of charge (excluding all shipping all shipping and handling costs) any products which have not been subject to misuse, abuse, or modification and
which in its sole determination were not manufactured in compliance with the warranty given above.
THE REMEDY PROVIDED FOR HEREIN SHALL BE THE EXCLUSIVE REMEDY FOR ANY BREACH OF WARRANTY OR ANY
CLAIM ARISING IN ANY WAY OUT OF THE MANUFACTURE, SALE, OR USE OF THESE PRODUCTS. In no event shall SPS
Technologies, Inc. be liable for consequential, incidental or any other damages of any nature whatsoever except those
specifically provided herein for any breach of warranty or any claim arising in any way out of the manufacture, sale, or
use of these products. No other person is authorized by SPS Technologies, Inc. to give any other warranty, written or oral,
pertaining to the products.

Copyright 1996, SPS Technologies

TABLE OF CONTENTS
UNBRAKO® Socket Screw Products
Page
Quick Selector Guide – Inch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Socket Head Cap Screws. . . . . . . Alloy Steel and Stainless Steel. . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Low Heads – Alloy Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Shoulder Screws . . . . . . . . . . . . . Alloy Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Flat Head Socket Screws . . . . . . Alloy Steel and Stainless Steel. . . . . . . . . . . . . . . . . . . . . . . 14, 16
Button Head Socket Screws . . . . Alloy Steel and Stainless Steel. . . . . . . . . . . . . . . . . . . . . . . 15, 16
Square Head Set Screws . . . . . . . Knurled Cup Point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Socket Set Screws . . . . . . . . . . . . Alloy Steel and Stainless Steel. . . . . . . . . . . . . . . . . . . . . . . . . . 18
Pressure Plugs . . . . . . . . . . . . . . . Dryseal Pressure Plugs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
LEVL-SEAL® Pressure Plugs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
PTFE/TEFLON*-coated Levl Seal Pressure Plugs . . . . . . . . . . . . 26
Dowel Pins . . . . . . . . . . . . . . . . . . Standard and Pull-Out Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Hexagon Keys . . . . . . . . . . . . . . . . Short Arm and Long Arm Wrenches . . . . . . . . . . . . . . . . . . . . . 32
Size Selector Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Thread Conversion chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Metric Table of Contents. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Metric Socket Head Cap Screws . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Metric Flat Head Cap Screws . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Metric Button Head Cap Screws . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
Metric Shoulder Screws . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
Metric Dowel Pins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
Metric Socket Set Screws . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
Metric Low Head Cap Screws . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
Metric Hexagon Keys and Size Selector Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
Metric Tolerances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
Metric Conversion Chart. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
Technical Section Table of Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

NOTE: The proper tightening of threaded fasteners can have a significant effect
on their performance.
Many application problems such as self-loosening and fatigue can be minimized
by adequate tightening.
The recommended seating torques listed in the catalog tables serve as guidelines
only.
Even when using the recommended seating torques, the induced loads obtained
may vary as much as ±25% depending upon the uncontrolled variables such as
mating material, lubrication, surface finish, hardness, bolt/joint compliance, etc.

LEVL-SEAL®, AND UNBRAKO® are registered trademarks of SPS Technologies
*Reg. Du Pont T.M.
1

PERFORMANCE (See Note 1)

TYPES

APPLICATIONS/FEATURES
COUNTERBORED

PROTRUDING

tensile
psi
(room temp.)

107 cycle
dynamic
fatigue (psi)

operating
temperatures
(unplated)

190,000

20,000

550°F

Socket Head
Cap Screws
1960 Series
Alloy Steel

4–10
Use alloy for maximum tensiles;
up to 190,000 psi, highest of any
socket cap screw

180,000

Socket Head
Cap Screws
1960 Series
Stainless
Steel

Use stainless for corrosive,
cryogenic or elevated temperature environments, hygienic
cleanliness.

95,000

30,000

800°F

4–10

Socket Head
Cap Screws
Low Head
Series

Use in parts too thin for standard
height heads and where clearance is limited

170,000

20,000

550°F

11

heat treat
level psi

shear
strength
in psi
550°F

12–13

Shoulder
Screws

Tool and die
industry standards;
also replace costly
special partsshafts, pivots,
clevis pins,
guides, trunnion
mountings, linkages, etc.

Flat Head
Socket
Screws
Alloy/
Stainless

Uniform, controlled 82° underhead angle for
maximum flushness and side wall
contact; non-slip
hex socket prevents marring of
material

Button Head
Cap Screws
Alloy/
Stainless

Low heads
streamline design,
use in materials
too thin to countersink; also for
non-critical loading requiring heat
treated screws

NOTE 1: Performance data listed are for standard production items only.
Non-stock items may vary due to variables in methods of manufacture.
It is suggested that the user verify performance on any non-standard parts
for critical applications.

2

page

160,000

96,000

160,000

96,000

550°F
14, 16
800°F

160,000

96,000

550°F
15–16
800°F

INCH QUICK SELECTOR GUIDE
PERFORMANCE (See Note 1)

TYPES

APPLICATIONS/FEATURES

hardness

operating
temperatures
(unplated)

page

Square Head
Set Screws

Half-dog or self-locking cup points
only. Use where maximum tightening torques are required

Rc 45 (min.)

450° F

17

Socket Set
Screws
Alloy Steel

Fasten collars, sheaves, gears,
knobs on shafts. Locate machine
parts. Cone, half-dog, flat, oval, cup
and self-locking cup points standard

Rc 45-53

450°F

18–23

Socket Set
Screws
Stainless
Steel

Use stainless for corrosive, cryogenic or elevated temperatures
environments. Plain cup point standard. Other styles on special order

Rb96-Rc33

800°F

18–23

Pressure
Plugs
3/4” Taper
Dryseal

Features common to 3/4” and 7/8”
tapers: Dryseal threads for positive
seal without sealing compound;
controlled chamfer for faster
starting

Rc 34-40

550°F

7/8” Taper
LEVL-SEAL®
Pressure Plug

LEVL-SEAL® plug features: controlled 7/8” tape in 3/4” taper hole
seats plug level, flush with surface
within 1/2 pitch. LEVL-SEAL plug is
an UNBRAKO original

PTFE/
TEFLON**
Coated

800°F
24, 26
Rb 82
Typical

400°F
Brass

Rc 35-40

550°F

Rb 82
Typical

400°F
Brass

PTFE/TEFLON coated plugs seal at
60% lower seating torques without
tape or compound; install faster at
lower cost; smaller sizes can be
power installed; LEVL-SEAL plug
type for 100% flush seating

Rc 35-40

450°F
(uncoated)

26–27

Hex Keys

Tough, ductile, for high torquing;
accurate fit in all types socket
screws; size marked for quick
identity

Rc 47-57

torsional shear
in-lb. min.
1.2 to 276.000

32–33

Dowel Pins
(Standard)

Formed ends, controlled heat treat;
close tolerances; standard for die
work; also used as bearings, gages,
precision parts, etc.

core:
Rc 50-58

For use in blind holes. Easily
removed without special tools.
Reusable, Save money. No need for
knock-out holes. Same physicals,
finish, accuracy and tolerances as
standard UNBRAKO dowel pins.

surface:
Rc 60
(min.)

Dowel Pins
Pull-Out
Type

25–27

calculated
shear psi

surface

150,000

8 microinch
(max)

150,000

8 microinch
(max)

28–29

30–31

NOTE 1: Performance data listed are for standard production items only.
Non-stock items may vary due to variables in methods of manufacture.
It is suggested that the user verify performance on any non-standard parts
for critical applications.

3

SOCKET HEAD CAP SCREWS. . . Why Socket Screws? Why UNBRAKO?
The most important reasons for
the increasing use of socket head
cap screws in industry are safety,
reliability and economy. All three
reasons are directly traceable to
the superior performance of socket
screws vs. other fasteners, and
that is due to their superior
strength and advanced design.

 Reliability, higher pressures,
stresses and speeds in todays
machines and equipment
demand stronger, more reliable
joints and stronger, more reliable fasteners to hold them
together.

 Rising costs make failure and
downtime intolerable. Bigger,
more complex units break down
more frequently despite every
effort to prevent it.

 This is why the reliability of
every component has become
critical. Components must stay
together to function properly,
and to keep them together joints
must stay tight.

 Joint reliability and safety with
maximum strength and fatigue
resistance. UNBRAKO socket
cap screws offer this to a greater
degree than any other threaded
fastener you can purchase
“off-the-self.”

 UNBRAKO socket cap screws
offer resistance to a greater
degree than any other threaded
fasteners you can purchase
“off-the-shelf.”

TENSILE STRENGTH

 U.S. standard alloy steel socket
head cap screws are made to
strength levels of 180,000 and
170,000 psi to current industry
standards. However, UNBRAKO
socket cap screws are consistently maintained at 190,000
and 180,000 psi (depending on
screw diameter).

 The higher tensile strength of
UNBRAKO socket screws can
be translated into savings.
Using fewer socket screws of
the same size can achieve the
same clamping force in the
joint. A joint requiring twelve
1-3/8” Grade 5 hex heads would
need only 7 UNBRAKO socket
head cap screws. Use them size
for size and there are fewer
holes to drill and tap and fewer
screws to buy and handle.
Smaller diameter socket head
cap screws vs. larger hex screws
cost less to drill and tap, take
less energy to drive, and there
is also weight saving.

 The size of the component
parts can be reduced since the
cylindrical heads of socket
screws need less space than
hex heads and require no
additional wrench space.
FATIGUE STRENGTH

 Joints that are subject to external
stress loading are susceptible to
fatigue failure. UNBRAKO socket
screws have distinct advantages
that give you an extra bonus of
protection against this hazard.

 Three major factors account for
the greater fatigue resistance of
UNBRAKO socket screws –
design improvements, mechanical properties and closely
controlled manufacturing
processes.

4

AUSTENITIC STAINLESS STEEL
STANDARD SERIES
UNBRAKO stainless socket screws
are made from austenitic stainless
steel. UNBRAKO stainless screws
offer excellent resistance to rust
and corrosion from acids, organic
substances, salt solutions and
atmospheres. Superior properties
attained with stainless steel include
retention of a high percentage of
tensile strength and good creep
resistance up to 800°F. without
scaling or oxidation, and good
shock and impact resistance to
temperatures as low as –300°F.
non-magnetic – Valuable in certain
electrical applications. Maximum
permeability is 1.2 Can be reduced
to 1.02 by bright annealing.
cleanliness – Corrosion resistant
characteristics of UNBRAKO
screws are useful in chemical,
food processing, appliance, paper,
textile, packaging and pharmaceutical industries, as well as laboratories, hospitals, etc.
eye-appeal – Bright, non-tarnishing
qualities add to appearance and
salability of many products; are
valuable assets to designers.
Standard processing of UNBRAKO
stainless steel socket screws
includes a passivation surface
treatment which removes any
surface contaminations.

SOCKET HEAD CAP SCREWS

Why Socket Screws?. . . Why UNBRAKO 
 “Profile” of Extra Strength
PROFILE OF EXTRA STRENGTH
Deep, accurate socket for high torque wrenching. Knurls for easier handling.
Marked for easier identification.
Head with increased bearing area for greater loading carrying capacity.
Precision forged for symmetrical grain flow, maximum strength.

Elliptical fillet doubles fatigue life at critical head-shank juncture.
“3-R” (radiused-root runout) increases fatigue life in this critical
head-shank juncture.

SHANK
ROOT
BODY

CONVENTIONAL THREAD
RUNOUT – Note sharp angle
at root where high stress concentration soon develops
crack which penetrates into
body of the screw.

UNBRAKO “3-R” (RADIUSED
ROOT RUNOUT) THREAD –
Controlled radius of runout
root provides a smooth form
that distributes stress and
increases fatigue life of
thread run-out as a much
as 300% in certain sizes.

Fully formed radiused thread increases fatigue life 100% over flat root
thread forms.
Controlled heat treatment produces maximum strength without brittleness.

Accurate control of socket depth
gives more wrench engagement
than other screws, permits full
tightening without cracking or
reaming the socket, yet provides
ample metal in the crucial fillet
area for maximum head strength.

Controlled head forging, uniform
grain flow, unbroken flow lines;
makes heads stronger; minimizes
failure in vital fillet area; adds to
fatigue strength.

Contour-following flow lines
provide extra shear strength in
threads, resist stripping and
provide high fatigue resistance.
The large root radius UNBRAKO
socket screw development
doubles fatigue life compared
to flat root thread forms.
5

SOCKET HEAD CAP SCREWS 
 1960 Series 
 Dimensions 
 Mechanical Properties

H
T

30°

G

UNB

"LT" THREAD LENGTH

A

F D

APPROX. 45°
LENGTH

J

Head markings may vary slightly depending
on manufacturing practice. Diamond knurls,
UNBRAKO, and UNB are recognized identifications for 1/4” diameter and larger.

DIMENSIONS
threads
per inch

A

D

G

head diameter

body diameter

max.

max.

T

H

J

F

LT

basic
screw
dia.

UNRC

UNRF

min.

min.

min.

max.

min.

max.

min

basic

#0
#1
#2

.060
.073
.086

–
64
56

80
72
64

.096
.118
.140

.091
.112
.134

.060
.073
.086

.0568
.0695
.0822

.020
.025
.029

.025
.031
.038

.060
.073
.086

.057
.070
.083

.050
.062
.078

.074
.087
.102

.051
.061
.073

.500
.625
.625

#3
#4
#5

.099
.112
.125

48
40
40

56
48
44

.161
.183
.205

.154
.176
.198

.099
.112
.125

.0949
.1075
.1202

.034
.038
.043

.044
.051
.057

.099
.112
.125

.095
.108
.121

.078
.094
.094

.115
.130
.145

.084
.094
.107

.625
.750
.750

#6
#8
#10

.138
.164
.190

32
32
24

40
36
32

.226
.270
.312

.218
.262
.303

.138
.164
.190

.1329
.1585
.1840

.047
.056
.065

.064
.077
.090

.138
.164
.190

.134
.159
.185

.109
.141
.156

.158
.188
.218

.116
.142
.160

.750
.875
.875

1/4
5/16
3/8

.250
.312
.375

20
18
16

28
24
24

.375
.469
.562

.365
.457
.550

.250
.3125
.375

.2435
.3053
.3678

.095
.119
.143

.120
.151
.182

.250
.312
.375

.244
.306
.368

.188
.250
.312

.278
.347
.415

.215
.273
.331

1.000
1.125
1.250

7/16
1/2
9/16

.437
.500
.562

14
13
12

20
20
18

.656
.750
.843

.642
.735
.827

.4375
.500
.5625

.4294
.4919
.5538

.166
.190
.214

.213
.245
.265

.437
.500
.562

.430
.492
.554

.375
.375
.438

.484
.552
.6185

.388
.446
.525

1.375
1.500
1.625

5/8
3/4
7/8

.625
.750
.875

11
10
9

18
16
14

.938
1.125
1.312

.921
1.107
1.293

.625
.750
.875

.6163
.7406
.8647

.238
.285
.333

.307
.370
.432

.625
.750
.875

.616
.740
.864

.500
.625
.750

.689
.828
.963

.562
.681
.798

1.750
2.000
2.250

1
1
1 1/8

1.000
1.000
1.125

8
–
7

12
14*
12

1.500
1.500
1.688

1.479
1.479
1.665

1.000
1.000
1.125

.9886
.9886
1.1086

.380
.380
.428

.495
.495
.557

1.000
1.000
1.125

.988
.988
1.111

.750
.750
.875

1.100
1.100
1.235

.914
.914
1.023

2.500
2.500
2.812

1 1/4
1 3/8
1 1/2

1.250
1.375
1.500

7
6
6

12
12
12

1.875
2.062
2.250

1.852
2.038
2.224

1.250
1.375
1.500

1.2336
1.3568
1.4818

.475
.523
.570

.620
.682
.745

1.250
1.375
1.500

1.236
1.360
1.485

.875
1.000
1.000

1.370
1.505
1.640

1.148
1.256
1.381

3.125
3.437
3.750

1 3/4
2
2 1/4

1.750
2.000
2.250

5
4 1/2
4 1/2

12
12
12

2.625
3.000
3.375

2.597
2.970
3.344

1.750
2.000
2.250

1.7295
1.9780
2.2280

.665
.760
.855

.870
.995
1.120

1.750
2.000
2.250

1.734
1.983
2.232

1.250
1.500
1.750

1.910
2.180
2.450

1.609
1.843
2.093

4.375
5.000
5.625

2 1/2
2 3/4
3

2.500
2.750
3.000

4
4
4

12
12
12

3.750
4.125
4.500

3.717
4.090
4.464

2.500
2.750
3.000

2.4762
2.7262
2.9762

.950
1.045
1.140

1.245
1.370
1.495

2.500
2.750
3.000

2.481
2.730
2.979

1.750
2.000
2.250

2.720
2.990
3.260

2.324
2.574
2.824

6.250
6.875
7.500

nom.
size

min.

head height

fillet diameter
nom.

Performance data listed are for standard production items only. Non-stock items may vary due to variables in methods of manufacture.
It is suggested that the user verify performance on any non-standard parts for critical applications.
* 1-14 is UNRS (special) standard thread form.

6

SOCKET HEAD CAP SCREWS

1960 Series 
 Dimensions 
 Mechanical Properties 
 Application Data
NOTES

Typical values for test specimens:
Alloy
Steel

Material: ASTM A574 – alloy steel
ASTM F837 – stainless steel

Stainless
Steel

Dimensions: ANSI/ASME B18.3

Elongation in 2 inches:

10% min. 10% min.

Hardness: Alloy Steel – Rc 38-43
Stainless Steel – Rb 80 – Rc 33

Reduction of area:

35% min. 30% min.

Concentricity: Body to head O.D. – within 2% of body diameter T.I.R. or .006 T.I.R.
whichever is greater. Body to hex socket – (sizes through 1/2”) – within 3% of
body diameter T.I.R. or .005 T.I.R. whichever is greater; (sizes over 1/2” – within
6% of body diameter).
The plane of the bearing surface shall be perpendicular to the axis of the screw
within a maximum deviation of 1°.
For body and grip lengths see pages 8 and 9.
Thread Class: #0 through 1” dia. – 3A; over 1” dia. – 2A.

MECHANICAL PROPERTIES
ALLOY STEEL
tensile strength
pounds
nom.
size

UNRC

UNRF

STAINLESS STEEL

minimum minimum single
shear
tensile
yield
strength strength strength
of body
psi
psi
lbs. min.
min.
min.

recommended seating torque* in-lbs
UNRC

UNRF

plain

plain

tensile strength
pounds
UNRC

UNRF

recommended seating torque* in-lbs
minimum minimum single
tensile
yield
shear
strength strength strength

UNRC

UNRF

plain

plain

#0
#1
#2

–
499
702

342
528
749

190,000 170,000
190,000 170,000
190,000 170,000

320
475
660

–
5
7

3
5
8

–
250
352

171
264
374

95,000
95,000
95,000

30,000
30,000
30,000

130
190
260

–
2.0
3.8

1.3
2.3
4

#3
#4
#5

925
1,150
1,510

994
1,260
1,580

190,000 170,000
190,000 170,000
190,000 170,000

875
1,120
1,400

12
18
24

13
19
25

463
574
756

497
628
789

95,000
95,000
95,000

30,000
30,000
30,000

350
440
550

5.7
8.0
12

6
9
14

#6
#8
#10

1,730
2,660
3,330

1,930
2,800
3,800

190,000 170,000
190,000 170,000
190,000 170,000

1,700
2,400
3,225

34
59
77

36
60
91

864
1,330
1,660

964
1,400
1,900

95,000
95,000
95,000

30,000
30,000
30,000

670
850
1,280

15
28
40

17
29
45

1/4
5/16
3/8

6,050
9,960
14,700

6,910
11,000
16,700

190,000 170,000
190,000 170,000
190,000 170,000

5,600
8,750
12,600

200
425
750

240
475
850

3,020
4,980
7,360

3,460
5,510
8,350

95,000
95,000
95,000

30,000
30,000
30,000

2,200
3,450
4,470

95
170
300

110
190
345

7/16
1/2
9/16

20,200
27,000
32,800

22,600
30,400
36,500

190,000 170,000
190,000 170,000
180,000 155,000

17,100
22,350
28,300

1,200
1,850
2,500

1,350
2,150
2,700

10,100
13,500
17,300

11,300
15,200
19,300

95,000
95,000
95,000

30,000
30,000
30,000

6,760
8,840
11,200

485
750
920

545
850
1,050

5/8
3/4
7/8

40,700
60,200
83,100

46,100
67,100
91,700

180,000 155,000
180,000 155,000
180,000 155,000

34,950
47,700
64,000

3,400
6,000
8,400

3,820
6,800
9,120

21,500
31,700
44,000

24,300
35,400
48,400

95,000
95,000
95,000

30,000
30,000
30,000

13,800
19,850
27,100

1,270
2,260
3,790

1,450
2,520
4,180

57,600

63,000

95,000

30,000

35,300

5,690

6,230

1
1
1-1/8

109,000 119,000
–
122,000
137,000 154,000

180,000 155,000
84,800
180,000 155,000 107,000
180,000 155,000 107,000

12,500
–
14,900

13,200
13,900
16,600

1-1/4
1-3/8
1-1/2

175,000 193,000
208,000 237,000
253,000 285,000

180,000 155,000 132,500
180,000 155,000 160,000
180,000 155,000 190,500

25,000
33,000
43,500

27,000
35,000
47,000

1-3/4
2
2-1/4

342,000 394,000
450,000 521,000
585,000 664,000

180,000 155,000 259,500 71,500 82,500
180,000 155,000 339,000 108,000 125,000
180,000 155,000 429,000 155,000 186,000

2-1/2
2-3/4
3

720,000 828,000 180,000 155,000 530,000 215,000 248,000
888,000 1,006,000 180,000 155,000 641,000 290,000 330,000
1,074,000 1,204,000 180,000 155,000 763,000 375,000 430,000

*Seating torques for alloy steel calculated in accordance with VDI 2230, “Systematic Calculation of High Duty Bolted Joints,” to induce approximately 120,000 PSI in the screw threads through 0.500-inch diameter, and 115,000 PSI over 0.500-inch diameter. Seating torques for stainless
steel are calculated to induce approximately 40,000 PSI stress. Values are for plain screws. For cadmium plated screws, multiply recommended
seating torque by .75; for zinc plated screws multiply by 1.40. See note, page 1.
See Technical Guidelines section for additional information on torques, installation, and hole preparation.

7

SOCKET HEAD CAP SCREWS 
 1960 Series 
 Body and Grip Lengths
LG

LENGTH TOLERANCES

LB

LENGTH

length

up to 1”
incl.

over 1” to over 2 1/2”
2 1/2” incl. to 6” incl.

#0 thru 3/8 incl.

–.03

–.04

–.06

–.12

7/16 to 3/4 incl.

–.03

–.06

–.08

–.12

7/8 to 1-1/2 incl.

–.05

–.10

–.14

–.20

–.18

–.20

–.24

over 1 1/2

BODY and GRIP LENGTHS
#0

diameter

#1

#2

#3

#4

#5

#6

LG

LB

LG

LB

LG

LB

LG

LB

LG

LB

LG

LB

3/4
7/8
1

.250
.250
.500

.187
.187
.437

LG

.250
.250

.172
.172

.250
.250

.161
.161

.250
.250

.146
.146

.250

.125

.250

.125

1 1/4
1 1/2
1 3/4

.750

.687

.625
.875

.547
.797

.625 .536 .625 .521
.875 .786 .875 .771
1.125 1.036 1.125 1.021

.250
.750
.750

.125
.625
.625

.250
.750
.750

.125 .500
.625 .500
.625 1.000

#8

over 6”

#10

LB

LG

.344
.344
.844

.375 .219
.375 .219
.875 .719

LB

1/4

LG

LB

LG

LB

.375
.375
.875

.167
.167
.667

.500
.500

.250
.250

2
2 1/4
2 1/2

.875 .667 1.000 .750
1.375 1.271 1.250 1.125 1.250 1.125 1.000 .844 .875 .719
1.250 1.125 1.500 1.344 1.375 1.219 1.375 1.167 1.000 .750
1.750 1.625 1.500 1.344 1.375 1.219 1.375 1.167 1.500 1.250

2 3/4
3
3 1/4

2.000 1.844 1.875 1.719 1.875 1.667 1.500 1.250
1.875 1.719 1.875 1.667 2.000 1.750
2.375 2.219 2.375 2.167 2.000 1.750

3 1/2
3 3/4
4

2.375 2.167 2.500 2.250
2.875 2.667 2.500 2.250
3.000 2.750

4 1/4
4 1/2
4 3/4

3.000 2.750
3.500 3.250
3.500 3.250

5
5 1/4
5 1/2

4.000 3.750
4.000 3.750

5 3/4
6
6 1/4
6 1/2
6 3/4
7
7 1/4
7 1/2
7 3/4
8
8 1/2
9
9 1/2
10
11
12
13
14
15
16
17
18
19
20

8

SOCKET HEAD CAP SCREWS

1960 Series 
 Body and Grip Lengths

LG is the maximum grip length and is the distance from
the bearing surface to the first complete thread.

total thread length including imperfect threads shall be
basic thread length plus five pitches. Lengths too short to
apply formula shall be threaded to head. Complete threads
shall extend within two pitches of the head for lengths
above the heavy line on sizes up to and including 5/8”
diameter. Larger diameters shall be threaded as close to the
head as practicable.

LB is the minimum body length and is the length of the
unthreaded cylindrical portion of the shank.
Thread length for the sizes up to and including 1” diameter
shall be controlled by the grip length and body length as
shown in the table.

Screws of longer lengths than those tabulated shall have
a thread length conforming to the formula for sizes larger
than 1”.

For sizes larger than 1” the minimum complete thread
length shall be equal to the basic thread length, and the

5/16

3/8

7/16

LG

LB

LG

LB

.625

.347

.500

.187

.625
1.125
1.125

.347
.847
.847

.500
1.000
1.000

1.625
1.625
2.125

1.187
1.347
1.847

2.125
2.625
2.625

1/2

9/16

5/8

3/4
LG

7/8
LB

LG

1
LB

LG

LB

LG

LB

LG

LB

LG

LB

LG

LB

.187
.687
.687

.625
.625
1.125

.268
.268
.768

.750
.750

.365
.365

.875

.458

.750

.295

1.500
1.500
2.000

1.187
1.187
1.687

1.125
1.625
1.625

.768
1.268
1.268

.750
1.500
1.500

.365
1.115
1.115

.875
.875
1.625

.458
.458
1.208

.750
.750
1.500

.295
.295
1.045

1.000
1.000

.500
.500

1.000

.444

1.847
2.347
2.347

2.000
2.500
2.500

1.687
2.187
2.187

2.125
2.125
2.625

1.768
1.768
2.268

1.500
2.250
2.250

1.115
1.865
1.865

1.625
1.625
2.375

1.208
1.208
1.958

1.500
1.500
2.250

1.045
1.045
1.795

1.000
1.000
2.000

.500
.500
1.500

1.000
1.000
1.000

.444
.444
.444

1.000
1.000
1.000

.375
.375
.375

3.125
3.125
3.625

2.847
2.847
3.347

3.000
3.000
3.500

2.687
2.687
3.187

2.625
3.125
3.125

2.268
2.768
2.768

2.250
3.000
3.000

1.865
2.615
2.615

2.375
2.375
3.125

1.958
1.958
2.708

2.250
2.250
3.000

1.795
1.795
2.545

2.000
2.000
2.000

1.500
1.500
1.500

2.000
2.000
2.000

1.444
1.444
1.444

1.000
2.000
2.000

.375
1.375
1.375

3.625
4.125
4.125

3.347
3.847
3.847

3.500
4.000
4.000

3.187
3.687
3.687

3.625
3.625
4.125

3.268
3.268
3.768

3.000
3.750
3.750

2.615
3.365
3.365

3.125
3.125
3.875

2.708
2.708
3.458

3.000
3.000
3.750

2.545
2.545
3.295

3.000
3.000
3.000

2.500
2.500
2.500

2.000
3.000
3.000

1.444
2.444
2.444

2.000
2.000
3.000

1.375
1.375
2.375

4.625
4.625
5.125

4.347
4.347
4.847

4.500
4.500
5.000

4.187
4.187
4.687

4.125
4.625
4.625

3.768
4.268
4.268

3.750
4.500
4.500

3.365
4.115
4.115

3.875
3.875
4.625

3.458
3.458
4.208

3.750
3.750
4.500

3.295
3.295
4.045

3.000
4.000
4.000

2.500
3.500
3.500

3.000
3.000
4.000

2.444
2.444
3.444

3.000
3.000
3.000

2.375
2.375
2.375

5.000
5.500
5.500

4.687
5.187
5.187

5.125
5.125
5.625

4.768
4.768
5.268

4.500
5.250
5.250

4.115
4.865
4.865

4.625
4.625
5.375

4.208
4.208
4.958

4.500
4.500
5.250

4.045
4.045
4.795

4.000
4.000
5.000

3.500
3.500
4.500

4.000
4.000
4.000

3.444
3.444
3.444

4.000
4.000
4.000

3.375
3.375
3.375

6.000

5.687

5.625
6.125
6.125

5.268
5.768
5.768

5.250
6.000
6.000

4.865
5.615
5.615

5.375
5.375
6.125

4.958
4.958
5.708

5.250
5.250
6.000

4.795
4.795
5.545

5.000
5.000
5.000

4.500
4.500
4.500

5.000
5.000
5.000

4.444
4.444
4.444

4.000
5.000
5.000

4.375
4.375
4.375

6.625
7.125
7.625

6.268
6.768
7.268

6.000
7.000
7.000

5.615
6.615
6.615

6.125
6.875
6.875

5.708
6.458
6.458

6.000
6.750
6.750

5.545
6.295
6.295

6.000
6.000
7.000

5.500
5.500
6.500

5.000
6.000
6.000

4.444
5.444
5.444

5.000
5.000
5.000

4.375
5.375
5.375

8.000
8.000

7.615
7.615

7.625
7.625
9.125

7.208
7.208
8.708

7.750
7.750
9.250

7.295
7.295
8.795

7.000
8.000
9.000

6.500
7.500
8.500

7.000
7.000
8.000

6.444
6.444
7.444

7.000
7.000
8.000

6.375
6.375
7.375

10.125 9.708

10.250

9.795 10.000 9.000 9.000 8.444 9.000 8.375
11.000 10.500 10.000 9.444 10.000 9.375
12.000 11.500 11.000 10.444 11.000 10.375
13.000 12.500 12.000 11.444 12.000 11.375
13.000 12.444 13.000 12.375
14.000 13.444 14.000 13.375
15.000 14.444 15.000 14.375
16.000 15.375
17.000 16.375

9

FEWER HOLES TO DRILL AND TAP
three screws do the
work of five

clearance for socket
wrench

no wrench clearance
necessary

old method

UNBRAKO method

old method

UNBRAKO method

5–3/8-16 screws @
120,000 psi tensile

3–3/8-16 screws @
190,000 psi tensile

16–3/4-16 socket head
cap

85,000 psi yield =

170,000 psi yield =

5 x 85,000 x .0775 =
33,000 lbs. max. load

3 x 170,000 x .0775 =
39,000 lbs. max. load

12–3/4-16 hexagon
head screws @
120,000 psi tensile
strength
Total strength =
537,000 lbs.

Total strength =
1,074,200 lbs.

screws @ 180,000 psi
tensile strength

HIGH TENSILE AND YIELD STRENGTH

HIGH SHEAR STRENGTH

ordinary bolts

socket head cap screws

ordinary bolts

socket head cap screws

old method

UNBRAKO method

old method

UNBRAKO method

120,000 psi. 1/2-20 bolt

120,000 psi. 1/2-20 bolt

tensile = 19,200 lbs.

190,000 psi 1/2-20
UNBRAKO

190,000 psi. 1/2-20
UNBRAKO

yield = 13,600 lbs.

tensile = 30,400 lbs.
yield = 27,200 lbs.
Extra UNBRAKO joint
strength:
tensile – 58% increase
yield – 100% increase

10

COMPACT SPACING

Shear strength =
14,100 lbs.

Shear strength =
22,400 lbs.
Extra UNBRAKO shear
strength = 8,300 lbs.
less wrenching space
needed

LOW HEAD CAP SCREWS
SOCKET HEAD CAP SCREWS 
 Low Head Type
Smooth, burr-free sockets, uniformly
concentric and usable to full depth
for correct wrench engagement.

High strength, precision fasteners
for use in parts too thin for standard
height socket cap screw and for
applications with limited clearances.

Low head height for thin parts and
limited space.
Fillet under head increases fatigue
life of head-to-shank junction.
Class 3A rolled threads with radiused
root to increase fatigue life of threads
by reducing stress concentrations
and avoiding sharp corners where
failures start.

LENGTH TOLERANCE

Highest standards of quality, material,
manufacture and performance.

UNB

MECHANICAL PROPERTIES
Hardness: Rc 38–43
Tensile Strength: 170,000 psi min.

W

to 1”

over 1”
to 2 1/2”

over
2 1/2”

All

–.03

–.04

–.06

NOTE: Performance data listed are
for standard production items only.
Non-stock items may vary due to
variables in methods of manufacture.
It is suggested that the user verify
performance on any non-standard
parts for critical applications.

Material: ASTM A574 – alloy steel

A

Diameter

Yield Strength: 150,000 psi min.

tensile strength – lbs. min.
nominal
size

UNRC

UNRF

#8
#10
1/4”
5/16”
3/8”
1/2”

2,380
2,980
5,410
8,910
13,200
24,100

2,500
3,400
6,180
9,870
14,900
27,200

single shear strength in threads
recommended*
(calculated lbs.)
seating torque
UNRC
UNRF
inch-lbs
1,450
1,700
3,090
4,930
7,450
13,600

1,570
2,140
3,900
6,210
9,400
17,100

25
35
80
157
278
667

DIMENSIONS
nom.
size

basic
screw
diameter

threads per inch

A

UNRC

UNRF

max.

#8
#10
1/4”
5/16”
3/8”
1/2”

.164
.190
.250
.312
.375
.500

32
24
20
18
16
13

36
32
28
24
24
20

.270
.312
.375
.437
.562
.750

R
fillet extension

B

F

H

W

min.

basic

min.

max.

min.

max.

min.

nom.

.265
.307
.369
.431
.556
.743

0.1640
0.1900
0.2500
0.3125
0.3750
0.5000

.060
.072
.094
.110
.115
.151

.085
.098
.127
.158
.192
.254

.079
.092
.121
.152
.182
.244

.012
.014
.014
.017
.020
.026

.007
.009
.009
.012
.015
.020

.0781
.0938
.1250
.1562
.1875
.2500

Thread Length: On all stock lengths the last complete (full form) thread measured with a thread ring gage extends to within two
threads of the head.
Threads: Threads are Class 3A UNRC and UNRF.
*Torque calculated to induce approximately 50,000 psi tensile stress in the screw threads (See Note, page 1).

11

SHOULDER SCREWS 
 Dimensions 
 Mechanical Properties 
 Seating Torques
Precision hex socket for maximum wrenching strength

Knurled head for sure finger grip and fast assembly

Neck to allow assembly with minimal chamfering

Controlled concentricity between head and body for easier, more
accurate assembly

Shoulder diameter held to .002 inch tolerance

Concentricity controlled between body and thread
Finished threads close to body for maximum holding power

Head sidewall may have straight knurls at mfrs. option

T

F

E

UNB

32

A

K

I

G

D

45°
APPROX.

J

H

LENGTH
± .005

DIMENSIONS
threads
per inch

A

D

T

H

J

K

nom.

min.

G

nom.
shoulder
diameter

thread
size

UNRC

max.

min.

max.

min.

min.

max.

1/4
5/16
3/8

.190
.250
.312

24
20
18

.375
.438
.562

.357
.419
.543

.248
.3105
.373

.246
.3085
.371

.094
.117
.141

.188
.219
.250

.177
.209
.240

.125
.156
.188

.227
.289
.352

.142
.193
.249

.133
.182
.237

1/2
5/8
3/4

.375
.500
.625

16
13
11

.750
.875
1.000

.729
.853
.977

.498
.623
.748

.496
.621
.746

.188
.234
.281

.312
.375
.500

.302
.365
.490

.250
.312
.375

.477
.602
.727

.304
.414
.521

.291
.397
.502

1
1-1/4
1-1/2

.750
.875
1.125

10
9
7

1.312
1.750
2.125

1.287
1.723
2.095

.998
1.248
1.498

.996
1.246
1.496

.375
.469
.656

.625
.750
1.000

.610
.735
.980

.500
.625
.875

.977
1.227
1.478

.638
.750
.964

.616
.726
.934

1-3/4
2

1.250
1.500

7
6

2.375
2.750

2.345
2.720

1.748
1.998

1.746
1.996

.750
.937

1.125
1.250

1.105
1.230

1.000
1.250

1.728
1.978

1.089
1.307

1.059
1.277

min.

max.

min.

NOTE: Performance data listed are for standard production items only. Non-stock items may vary due to variables in methods of manufacture.
It is suggested that the user verify performance on any non-standard parts for critical applications.

12

SHOULDER SCREWS

Dimensions 
 Mechanical Properties 
 Seating Torques
APPLICATIONS

moving shaft or pivot

stationary guide

pulley shaft uses

Shoulder screws have an undercut
portion between the thread and
shoulder, allowing a close fit. They’re
used for a wide range of punch and
die operations, such as the location
and retention of stripper plates, and
act as a guide in blanking and forming presses. Other applications for
shoulder screws include: bearing pins
for swing arms, links and levers,
shafts for cam rolls and other rotating
parts, pivots, and stud bolts. Shoulder
screws are sometimes referred to as
stripper bolts, resulting from their use
with stripper plates and springs.

MECHANICAL PROPERTIES AND SEATING TORQUES
E

F

I

T

ult.
tensile
strength
lbs. min.

single
shear
strength
of body
lbs. min.

recom.-*
mended
seating
torque
inch-lbs.

thread
length

max.

max.

+.000
–.020

.375
.438
.500

.093
.093
.093

.083
.100
.111

.094
.117
.141

2,220
4,160
7,060

4,710
7,360
10,500

45
112
230

.625
.750
.875

.093
.093
.093

.125
.154
.182

.188
.234
.281

10,600
19,810
31,670

18,850
29,450
42,410

388
990
1,975

1.000
1.125
1.500

.125
.125
.125

.200
.222
.286

.375
.469
.656

47,680
66,230
110,000

75,400
117,800
169,500

3,490
5,610
12,000

1.750
2.000

.125
.125

.286
.333

.750
.937

141,000
205,000

231,000
301,500

16,000
30,000

*See Note, page 1

NOTES
Material: ANSI/ASME B18.3,ASTM A574 – alloy steel
Heat treatment: Rockwell C 36-43; 160,000 psi tensile strength.
Dimensions: ANSI/ASME B18.3
Concentricity: Head to body – within .005 T.I.R. when checked in
“V” block equal to or longer than body length. Pitch diameter to
body – within .004 T.I.R. when held in threaded bushing and
checked at a distance of 3/16” from shoulder at threaded end.
Shoulder must rest against face of shoulder of standard “GO” ring
gage. Bearing surface of head – perpendicular to axis of body within 2° maximum deviation.
Tensile strength based on minimum neck area “G.” Shear strength
based on shoulder diameter “D.”
Thread class: 3A
Screw point chamfer: The point shall be flat or slightly concave,
and chamfered. The plane of the point shall be approximately normal to the axis of the screw. The chamfer shall extend slightly
below the root of the thread, and the edge between flat and
chamfer may be slightly rounded. The included angle of the point
should be approximately 90°.

13

FLAT HEAD SOCKET SCREWS
Dimensions
Deep, accurate socket for maximum key engagement
Uniform 82° angle under head for maximum contact
Fully formed threads for greater strength and precision fit
Continuous grain flow throughout the screw for increased strength
Heat treated alloy steel for maximum strength without brittleness or
decarburization
See page 16 for mechanical properties and applications.
LENGTH TOLERANCE
Diameter

to 1”

over 1”
to 2 1/2”

over 2 1/2”
to 6”

#0 to 3/8” incl.
7/16 to 3/4” incl.
7/8 to 1” incl.

–.03
–.03
–.05

–.04
–.06
–.10

–.06
–.08
–.14

Dimensions: ANSI/ASME B18.3
Thread Class: 3A

SOCKET DEPTH
H

(GUAGE DIA.)

UNB

T

82°
+0° A
-2°

T

THREAD LENGTH
MAX. -2 IMPERFECT THREADS

D G

APPROX. 45°

F

MACHINED SOCKET
(MANUFACTURER’S OPTION)

LENGTH
P

J

SOCKET
DEPTH

HEAD PROTRUSION

DIMENSIONS and APPLICATION DATA
nom.
size

basic
screw
dia.

threads
per inch
UNRC

UNRF

A
head diameter

D
body diameter

max.*

min.**

max.

min.

min.

T

min.

H
max.
ref.

***
thd-to-hd
max.
ref.

max.

G protrusion
gage diameter
max.

P
protrusion

F

J

min.

max.

nom.

#0
#1
#2

.060
.073
.086

–
64
56

80
72
64

.138
.168
.197

.117
.143
.168

.060
.073
.086

.0568
.0695
.0822

.025
.031
.038

.078
.101
.124

.077
.100
.123

.044
.054
.064

.500
.750
.750

.034
.038
.042

.029
.032
.034

.006
.008
.010

.035
.050
.050

#3
#4
#5

.099
.112
.125

48
40
40

56
48
44

.226
.255
.281

.193
.218
.240

.099
.112
.125

.0949
.1075
.1202

.044
.055
.061

.148
.172
.196

.147
.171
.195

.073
.083
.090

.750
.875
.875

.044
.047
.048

.035
.037
.037

.010
.012
.014

.0625
.0625
.0781

#6
#8
#10

.138
.164
.190

32
32
24

40
36
32

.307
.359
.411

.263
.311
.359

.138
.164
.190

.1329
.1585
.1840

.066
.076
.087

.220
.267
.313

.219
.266
.312

.097
.112
.127

.875
1.000
1.250

.049
.051
.054

.037
.039
.041

.015
.015
.015

.0781
.0937
.1250

1/4
5/16
3/8

.250
.312
.375

20
18
16

28
24
24

.531
.656
.781

.480
.600
.720

.250
.3125
.375

.2435
.3053
.3678

.111
.135
.159

.424
.539
.653

.423
.538
.652

.161
.198
.234

1.250
1.500
1.750

.059
.063
.069

.046
.050
.056

.015
.015
.015

.1562
.1875
.2187

7/16
1/2
5/8

.437
.500
.625

14
13
11

20
20
18

.844
.937
1.188

.781
.872
1.112

.4375
.500
.625

.4294
.4919
.6163

.159
.172
.220

.690
.739
.962

.689
.738
.961

.234
.251
.324

2.000
2.250
2.500

.084
.110
.123

.071
.096
.108

.015
.015
.015

.2500
.3125
.3750

3/4
7/8
1

.750
.875
1.000

10
9
8

16
14
12

1.438
1.688
1.938

1.355
1.605
1.855

.750
.875
1.000

.7406
.8647
.9886

.220
.248
.297

1.186
1.411
1.635

1.185
1.410
1.634

.396
.468
.540

3.000
3.250
3.750

.136
.149
.162

.121
.134
.146

.015
.015
.015

.5000
.5625
.6250

* maximum – to theoretical sharp corners
** minimum – absolute with A flat
*** maximum product length, thread to head

14

NOTE: Performance data listed are for standard production items
only. Non-stock items may vary due to variables in methods of
manufacture. It is suggested that the user verify performance on
any non-standard parts for critical application.

BUTTON HEAD CAP SCREWS
Dimensions
Precision hex socket for maximum key engagement

Low head height for modern streamline design

Fully formed threads rolled under extreme pressure provide
greater strength
Continuous grain flow makes the whole screw stronger
Heat treated alloy steel for maximum strength without brittleness
or decarburization

See page 16 for mechanical properties and applications.
LENGTH TOLERANCE
Diameter

to 1”
incl.

over 1”
to 2” incl.

To 1” incl.
Over 1” to 2”

–.03
–.03

–.04
–.06

Dimensions: ANSI/ASME B18.3
Thread Class: 3A

SOCKET
DEPTH

SOCKET
DEPTH
THREAD LENGTH
2 IMPERFECT THREADS

UNB

T

T

D

A
R

APPROX. 45°
F
MACHINED SOCKET
(MANUFACTURER’S OPTION)

S
J

H

LENGTH

DIMENSIONS and APPLICATION DATA
threads
per inch

A
head diameter

D
body diameter

UNRF

max.

min.

max.

min.

min.

max.

–
64
56

80
72
64

.114
.139
.164

.104
.129
.154

.060
.073
.086

.0568
.0695
.0822

.020
.028
.028

.099
.112
.125

48
40
40

56
48
44

.188
.213
.238

.176
.201
.226

.099
.112
.125

.0949
.1075
.1202

#6
#8
#10

.138
.164
.190

32
32
24

40
36
32

.262
.312
.361

.250
.298
.347

.138
.164
.190

1/4
5/16
3/8

.250
.312
.375

20
18
16

28
24
24

.437
.547
.656

.419
.527
.636

1/2
5/8

.500
.625

13
11

20
18

.875
1.000

.851
.970

basic
screw
dia.

UNRC

#0
#1
#2

.060
.073
.086

#3
#4
#5

nom.
size

T

H
head height

S

R

min.

thd-to-hd
max.
ref.

ref.

F
fillet dia.
max.

max.

.032
.039
.046

.026
.033
.038

.500
.500
.500

.035
.035
.044

.052
.059
.066

.044
.051
.058

.1329
.1585
.1840

.044
.052
.070

.073
.087
.101

.250
.3125
.375

.2435
. 3053
.3678

.087
.105
.122

.500
.625

.4919
.6163

.175
.210

J
min.

.010
.010
.010

.070
.080
.099

.080
.093
.106

.035
.050
.050

.500
.500
.500

.010
.015
.015

.110
.135
.141

.119
.132
.145

.0625
.0625
.0781

.063
.077
.091

.625
.750
1.000

.015
.015
.020

.158
.185
.213

.158
.194
.220

.0781
.0937
.1250

.132
.166
.199

.122
.152
.185

1.000
1.000
1.250

.031
.031
.031

.249
.309
.368

.290
.353
.415

.1562
.1875
.2187

.265
.331

.245
.311

2.000
2.000

.046
.062

.481
.523

.560
.685

.3125
.3750

15

FLAT HEAD AND BUTTON HEAD SOCKET SCREWS
Mechanical Properties
NOTES
Material: ASTM F835 – alloy steel
ASTM F879 – stainless
Hardness: Rc 38–43 for alloy steel
Rb 96–Rc 33 for stainless steel
Tensile Strength: 160,000 PSI min. ultimate tensile strength for alloy steel
90,000 PSI min. ultimate tensile strength for stainless steel
Heat Treatment: Stainless steel is in cold-worked (CW) condition unless
otherwise requested.
GENERAL NOTE
Flat, countersunk head cap screws and button head cap screws are designed
and recommended for moderate fastening applications: machine guards,
hinges, covers, etc. They are not suggested for use in critical high strength
applications where socket head cap screws should be used.

MECHANICAL PROPERTIES
ALLOY STEEL
nom.
size

ultimate
strength lbs.
UNRC

UNRF

single shear
strength of
body lbs. min.

STAINLESS STEEL
seating torque
inch-lbs.*

ultimate tensile
strength lbs.

UNRC

UNRF

–
2.5
4.5

1.5
2.5
4.5

#0
#1
#2

–
390
555

265
390
555

271
402
556

#3
#4
#5

725
1,040
1,260

725
1,040
1,310

739
946
1,180

7
8
12

#6
#8
#10

1,440
2,220
2,780

1,620
2,240
3,180

1,440
2,030
2,770

1/4
5/16
3/8

5,070
8,350
12,400

5,790
9,250
14,000

7/16
1/2
9/16

16,900
22,800
28,900

5/8
3/4
7/8
1

36,000
53,200
73,500
96,300

UNRC

UNRF

single shear
strength of
body lbs. min.

seating torque
inch-lbs.*
UNRC

UNRF

–
237
333

162
250
355

93
137
191

–
1.7
2.8

1.0
1.8
3.0

7
8
13

438
544
716

471
595
747

253
325
403

4.3
6.0
8.9

4.6
6.6
9.3

15
30
40

17
31
45

818
1,260
1,575

913
1,327
1,800

491
693
931

11
20
30

12
21
34

4,710
7,360
10,600

100
200
350

110
220
400

2,862
4,716
6,975

3,276
5,220
7,900

1,610
2,520
3,620

71
123
218

81
136
247

18,900
25,600
32,300

14,400
18,850
23,900

560
850
1,200

625
1,000
1,360

9,570
12,770
16,300

10,680
14,390
18,300

4,930
6,440
8,150

349
532
767

388
600
856

40,800
59,300
81,000
106,000

29,450
42,400
57,700
75,400

1,700
3,000
5,000
7,200

1,900
3,200
5,400
7,600

20,300
30,100
41,500
54,500

23,000
33,600
45,800
59,700

10,100
14,500
19,700
25,800

1,060
1,880
3,030
4,550

1,200
2,100
3,340
5,000

*Torques values listed are for plain screws to induce 65,000 psi stress in alloy steel and 30,000 psi tensile stress in stainless steel screw
threads. For cadmium plated screws, multiply recommended seating torque by .75; for zinc plated screws multiply by 1.40.
See Note, page 1.

16

SQUARE HEAD SET SCREWS

Dimensions 
 Application Data

Heat treated alloy steel for
maximum strength without
brittleness or decarburization

VIBRATIONAL HOLDING POWER
vs. SEATING TORQUE
Size: 5/16”–18x1/2”
GREATER TIGHTENING TORQUES
of UNBRAKO Square Heads,
made of high quality alloy steel,
provide 50% more axial holding
power than ordinary carbon steel
square heads. And, because of
the increased torque plus the
Knurled Cup Point, UNBRAKO
Square Head Set Screws deliver
up to 400 percent more
Vibrational Holding Power.

Fully formed threads with continuous grain flow for greater strength
and precision fit

Knurled cup point for positive
self-locking and vibration resistance

Seconds

1000

800

600

400

VIBRATIONAL HOLDING POWER (Endurance)

Threads per ANSI B 1.1;
Handbook H-28. ANSI B18.6.2

UNBRAKO
ALLOY STEEL
SQUARE HEADS

400% more
Vibrational
Holding Power

CARBON STEEL
SQUARE HEADS
(Plain Cup Point)

200

SEATING TORQUE (Inch Lbs.)

0

100

200

300

400

LENGTH TOLERANCE
Diameter
up to 5/8”
3/4” and over

SELF-LOCKING KNURLED CUP POINT
INTERNAL OR EXTERNAL KNURL
FURNISHED AT UNBRAKO OPTION

basic
screw
diameter

threads
per
inch

#10
1/4
5/16

.190
.250
.312

3/8
7/16
1/2

A

–.03
–.06

–.06
–.12

–.09
–.18

NOTES
Material: ASTM A574 – alloy steel
Heat treatment: Rc 45 min., through heat treated
Thread: Class 2A.
These torques are appreciably higher than socket set screw torque
values, therefore thread stripping strength of mating material
must be considered.

DIMENSIONS and APPLICATION DATA
nom.
size

up to 1” 1” to 2” 2” and
incl.
incl.
over

C

D

min.

max.

min.

max.

24
20
18

.247*
.331
.415

.102
.132
.172

.88
.118
.156

.375
.437
.500

16
14
13

.497
.581
.665

.212
.252
.291

9/16
5/8
3/4

.562
.652
.750

12
11
10

.648
.833
1.001

7/8
1
1 1/8

.875
1.000
1.125

9
8
7

1 1/4
1 3/8
1 1/2

1.250
1.375
1.500

7
6
6

F

H

min.

±.010

max.

.127
.156
.203

.120
.149
.195

.045
.063
.078

.148
.196
.245

.194
.232
.270

.250
.297
.344

.241
.287
.334

.094
.109
.125

.332
.371
.450

.309
.347
.425

.391
.469
.563

.379
.456
.549

1.170
1.337
1.505

.530
.609
.689

.502
.579
.655

.656
.750
.844

1.674
1.843
2.010

.767
.848
.926

.733
.808
.886

.938
1.031
1.125

R
min.

W

recom.**
torque
inch-lbs.

nom.

max.

min.

.134*
.178
.224

31/64
5/8
25/32

.188
.250
.312

.180
.241
.302

100
212
420

.293
.341
.389

.270
.315
.361

15/16
1 3/32
1 1/4

.375
.437
.500

.362
.423
.484

830
1,350
2,100

.140
.156
.188

.437
.485
.582

.407
.452
.544

1 13/32
1 9/16
1 7/8

.562
.625
.750

.545
.606
.729

2,850
4,250
7,700

.642
.734
.826

.219
.250
.281

.678
.774
.870

.635
.726
.817

2 3/16
2 1/2
2 13/16

.875
1.000
1.125

.852
.974
1.096

12,600
16,600
20,800

.920
1.011
1.105

.312
.344
.375

.966
1.063
1.159

.908
1.000
1.091

3 1/8
3 7/16
3 3/4

1.250
1.375
1.500

1.219
1.342
1.464

25,000
32,000
44,000

*#10 may have head dimensions from 1/4 nominal size furnished at Unbrako option. **See Note, page 1.

17

SOCKET SET SCREWS 
 Dimensions 
 Application Data 
 Seating Torques

T

LENGTH TOLERANCE

APPROX. 45°

C

A

APPROX.
45°

.63 and
under

over .63
to 2”

over 2”
to 6”

over 6”

All

±.01

±.02

±.03

±.06

118°

SEE NOTE**

APPROX.
30°

J

Diameter

LENGTH – SEE NOTE

APPROX.
45°

APPROX.
45°

Q
R
C

C

P

90°

C

FLAT

HALF-DOG

SEE NOTE 1
CONE

SEE NOTE 2
KNURLED CUP

118°

118°

SEE NOTE**
PLAIN CUP

OVAL

DIMENSIONS
basic
screw
diameter

UNRC

UNRF

max.

UNRC

UNRF

max.

min.

max.

min.

#0
#1
#2

.060
.073
.086

–
64
56

80
72
64

.0600
.0730
.0860

–
.0692
.0819

.0568
.0695
.0822

.033
.040
.047

.027
.033
.039

.040
.049
.057

.037
.045
.053

#3
#4
#5

.099
.112
.125

48
40
40

56
48
44

.0990
.1120
.1250

.0945
.1069
.1199

.0949
.1075
.1202

.054
.061
.067

.045
.051
.057

.066
.075
.083

.062
.070
.078

#6
#8
#10

.138
.164
.190

32
32
24

40
36
32

.1380
.1640
.1900

.1320
.1580
.1825

.1329
.1585
.1840

.074
.087
.102

.064
.076
.088

.092
.109
.127

.087
.103
.120

1/4
5/16
3/8

.250
.312
.375

20
18
16

28
24
24

.2500
.3125
.3750

.2419
.3038
.3656

.2435
.3053
.3678

.132
.172
.212

.118
.156
.194

.156
.203
.250

.149
.195
.241

7/16
1/2
9/16

.437
.500
.562

14
13
12

20
20
18

.4375
.5000
.5625

.4272
.4891
.5511

.4294
.4919
.5538

.252
.291
.332

.232
.207
.309

.297
.344
.390

.287
.334
.379

5/8
3/4
7/8

.625
.750
.875

11
10
9

18
16
14

.6250
.7500
.8750

.6129
.7371
.8611

.6163
.7406
.8647

.371
.450
.530

.347
.425
.502

.469
.562
.656

.456
.549
.642

1
1 1/8
1 1/4

1.000
1.125
1.250

8
7
7

12
12
12

1.0000
1.1250
1.2500

.9850
1.1086
1.2336

.9886
1.1136
1.2386

.609
.689
.767

.579
.655
.733

.750
.844
.938

.734
.826
.920

1 3/8
1 1/2

1.375
1.500

6
6

12
12

1.3750
1.5000

1.3568
1.4818

1.3636
1.4886

.848
.926

.808
.886

1.031
1.125

1.011
1.105

nom.
size

threads per inch

A

C

NOTE: Performance data listed are for standard production items only. Non-stock
items may vary due to variables in methods of manufacture. It is suggested that
the user verify performance on any non-standard parts for critical applications.

18

P

SOCKET SET SCREWS

Dimensions 
 Application Data 
 Seating Torques
Deep socket – Key fits deeply into socket to provide extra wrenching area for
tighter tightening without reaming the socket or rounding off corners of key

Continuous grain flow – Flow lines of rolled threads follow closely the contour
of the screw

Fully formed threads – are rolled, not cut or ground. Metal is compressed, making
it extra strong. Threads resist shearing, withstand higher tightening torques
Class 3A threads – Formed with closest interchangeable fit for maximum crosssection with smooth assembly. Assure better mating of parts

Counterbored knurled cup point

NOTES
Material: ASTM F912 – alloy steel
ASTM F880 – stainless steel
Dimensions: ASME/ANSI B18.3
Hardness: Rc 45-53 (alloy steel only),
Rb 96-Rc 33 (stainless steel)
Thread class: 3A
DIMENSIONS

1. When length equals nominal diameter or less, included angle is 118°.
(#4 x 1/8 and #8 x 3/16 also have
118 angle)
2. When length equals nominal diameter or less, included angle is 130°.

RECOMMENDED SEATING TORQUES – INCH-LBS.**

Q

T*

J

R

Applicable only to nominal minimum lengths shown or longer

nom. min.
screw length

min. key
engagement

.4
1.2
1.2

3/32
1/8
1/8

.050
.060
.060

5
5
10

4
4
7

5/32
5/32
5/32

.070
.070
.080

.104
.123
.142

10
20
36

7
16
26

3/16
3/16
3/16

.080
.090
.100

.125
.1562
.1875

.188
.234
.281

87
165
290

70
130
230

5/16
3/8
7/16

.125
.156
.188

.190
.210
.265

.2187
.250
.250

.328
.375
.422

430
620
620

340
500
500

1/2
9/16
5/8

.219
.250
.250

.148
.180
.211

.265
.330
.450

.3125
.375
.500

.469
.562
.656

1,325
2,400
3,600

980
1,700
3,000

11/16
3/4
3/4

.312
.375
.500

.260
.291
.323

.240
.271
.303

.550
.650
.700

.5625
.5625
.625

.750
.844
.938

5,000
7,200
9,600

4,000
5,600
7,700

7/8
1
1 1/8

.562
.562
.625

.354
.385

.334
.365

.700
.750

.625
.750

1.031
1.125

9,600
11,320

7,700
9,100

1 1/4
1 1/4

.625
.750

max.

min.

min.

nom.

basic

.017
.021
.024

.013
.017
.020

.035
.035
.035

.028
.035
.035

.045
.055
.064

.027
.030
.033

.023
.026
.027

.060
.075
.075

.050
.050
.0625

.074
.084
.094

.038
.043
.049

.032
.037
.041

.075
.075
.105

.0625
.0781
.0937

.067
.082
.099

.059
.074
.089

.105
.140
.140

.114
.130
.146

.104
.120
.136

.164
.196
.227

alloy steel
1.0
1.8
1.8

*CAUTION: Values shown in column T are for minimum stock length cup point screws.
Screws shorter than nominal minimum length shown do not have sockets deep enough
to utilize full key capability which can result in failure of socket, key or mating threads.

stainless

**See Note, page 1.

19

SOCKET SET SCREWS 
 Point Selection According to Application
Socket set screws offer three types of
holding power: torsional (resistance
to rotation); axial (resistance to lateral
movement); and vibrational.
Size selection is an important factor
in holding power. The screw diameter
should be roughly 1/2 that of the
shaft as a rule-of-thumb. (For more
specific size data see pages 18–19.)
Additional design considerations
appear below.
Holding power is almost directly proportional to seating torque in a cup,
flat, and oval point screws. Holding
power can be increased by increasing
seating torque. Greater holding
power reduces the number of screws
required and the assembled cost of
the application.
By its penetration, the set screw point
can add as a much as 15% to total
holding power. Cone points, with

deepest penetration, give the greatest
increase; oval points, with minimum
penetration, the least. Making 1 the
index for cup point, holding power
values from tables on pages 22 and
23 can be multiplied by 1.07 for cone
point, 0.92 for flat or dog points, and
0.90 for oval point.
Relative hardness between set screw
and shaft is also a factor. A 10-point
differential between the screw’s normal Rockwell C 50 and shaft should
be maintained for full holding power.
As much as 15% loss in holding
power can result from a lower differential.
Vibration resistance can be achieved
by correct size and proper tightening.
The UNBRAKO knurl cup set screw
offers additional mechanical locking
resistance when required.

POINT SELECTION
According to Application
Point selection is normally determined by
the nature of the application – materials, their
relative hardness, frequency of assembly
and re-assembly and other factors. Reviewed
here are standard point types, their general
features and most frequent areas of application
of each type.

20

knurled cup

plain cup

For quick and permanent
location of gears, collars,
pulleys or knobs on shafts.
Exclusive counterclockwise
locking knurls resist screw
loosening, even in poorly
tapped holes. Resists most
severe vibration.

Use against hardened
shafts, in zinc, die castings
and other soft materials
where high tightening
torques are impractical.

SOCKET SET SCREWS
Point Selection According to Application
STAINLESS STEEL ADVANTAGES

 Corrosion resistance, Wide temperature range (–300° F to +800° F), Freedom
from scaling or oxidation.

 Non-magnetic, a valuable property in certain electrical and electronic applications. (Maximum permeability is 1.2 and can be reduced to 1.02 by bright
annealing.) Corrosion-resistance useful where cleanliness is important.

 Standard processing of these socket set screws includes a passivation treatment which neutralizes surface contamination.

flat

oval

cone

half dog

Use where parts must be
frequently re-set, as it
causes little or no damage
to part it bears against.
Can be used against hardened shafts (usually with
ground flat for better contact) and as adjusting
screw. Preferred for thin
wall thickness and on soft
plugs.

Use for frequent adjustment without deformation
of part it bears against,
also for seating against
an angular surface.
Circular U-grooves or
axial V-grooves sometimes
put in shaft to permit
rotational or longitudinal
adjustment.

For permanent location of
parts. Deep penetration
gives highest axial and
holding power. In material
over Rockwell C15 point is
spotted to half its length
to develop shear strength
across point. Used for
pivots and fine adjustment.

Used for permanent location of one part to another.
Point is spotted in hole
drilled in shaft or against
flat (milled). Often replaces
dowel pins. Works well
against hardened members
or hollow tubing.

21

SOCKET SET SCREWS 
 Torsional and Axial Holding Power
HOLDING POWER (percent of single set screw assembly)

SIZE SELECTION OF SOCKET SET SCREWS
The user of a set-screw-fastened assembly is
primarily buying static holding power. The
data in this chart offers a simplified means for
selecting diameter and seating torque of a set
screw on a given diameter shaft.
Torsional holding power in inch-pounds
and axial holding power in pounds are tabulated for various cup point socket screws,
seated at recommended installation torques.
Shafting used was hardened to Rockwell C15.
Test involved Class 3A screw threads in Class
2B tapped holes. Data was determined experimentally in a long series of tests in which
holding power was defined as the minimum
load to produce 0.010 inch relative movement
of shaft and collar.

200

a
100

60

120

180

Fig. 1 ANGLE BETWEEN SCREWS, a (deg.)

From this basic chart, values can be modified by percentage factors to yield suitable
design data for almost any standard set screw
application.

NOTES
Tabulated axial and torsional holding powers are typical strengths and should be used accordingly, with specific safety factors appropriate
to the given application and load conditions. Good results have been obtained with a factor of 1.5-2.0 under static load conditions (i.e.,
where a collar is supporting a vertical load on a post) and of 4.0-8.0 for various dynamic situations.
Values in bold type in the chart indicate recommended set screw sizes on the basis that screw diameter should be roughly one-half shaft
diameter.

TORSIONAL and AXIAL HOLDING POWER (Based on Recommended Seating Torques – Inch-Lbs.)

nom.
size
#0
#1
#2

axial
seating holding
power
torque
inch-lbs. (pounds)
1.0
1.8
1.8

3/32

1/8

5/32

3/16

7/32

1/4

5/16

3/8

7/16

1/2

26.3
35.0
43.7

40.0
50.0

9/16

torsional holding power inch-lbs.
1.5
2.0
2.6

2.3
3.0
4.0

3.1
4.0
5.3

3.9
5.0
6.6

4.7
6.1
8.0

5.4
7.1
9.3

6.2
8.1
10.6

10.0
13.2

16.0

3.2

5.6
7.5

7.5
10.0
12.5

9.3
12.5
15.6

11.3
15.0
18.7

13.0
17.5
21.8

15.0
20.0
25.0

18.7
25.0
31.2

22.5
30.0
37.5

19
30

23
36
51

27
42
59

31
48
68

39
60
84

47
72
101

55
84
118

62
96
135

70
108
152

125

156
234

187
280
375

218
327
437

250
375
500

281
421
562

545

625
750

702
843
985

5
5
10

120
160
200

#6
#8
#10

10
20
36

250
385
540

1/4
5/16
3/8

87
165
290

1,000
1,500
2,000

7/16
1/2
9/16

430
620
620

2,500
3,000
3,500

1,325
2,400
3,600
5,000

4,000
5,000
5,600
6,500

22

1/16

50
65
85

#3
#4
#5

5/8
3/4
7/8
1

shaft diameter (shaft hardness Rc 15 to Rc 35)

56.2

SOCKET SET SCREWS
Torsional and Axial Holding Power
If you know set screws, you know
that the tighter you can tighten them,
the better they hold and the more
they resist loosening from vibration.
But there’s a limit to how much you
can tighten the average socket set
screw. If you’re not careful, you can
ream or crack the socket, and in some
cases, even strip the threads. So
you’re never quite sure whether or
not it will actually stay tight.
With UNBRAKO set screws it’s a
different story. A unique combination
of design and carefully controlled
manufacturing and heat treating
gives these screws extra strength that
permits you to tighten them appreciably tighter than ordinary screws with
minimal fear of reaming or cracking
the socket. this extra strength represents a substantial bonus of extra
holding power and the additional
safety and reliability that goes with it.
Design – Deeper UNBRAKO sockets give more key engagement to let
you seat the screws tighter. Corners
are radiused to safeguard against
reaming or cracking the socket when
the extra tightening torque is applied.
The sharp corners of other set screws
create high stress concentrations and

can cause cracking, even at lower
tightening torques. By eliminating the
corners, the radii distribute tightening
stresses to reduce the chance of splitting to a minimum.
Controlled Manufacturing – The
fully-formed threads of UNBRAKO set
screws are rolled under extreme pressure to minimize stripping and handle
the higher tightening torques. Also,
with rolled threads, tolerances can be
more closely maintained. UNBRAKO
set screws have Class 3A threads,
closest interchangeable fit, giving
maximum cross-section with smooth
assembly. The thread form itself has
the radiused root that increases the
strength of the threads and resistance
to shear.
Controlled Heat Treatment – This
is the third element of the combination. Too little carbon in the furnace
atmosphere (decarburization) makes
screws soft, causing reamed sockets,
stripped threads and sheared points
when screws are tightened. Too much
carbon (carburization) makes screws
brittle and liable to crack or fracture.
The heat treatment is literally tailored
to each “heat” of UNBRAKO screws,
maintaining the necessary controlled

Rc 45-53 hardness for maximum
strength.
Finally, point style affects holding
power. As much as 15% more can be
contributed, depending on the depth
of penetration. The cone point (when
used without a spotting hole in the
shaft) gives greatest increase because
of its greater penetration. The oval
point, with the least contact area,
affords the least. The cup point lies
in between, but is by far the most
commonly used, because of the wide
range of applications to which it is
adaptable.
However, there is one cup point
that can give you both a maximum of
holding power and of resistance to
vibration. It is the exclusive UNBRAKO
knurled cup point, whose locking
knurls bite into the shaft and resist
the tendency of the screw to back out
of the tapped hole. The chart on this
page shows clearly how much better
the UNBRAKO set screws resist
vibration in comparison with plain
cup point set screws. UNBRAKO
knurled cup point self-locking set
screws give you excellent performance under conditions of extreme
vibration.

UNBRAKO SOCKET SET SCREWS – UNRC or UNRF Thread – Seated Against Steel Shaft

nom.
size
#0
#1
#2

axial
seating holding
torque
power
inch-lbs. (pounds)
1.0
1.8
1.8

shaft diameter (shaft hardness Rc 15 to Rc 35)
5/8

3/4

7/8

1

1 1/4

1 1/2

1 3/4

2

2 1/2

3

3 1/2

torsional holding power inch-lbs.

50
65
85

#3
#4
#5

5
5
10

120
160
200

62

#6
#8
#10

10
20
36

250
385
540

78
120
169

94
144
202

109
168
236

192
270

338

1/4
5/16
3/8

87
165
290

1,000
1,500
2,000

312
468
625

357
562
750

437
656
875

500
750
1000

625
937
1250

750
1125
1500

1310
1750

1500
2000

7/16
1/2
9/16

430
620
620

2,500
3,000
3,500

780
937
1090

937
1125
1310

1095
1310
1530

1250
1500
1750

1560
1875
2190

1875
2250
2620

2210
2620
3030

2500
3000
3500

3125
3750
4370

4500
5250

6120

1,325
2,400
3,600
5,000

4,000
5,000
5,600
6,500

1250

1500
1875

1750
2190
2620

2000
2500
3000
3500

2500
3125
3750
4375

3000
3750
4500
5250

3750
4500
5250
6120

4000
5000
6000
7000

5000
6250
7500
8750

6000
7500
9000
10500

7000
8750
10500
12250

5/8
3/4
7/8
1

4

8000
10000
12000
14000

23

PRESSURE PLUGS
DRYSEAL TYPE with 3/4-inch taper per foot
Precision hex socket with maximum depth for positive wrenching
at higher seating torques

Dryseal-thread form achieves a seal without need for compound
Heat treated alloy steel for strength
Roundness-closely controlled for better sealing
Uniform taper of 3/4 inch per foot

Controlled chamfer for faster starting

Threads NPTF per ANSI B1.20.3

See Notes on page 25
See Application Data on page 26

DIMENSIONS
nominal
thread
size

A
ref.

F
min.

G
min.

L
±.010

W
nom.

X
note 4

E0

E1

L1

1/16
1/8
1/4

.062
.125
.250

27
27
18

.318
.411
.545

.062
.062
.073

.140
.140
.218

.312
.312
.437

.156
.188
.250

.003
.003
.003

.27118
.36351
.47739

.28118
.37360
.49163

.160
.1615
.2278

3/8
1/2
3/4

.375
.500
.750

18
14
14

.684
.847
1.061

.084
.095
.125

.250
.312
.312

.500
.562
.625

.312
.375
.562

.005
.005
.007

.61201
.75843
.96768

.62701
.77843
.98887

.240
.320
.339

1.000
1.250
1.500
2.000

11 1/2
11 1/2
11 1/2
11 1/2

1.333
1.679
1.918
2.395

.125
.156
.156
.156

.375
.437
.437
.437

.750
.812
.812
.875

.625
.750
1.000**
1.000**

.007
.010
.010
.010

1.21363
1.55713
1.79609
2.26902

1.23863
1.58338
1.82234
2.29627

.400
.420
.420
.436

1
1 1/4
1 1/2
2

**.750 for LEVL-SEAL

24

basic thread dimension

threads
per in.

PRESSURE PLUGS
LEVL SEAL TYPE Dryseal Thread Form with 7/8-inch per foot
®

Precision hex socket with maximum depth for positive
wrenching at higher seating torques
Heat treated alloy steel for strength
Roundness closely controlled for better sealing
High pressure is developed through a deliberate difference of
taper between the plug and the tapped hole having standard
3/4” taper
Flush seating is achieved through closer control of thread
forms, sizes and taper-improves safety and appearance.
Fully formed PTF dryseal threads for better sealing without
the use of a compound
Controlled chamfer for faster

See Application Data on page 27

DIMENSIONS*
nominal
thread
size

A
ref.

F
min.

G
min.

L
+.000
–.015

E
note 3
ref.

1/16
1/8
1/4

.062
.125
.250

.307
.401
.529

.052
.049
.045

.141
.141
.266

.250
.250
.406

.28118
.37360
.49163

3/8
1/2
3/4

.375
.500
.750

.667
.830
1.041

.040
.067
.054

.266
.329
.329

.406
.531
.531

.62701
.77843
.98887

1.000
1.250
1.500
2.000

1.302
1.647
1.885
2.360

.112
.102
.102
.084

.360
.360
.360
.360

.656
.656
.656
.656

1.23863
1.58338
1.82234
2.29627

1
1 1/4
1 1/2
2

See page 24 for threads per inch, w nom., and X.
*Dimensions before coating for PTFE/TEFLON-coated LEVL-SEAL
pressure plugs.

NOTES
1. Material: ASTM A574 alloy steel, austenitic stainless
steel or brass.
2. Hardness: Rc 35-40 for steel.
3. Basic pitch diameter: E-pitch dia. at a distance of
one-half pitch from large end of plug.
PTF thread from 7/8-inch taper per foot.
E0 – pitch diameter at small end of plug;
E1 – pitch diameter at L1 distance from end of plug;
L1 – length of hand-tight engagement.
4. Bottom of plug to be flat within “X” T.I.R.
DRY-SEAL and LEVL-SEAL: Small end of plug to be
flush with face of standard NPTF ring gages within one
thread (L1, L2 and tapered ring).
Large end of plug to be flush with face of special 7/8
taper ring gages within one-half thread.
5. Undercut in socket at mfrs. option
6. Six equally spaced identification grooves (1/16-27
plug to have 3 identification grooves) on alloy steel
plugs. (LEVL-SEAL)
7. Dimensions apply before plating and/or coating.

25

PRESSURE PLUGS 
 Application Data
Pressure plugs are not pipe plugs.
Pipe plugs (plumber’s fittings) are
limited to pressures of 600 psi, are
sealed with a compound, and are
made of cast iron with cut threads
and protruding square drive.

possible tightening torque. Galling
and seizure are caused by metal
pickup on the mating surfaces and
are directly related to force on the
surface, material hardness, lubrication used, and thread finish.

TYPES OF PRESSURE
PLUG THREADS

Pressure plugs are made to
closer tolerances, are generally of
higher quality, and almost all have
taper threads. Properly made and
used, they will seal at pressures to
5000 psi and without a sealing compound (pressure tests are usually at
20,000 psi.) they are often used in
hydraulic and pneumatic designs.

How Pressure Plugs Seal

NPT: National Pipe thread, Tapered.
This is the thread form commonly
used for commercial pipe and fittings
for low pressure applications. A lubricant and sealer are generally used.

Performance Requirements
Pressure plugs used in industrial
applications should:

 not leak at pressures to 5000 psi

 need no sealing compounds

 be reusable without seizure

 give a good seal when reused

 seal low viscosity fluids

 require minimum seating torque

 require minimum re-tooling or
special tools.
For a satisfactory seal, the threads of
the plug and those in the mating hole
must not gall or seize up to maximum

Sealing is achieved by crushing the
crest of one thread against the root
of the mating thread. If too much
of compressive force is required to
torque the plug, it will tend to gall
in the hole. Too little force will not
deform the crest of threads enough
to produce a seal. Increasing the
hardness of the material will reduce
galling but will also increase the
required sealing force. Generally a
hardness range of Rc 30 to 40 will
meet most requirements. The tightening force must be low enough to
cause no galling in this range.
Cost Considerations
Dryseal plugs are more frequently
used, especially where reuse is frequent. Reason: more threads are
engaged and they therefore resist
leakage better. They are also preferred in soft metals to reduce of
over-torquing.

Three thread forms are commonly
used for pipe plugs and pressure
plugs:

ANPT: Aeronautical National Pipe
thread, Tapered. Covered by MIL-S7105, this thread form was developed
for aircraft use. It is basically the
same as the NPT thread except that
tolerances have been reduced about
50 percent. Plugs made with this
thread should be used with lubricants
and sealers. They are not to be used
for hydraulic applications.
NPTF: National Pipe thread, Tapered,
Fuel. This is the standard thread for
pressure plugs. They make pressuretight joints without a sealant.
Tolerances are about 1/4 those for
NPT threads. The standard which
applies is ANSI B1.20.3. Applicable
for fluid power applications.

APPLICATION DATA – DRYSEAL TYPE
nom.
size

threads
per
inch

1/16
1/8
1/4

27
27
18

15/64
21/64
27/64

1/4
11/32
7/16

150
250
600

3/8
1/2
3/4

18
14
14

9/16
11/16
57/64

37/64
23/32
59/64

1200
1800
3000

1 1/8
37.5mm
43.5mm
2 3/16

1 5/32
–
–
–

4200
5400
6900
8500

1
1 1/4
1 1/2
2

26

11 1/2
11 1/2
11 1/2
11 1/2

tap
drill
size+

tap
drill
size ++

recommended

torque
in.-lbs*

Unbrako recommends using a tapered reamer with corresponding
size tap drill (see page 27).
+With use of reamer (taper thread).
++Without use of tapered reamer.
*Recommended torques for alloy steel only. Multiply by .65 for
stainless steel and .50 for brass.
NPTF fully formed Dryseal threads achieve seal in tapped holes
without need for sealing compounds.

PRESSURE PLUGS
PTFE/TEFLON-Coated LEVL-SEAL Type Dryseal Thread Form with 7/8-inch Taper per Foot
Deliberate difference in taper between
the plug and the tapped hole. Ideal
for use in assemblies where clearance
is limited and in hydraulic lines near

moving parts. Designed for use in
hard materials and in thick-walled
sections as well as for normal plug
applications.
internal root crushes
external crest here
NPTF
INTERNAL THREADS
(HOLE)

PLUG

external root crushes
internal crest here

High pressure seal – Achieved
through metal-to-metal contact at the
large end of the plug. High load
placed on the few mating threads
near the top of the hole.

Flush seating – Design of LEVL-SEAL
plug permits seating within half a
pitch in a normally tapped hole.
Conventional plugs have the greater
tolerance of a full pitch and usually
protrude above the surface.

PTF fully formed Dryseal threads
designed to achieve seal in tapped
holes without need for sealing compounds.

PTFE/TEFLON Coated
LEVL-SEAL Type

of tapes or sealing compounds, even
with liquids of very low viscosity.
SPS Laboratories have tested these
plugs with surges up to 13,500 psi 8
times in 5 minutes, then held peak
pressure for 6 full hours without trace
of leakage.

combination of extra hardness and
abrasion resistance which permit
reuse up to 5 times without appreciable loss of seal.

Typical thickness is 0.0005-inch
LEVL-SEAL precision coated with
tough, corrosion-resistant
PTFE/TEFLON.
Installation of the new plugs is faster
with the coating of PTFE/TEFLON
which acts as a lubricant as well as
seal. Power equipment can be used
to install the smaller sizes instead of
the manual wrenching required by
higher torques of uncoated plugs.
Suited for in assembly line production.
Higher hydraulic and pneumatic
working pressures can be effectively
sealed. Seal is effective without use

Flush seating improves appearance and adds safety. LEVL-SEAL
plugs seat flush because of a combination of (1) gaging procedures, and
(2) a deliberate difference in taper
between the plug and a normally
tapped NPTF hole. (The taper of the
plug is 7/8” per foot , while that of the
hole is 3/4” per foot.)
PTFE/TEFLON was selected for
the coating material because of its

The coating is serviceable to
+450°F without deterioration.
Temperatures lower than –100°F
require the use of stainless steel plugs.
These are available in the same range
of sizes as the alloy steel plugs.
With no tape or sealing compound
involved, there is no danger of foreign
matter entering and contaminating
the system or equipment. The coating
reduces any tendency of the plug to
“freeze” in the hole because of rust
or corrosion.

APPLICATION DATA – LEVL-SEAL and LEVL-SEAL with PTFE/TEFLON
recommended hole diameter
nom.
size

threads
per inch

tapping information
tap projection thru L1 ring

max.

min.

max.

min.

imperfect
threads
allowable

tap* drill
size

recommended
torque (inch-lbs.)
alloy steel**

1/16
1/8
1/4

27
27
18

.2374
.3311
.4249

.2334
.3271
.4209

.375
.375
.521

.250
.250
.397

4
4
4

15/64
21/64
27/64

150
250
600

3/8
1/2
3/4

18
14
14

.5655
.6905
.8936

.5615
.6865
.8896

.516
.641
.627

.392
.517
.503

4
4
4

9/16
11/16
57/64

1200
1800
3000

1
1 1/4
1 1/2
2

11 1/2
11 1/2
11 1/2
11 1/2

1.1280
1.4794
1.7165
2.1905

1.1240
1.4754
1.7116
2.8165

.772
.780
.793
.761

.584
.592
.605
.573

4
4
4
4

1 1/8
37.5mm
43.5mm
2 3/16

4200
5400
6900
8500

*For taper thread (using tapered reamer). For tap drill size
(without using tapered reamer) see table and corresponding
comment on page 26.

**Maximum for PTFE/TEFLON-coated but can be reduced as
much as 60% in most applications.

27

DOWEL PINS 
 Dimensions 
 Application Data
Formed ends resist chipping
Surface hardness: Rockwell “C” 60 minimum
Surface finish: 8 microinch maximum
Core hardness: Rockwell “C” 50–58
Case depth: .020-inch minimum
Shear strength: 150,000 psi (calculated based on conversion from hardness)
Heat treated alloy steel for strength and toughness
Material, Heat Treatment, Dimensions: ASME B18.8.2
.0002 – inch oversize typically used for first installation.
.0010 – inch oversize typically used after hole enlarges.
Installation Warning – Do not strike. Use safety shield or glasses when pressing
chamfered end in first.
Single shear load calculated as 150,000 psi x π (nom. A)2  4

4° - 16°
8

D

R
A

B

LENGTH - ± .010

DIMENSIONS and APPLICATION DATA
A
.0002 over nom.

28

B

D

R

.001 over nom.

calculated
single shear
strength
(pounds)

recommended
hole size
(.0002 over nom.)

nom.

max.

min.

max.

min.

max.

min.

min.

max.

min.

1/16
3/32
1/8

.0628
.0941
.1253

.0626
.0939
.1251

.0636
.0949
.1261

.0634
.0947
.1259

.058
.089
.120

.016
.018
.022

.008
.012
.016

465
1,035
1,845

.0625
.0937
.1250

.0620
.0932
.1245

5/32
3/16
1/4

.1565
.1878
.2503

.1563
.1876
.2501

.1573
.1886
.2511

.1571
.1884
.2509

.150
.180
.240

.022
.023
.031

.020
.023
.031

2,880
4,140
7,370

.1562
.1875
.2500

.1557
.1870
.2495

5/16
3/8
7/16

.3128
.3753
.4378

.3126
.3751
.4376

.3136
.3761
.4386

.3134
.3759
.4384

.302
.365
.424

.034
.038
.047

.039
.047
.055

11,500
16,580
22,540

.3125
.3750
.4375

.3120
.3745
.4370

1/2
9/16
5/8

.5003
.5628
.6253

.5001
.5626
.6251

.5011
.5636
.6261

.5009
.5634
.6259

.486
.548
.611

.047
.047
.047

.063
.070
.078

29,460
37,270
46,020

.5000
.5625
.6250

.4995
.5620
.6245

3/4
7/8
1

.7503
.8753
1.0003

.7501
.8751
1.0001

.7511
.8761
1.0011

.7509
.8759
1.0009

.735
.860
.980

.059
.059
.059

.094
.109
.125

66,270
90,190
117,810

.7500
.8750
1.0000

.7495
.8745
.9995

DOWEL PINS

Dimensions 
 Application Data
Continuous grain flow resists chipping of ends. Precision heat treated
for greater strength and surface
hardness.
Chamfered end provides easier
insertion in hole. Surface finish to
8 microinch maximum.

APPLICATIONS
Widely used as plug gages in various
production operations, and as guide
pins, stops, wrist pins, hinges and
shafts. Also used as position locators
on indexing machines, for aligning
parts, as feeler gages in assembly
work, as valves and valve plungers
on hydraulic equipment, as fasteners
for laminated sections and machine
parts, and as roller bearings in casters
and truck wheels.

29

PULL-OUT DOWEL PINS 
 Dimensions 
 Application Data
Tapped hole for easy pull-out (ANSI B1.1)
Exclusive spiral grooves afford uniform relief for insertion and removal,
reduce chances of hole-scoring
Surface hardness-Rockwell C60 minimum
Surface finish-8 microinch maximum
Core hardness-Rockwell C 50–58
Shear strength: 150,000 psi (calculated based on conversion from hardness)
Heat treated alloy steel for strength and toughness
Held to precise tolerance – .0002-inch on diameter and roundness to 50
millionths of an inch (T.I.R.)

Formed ends resist chipping
Material and Heat Treatment: ASME B18.8.2
Single shear load calculated as 150,000 psi x π (nom. A)2  4

4°–16°

8

B DIA.

A DIA.

THREADS PER ANSI B1.1

DIMENSIONS and APPLICATION DATA

size
nom.

thread
size

max.

max.

min.

min.

note 1
max.

min.

note 2

calculated
single
shear
strength
(pounds)

1/4
5/16
3/8

#8-32 UNC-2B
#10-32 UNF-2B
#10-32 UNF-2B

.237
.302
.365

.2503
.3128
.3753

.2501
.3126
.3751

.031
.034
.038

.500
.625
.625

.212
.243
.243

7/16
1/2
9/16

7,370
11,500
16,580

.2500
.3125
.3750

.2495
.3120
.3745

7/16
1/2
5/8

#10-32 UNF-2B
1/4-20 UNC-2B
1/4-20 UNC-2B

.424
.486
.611

.4378
.5003
.6253

.4376
.5001
.6251

.047
.047
.047

.625
.750
.750

.243
.315
.315

5/8
3/4
3/4

22,540
29,460
46,020

.4315
.5000
.6250

.4370
.4995
.6245

3/4
7/8
1

5/16-18 UNC-2B
3/8-16 UNC-2B
3/8-16 UNC-2B

.735
.860
.980

.7503
.8753
1.0003

.7501
.8751
1.0001

.059
.059
.059

.875
.875
.875

.390
.390
.390

13/16
13/16
13/16

66,270
90,190
117,810

.7500
.8750
1.0000

.7495
.8745
.9995

B

A

NOTES
1. Lengths equal to or shorter than “P” max. values may be drilled
through.

30

D

P

T

X

recommended
hole diameter
max.

min.

2. Point angle (approx.) 5° on point for lengths equal to or
longer than X. For shorter lengths, use 15° angle.

PULL-OUT DOWEL PINS
Dimensions 
 Application Data

5 WAYS TO SAVE
UNBRAKO Pull-Out Dowel Pins are
easier, more accurate and more
economical than “do-it-yourself”
modifications of standard dowels.
They save you money FIVE ways:
1. YOU SAVE COST OF SEPARATE KNOCK-OUT HOLES IN
BLIND HOLES WHERE PINS MUST BE REMOVED.
UNBRAKO pull-out pins are easy to install in blind holes,
easy to remove. Exclusive spiral grooves release trapped
air for insertion or removal without danger of hole-scoring.
2. YOU MUST SAVE COST OF NEW PINS EACH TIME DIE
IS SERVICED OR DISMANTLED.
UNBRAKO pull-out dowel pins are reusable. The hole
tapped in one end for a removal screw or threaded “puller”
makes it easy and fast to remove the pin without damage
to pin or hole, permits repeated re-use.
3. YOU SAVE MONEY IN REDUCED DOWNTIME AND
LOSS OF PRODUCTION
UNBRAKO pull-out dowel pins speed up die servicing and
reworking. You can remove them without turning the die
over, and you can take out individual sections of the die for
rework or service without removing entire die assembly
from the press.

4. YOU SAVE MODIFICATIONS COSTS, YOU AVOID
HEADACHES AND YOU SAVE YOUR SKILLED PEOPLE
FOR PROFITABLE WORK.
UNBRAKO pull-out dowel pins have tapped holes and
relief grooves built in. Time-consuming “do-it-yourself”
modification of standard pin eliminated. No need for
annealing (to make pins soft enough to drill and tap)
and re-hardening, which can result in damage to finish,
and in inaccuracies and distortion.
5. YOU SAVE TIME AND MONEY BECAUSE OF THIS
QUALITY “REPEATABILITY.” NO SPECIAL PREPARATION
OF INDIVIDUAL HOLES NEEDED-YOU CAN BE SURE OF
ACCURATE FIT EVERY TIME.
UNBRAKO pull-out dowel pins are identical and interchangeable with standard UNBRAKO dowels. They have
the same physical, finish, accuracy and tolerances. And
they are consistently uniform. Their exclusive spiral relief
grooves provide more uniform relief than other types of
removable pins, assuring more uniform pull-out values.
You don’t need any special tools to remove UNBRAKO
pull-out dowels-just an ordinary die hook and a socket
head cap or button head socket screw.

31

HEX KEYS 
 Dimensions 
 Mechanical Properties 
 Screw Size Table

Accurately sized across flats and corners to insure snug fit and full wall contact
Heat treated alloy steel-key is hard, tough and ductile clear through for longer
life and retention of dimensional accuracy
Size stamped for easy identification – 5/64”-1” across flats
Square cut end engages the socket full depth for better tightening of screw
GGG-K-275. ANSI B18.3

B
C

DIMENSIONS

MECHANICAL PROPERTIES

key size
W

32

max.

min.

.028
.035
.050

.0275
.0345
.049

1/16
5/64
3/32

dash
number
size
page 87

C
length of long arm
short series

B
length of short arm

long series
max.

min.

torsional
shear
torsional
strength
yield
inch-lbs. min. inch-lbs. min.

max.

min.

max.

min.

1
2
3

1.312
1.312
1.750

1.125
1.125
1.562

2.688
2.766
2.938

2.500
2.578
2.750

.312
.438
.625

.125
.250
.438

1.1
2.3
6.5

.9
2.0
5.6

.0615
.0771
.0927

4
5
6

1.844
1.969
2.094

1.656
1.781
1.906

3.094
3.281
3.469

2.906
3.094
3.281

.656
.703
.750

.469
.516
.562

12.2
25
43

10.5
21
35

7/64
1/8
9/64

.1079
.1235
.1391

7
8
9

2.219
2.344
2.469

2.031
2.156
2.281

3.656
3.844
4.031

3.469
3.656
3.844

.797
.844
.891

.609
.656
.703

68
98
146

60
85
125

5/32
3/16
7/32

.1547
.1860
.2172

10
11
12

2.594
2.844
3.094

2.406
2.656
2.906

4.219
4.594
4.969

4.031
4.406
4.781

.938
1.031
1.125

.750
.844
.938

195
342
535

165
295
460

1/4
5/16
3/8

.2485
.3110
.3735

13
14
15

3.344
3.844
4.344

3.156
3.656
4.156

5.344
6.094
6.844

5.156
5.906
6.656

1.219
1.344
1.469

1.031
1.156
1.281

780
1,600
2,630

670
1,370
2,260

7/16
1/2
9/16

.4355
.4975
.5600

16
17
18

4.844
5.344
5.844

4.656
5.156
5.656

7.594
8.344
9.094

7.406
8.156
8.906

1.594
1.719
1.844

1.406
1.531
1.656

4,500
6,300
8,900

3,870
5,420
7,650

5/8
3/4
7/8

.6225
.7470
.8720

19
20
21

6.344
7.344
8.344

6.156
7.156
8.156

9.844
11.344
12.844

9.656
11.156
12.656

1.969
2.219
2.469

1.781
2.031
2.281

12,200
19,500
29,000

10,500
16,800
24,900

1
1 1/4
1 1/2

.9970
1.243
1.493

22
23
24

9.344
11.500
13.500

9.156
11.000
13.000

14.344

14.156

2.719
3.250
3.750

2.531
2.750
3.250

43,500
71,900
124,000

37,400
62,500
108,000

1 3/4
2

1.743
1.993

25
26

15.500
17.500

15.000
17.000

4.250
4.750

3.750
4.250

198,000
276,000

172,000
240,000

HEXAGON KEYS
Why UNBRAKO keys tighten socket screws tighter, safely
An UNBRAKO key is not an ordinary
hexagon key – it is a precision internal wrenching tool of great strength
and ductility. With an UNBRAKO key,
far more tightening torque than is
needed can be applied without damaging the screw or the key, and it can
be done safely. This is an important
feature, especially true of the smaller
sizes (5/32” and under) which are
normally held in the hand.

Photographs of a destruction test
show what we mean. Under excessive torque a 5/64” UNBRAKO key
twists but does not shear until a
torque has been reached that is
approximately 20% greater than can
be applied with an ordinary key. At
this point it shears off clean, flush
with the top of the socket, leaving
no jagged edge to gash a hand.

Still the UNBRAKO screw has not
been harmed. The broken piece of
the key is not wedged into the socket.
It can be lifted out with a small
magnet, convincing proof that the
socket has not been reamed or
otherwise damaged.

A 5/64” UNBRAKO key will twist up to
180° without weakening.

Twisted to about 270°, the key shears
off clean. Note the extension bar illustrated for test purposes only.

The socket hasn’t been reamed or
damaged. Broken section can be
lifted out with a magnet.

NOTE: The use of an extension in
these illustrations is for demonstration
purposes only. The manufacturer does
not recommend the use of extensions
with any hex key product under
normal conditions.

SCREW SIZE SELECTOR TABLE

button
head
screws

#0

#0
#1,#2

#0
#1,#2

#0
#1, #2
#3, #4

#1
#2,#3
#4,#5

#3,#4
#5,#6
#8

#3, #4
#5,#6
#8

#5,#6
#8
#10

#10

#10

1/4

1/4

1/4
5/16
3/8

1/4
5/16
3/8

5/16
3/8

5/16
3/8
7/16

1/16
1/8

1/2
5/8

7/16
1/2, 9/16
5/8

1/2
5/8
3/4

1/2
5/8
3/4

1/4
3/8
1/2

3/4
7/8

7/8, 1

7/8
1, 1/8

3/4

1

1 1/4

1 1/4, 1 3/8
1 1/2

1
1-1/4, 1-1/2

flat
head
screws

shoulder
screws

low heads
and socket
set
screws

NOTES

1960 Series
socket
head
cap screws

Material: ANSI B18.3, alloy steel
pressure*
plugs

Heat treat: Rc 47-57

#6
#8
#10
1/4
5/16
3/8
7/16,1/2
9/16
5/8
3/4
7/8,1
1 1/8, 1 1/4
1 3/8, 1 1/2
1 3/4
2

1 1/2
1 3/4
2

1/2,2

2 1/4, 2 1/2
2 3/4

33

THREAD CONVERSION CHARTS
DIAMETER/THREAD PITCH COMPARISON
INCH SERIES
Dia. (In.)

TPI

#0

0.060

80

#1

#2

34

METRIC

Size

0.073

0.096

Size

Dia.(In.)

Pitch (mm)

TPI (Approx)

M1.6

0.063

0.35

74

M2

0.079

0.4

64

M2.5

0.098

0.45

56

M3

0.118

0.5

51

M4

0.157

0.7

36

M5

0.196

0.8

32

M6

0.236

1.00

25

M8

0.315

1.25

20

M10

0.393

1.5

17

M12

0.472

1.75

14.5

M14

0.551

2

12.5

M16

0.63

2

12.5

M20

0.787

2.5

10

M24

0.945

3

8.5

M27

1.063

3

8.5

64

56

#3

0.099

48

#4

0.112

40

#5

0.125

40

#6

0.138

32

#8

0.164

32

#10

0.190

24

1/4

0.250

20

5/16

0.312

18

3/8

0.375

16

7/16

0.437

14

1/2

0.500

13

5/8

0.625

11

3/4

0.750

10

7/8

0.875

9

1

1.000

8

TABLE OF CONTENTS
UNBRAKO® Socket Screw Products (Metric)
Page
Metric Standards. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
Socket Head Cap Screws. . . . . . . Standards – Alloy Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Low Heads – Alloy Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
Flat Head Socket Screws . . . . . . Alloy Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Button Head Socket Screws . . . . Alloy Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
Shoulder Screws . . . . . . . . . . . . . Alloy Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
Dowel Pins . . . . . . . . . . . . . . . . . . Standards – Alloy Steel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
Socket Set Screws . . . . . . . . . . . . Alloy Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
Low Head Cap Screws . . . . . . . . . Low Heads – Alloy Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
Hex Keys . . . . . . . . . . . . . . . . . . . . Alloy Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
ISO Tolerances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
Conversion Chart. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

35

UNBRAKO Metric Fasteners
UNBRAKO Metric Fasteners are
the strongest off-the-shelf threaded fasteners you can buy. Their
exclusive design features and
closely controlled manufacturing
processes insure the dimensional
accuracy, strength and fatigue
resistance needed for reliability
in today’s advanced technology.
They are manufactured with the
same methods and features as
their inch-series counterpart.
Strength
UNBRAKO metric socket head cap
screws are made into property
class 12.9 with a minimum ultimate
tensile strength of 1300 or 1250
MPa depending on screw diameter. Precision in manufacturing
and careful control in stress areas
insure strength in such critical
areas as heads, sockets, threads,
fillets, and bearing areas.

36

When you purchase UNBRAKO
metric socket screw products, you
can be sure that they meet or
exceed the strength levels of all
current standards, including the
three most common-ANSI, ISO
and DIN. Unbrako is represented
on several ASME, ANSI, ASTM
and ISO committees.

 ANSI (American National
Standards Institute) documents
are published by ASME (The
American Society of Mechanical
Engineers) and are familiar to
almost all users of socket screw
products in the U.S.A.

 ASTM (American Society for
Testing and Materials). Many
ANSI documents list dimensional information but refer
to ASTM specifications for
materials, mechanical properties, and test criteria.


 ISO (International Standards
Organization) is a standards
group comprising 70 member
nations. Its objective is to
provide standards that will
be completely universal and
common to all countries
subscribing.

 DIN (Deutsche Industries
Normen) is the German
standards group.
NOTE: The proper tightening
of threaded fasteners can have
a significant effect on their
performance.

A WARNING TO METRIC FASTENER USERS
Metric socket cap screws are NOT sold in a single strength level
like U.S. inch socket screws.

Property Class

General Material

Strength Level, UTS min. MPa (KSI)

International Standards Organization, ISO
Property Class 8.8

Carbon Steel

Property Class 10.9
Property Class 12.9

Alloy Steel
Alloy Steel

800 (116) < M16
830 (120) ≥ M16
1040 (151)
1220 (177)

USA Standards
ASTM A574M

Alloy Steel

1220 (177)

Unbrako Standards
ASTM A574M

Alloy Steel

1300 (189) ≤ M16
1250 (181) > M16

STANDARDS
The use of metric fasteners in the worldwide market has led to the creation of many standards. These
standards specify the fastener requirements: dimensions, material, strength levels, inspection, etc. Different
standards are the responsibility of various organizations and are not always identical. Unbrako supplies metric
fasteners for maximum interchangeability with all standards. This Engineering Guide was published with the
most current values, which are however subject to change by any standards organization at any time.

37

METRIC SOCKET HEAD CAP SCREWS
Dimensions
Threads: ANSI B1.13M, ISO
261, ISO 262 (coarse series
only)

L

H
J

THREAD LENGTH
SEE STOCK TABLE

T
G

APPROX 45°

Property Class: 12.9-ISO 898/1
D

THREAD
SIZE

A

30°

NOTES

LENGTH TOLERANCE

1. Material: ASTM A574M,
DIN ENISO4762-alloy steel

nominal screw diameter
M1.6
thru
M10

2. Hardness: Rc 38-43
3. Tensile Stress: 1300 MPa thru M16 size.
1250 MPa over M16 size.
4. Yield Stress: 1170 MPa thru M16 size.
1125 MPa over M16 size.

nominal
screw length

DIMENSIONS
thread
size
nom.
M1.6
M2
M2.5

±0.3
±0.4
±0.7
±1.0
±2.0

±0.3
±0.4
±1.0
±1.5
±2.5

–
±0.7
±1.5
±2.0
±3.0

APPLICATION
DATA

MECHANICAL PROPERTIES
UTS
min.
MPa

tensile
strength
min.

single shear
strength of body
min.

A

D

H

J

G

T

pitch

max.

max.

max.

nom.

min.

min.

0.35
0.40
0.45

3.0
3.8
4.5

1.6
2.0
2.5

1.6
2.0
2.5

1.5
1.5
2.0

0.54
0.68
0.85

0.80
1.0
1.25

1300
1300
1300

1.65
2.69
4.41

370
605
990

M3
M4
M5

0.5
0.7
0.8

5.5
7.0
8.5

3.0
4.0
5.0

3.0
4.0
5.0

2.5
3.0
4.0

1.02
1.52
1.90

1.5
2.0
2.5

1300
1300
1300

6.54
11.4
18.5

1,470
2,560
4,160

M6
M8
M10

1.0
1.25
1.5

10.0
13.0
16.0

6.0
8.0
10.0

6.0
8.0
10.0

5.0
6.0
8.0

2.28
3.2
4.0

3.0
4.0
5.0

1300
1300
1300

26.1
47.6
75.4

5,870
10,700
17,000

M12
*(M14)
M16

1.75
2.0
2.0

18.0
21.0
24.0

12.0
14.0
16.0

12.0
14.0
16.0

10.0
12.0
14.0

4.8
5.6
6.4

6.0
7.0
8.0

1300
1300
1300

110
150
204

24,700
33,700
45,900

M20
M24
*M30

2.5
3.0
3.5

30.0
36.0
45.0

20.0
24.0
30.0

20.0
24.0
30.0

17.0
19.0
22.0

8.0
9.6
12.0

10.0
12.0
15.0

1250
1250
1250

306
441
701

68,800
99,100
158,000

*M36
*M42
*M48

4.0
4.5
5.0

54.0
63.0
72.0

36.0
42.0
48.0

36.0
42.0
48.0

27.0
32.0
36.0

14.4
16.8
19.2

18.0
21.0
24.0

1250
1250
1250

1020
1400
1840

kN

All dimensions in millimeters.
Sizes in brackets not preferred for new designs.
*Non-stock diameter.
**Torque calculated in accordance with VDI 2230, “Systematic Calculation of High Duty
Bolted Joints,” to induce approximately 800 MPa stress in screw threads. Torque values
listed are for plain screws. (See Note, page 1.)

38

over
20

tolerance on lgth., mm

Up to 16 mm, incl.
Over 16 to 50 mm, incl.
Over 50 to 120 mm, incl.
Over 120 to 200 mm, incl.
Over 200 mm

5. Thread Class: 4g 6g

M12
thru
M20

lbs.

recommended **
seating torque
plain finish

kN

lbs.

1.57
2.45
3.83

352.5
550
860

0.29
0.60
1.21

2.6
5.3
11

5.5
9.8
15.3

1240
2,205
3,445

2.1
4.6
9.5

19
41
85

22.05
39.2
61

4,960
8,800
13,750

16
39
77

140
350
680

19,850
27,000
35,250

135
215
330

1,200
1,900
2,900

53,000 650
76,500 1100
119,000 2250

5,750
9,700
19,900

171,500 3850
233,500 6270
305,000 8560

34,100
55,580
75,800

88
120
157
235.5
339
530

229,000 756
315,000 1040
413,000 1355

N-m

in-lbs.

SOCKET HEAD CAP SCREWS 
 Metric 
 Body and Grip Lengths
LG
LB

LG is the maximum grip length and is the distance from
the bearing surface to the first complete thread.
LB is the minimum body length and is the length of the
unthreaded cylindrical portion of the shank.

LENGTH

BODY and GRIP LENGTHS

BODY AND GRIP LENGTH DIMENSIONS FOR METRIC SOCKET HEAD CAP SCREWS
Nominal
Size
Nominal
Length

M1.6
LG

M2

LB

LG

M2.5

LB

LG

M3

LB

LG

M4

LB

M5

LG

LB

10.0

6.5

LG

M6

LB

LG

M8

LB

LG

LB

M10
LG

LB

M12
LG

LB

M14
LG

LB

M16
LG

LB

M20
LG

LB

M24
LG

LB

20

4.8

3.0

4.0

2.0

25

9.8

8.0

9.0

7.0

8.0

5.7

7.0

4.5

30

14.8

13.0

14.0 12.0

13.0

10.7

12.0

9.5

35

...

...

19.0 17.0

18.0

15.7

17.0 14.5

15.0 11.5

13.0

9.0

40

...

...

24.0 22.0

23.0

20.7

22.0 19.5

20.0 16.5

18.0

14.0

16.0 11.0

45

...

...

...

...

28.0

25.7

27.0 24.5

25.0 21.5

23.0

19.0

21.0 16.0

17.0

10.7

50

...

...

...

...

33.0

30.7

32.0 29.5

30.0 26.5

28.0

24.0

26.0 21.0

22.0

15.7

18.0 10.5

55

...

...

...

...

...

...

37.0 34.5

35.0 31.5

33.0

29.0

31.0 26.0

27.0

20.7

23.0 15.5

60

...

...

...

...

...

...

42.0 39.5

40.0 36.5

38.0

34.0

36.0 31.0

32.0

25.7

28.0 20.5

24.0 15.2

65

...

...

...

...

...

...

47.0 44.5

45.0 41.5

43.0

39.0

41.0 36.0

37.0

30.7

33.0 25.5

29.0 20.2

25.0 15.0

70

...

...

...

...

...

...

...

...

50.0 46.5

48.0

44.0

46.0 41.0

42.0

35.7

38.0 30.5

34.0 25.2

30.0 20.0

26.0 16.0

80

...

...

...

...

...

...

...

...

60.0 56.5

58.0

54.0

56.0 51.0

52.0

45.7

48.0 40.5

44.0 35.2

40.0 30.0

36.0 26.0

90

...

...

...

...

...

...

...

...

...

...

68.0

64.0

66.0 61.0

62.0

55.7

58.0 50.5

54.0 45.2

50.0 40.0

46.0 36.0

38.0

25.5

100

...

...

...

...

...

...

...

...

...

...

78.0

74.0

76.0 71.0

72.0

65.7

68.0 60.5

64.0 55.2

60.0 50.0

56.0 46.0

48.0

35.5

40.0

25.0

110

...

...

...

...

...

...

...

...

...

...

...

...

86.0 81.0

82.0

75.7

78.0 70.5

74.0 65.2

70.0 60.0

66.0 56.0

58.0

45.5

50.0

35.0

120

...

...

...

...

...

...

...

...

...

...

...

...

96.0 91.0

92.0

85.7

88.0 80.5

84.0 75.2

80.0 70.0

76.0 66.0

68.0

55.5

60.0

45.0

130

...

...

...

...

...

...

...

...

...

...

...

...

...

...

102.0

95.7

98.0 90.5

94.0 85.2

90.0 80.0

86.0 76.0

78.0

65.5

70.0

55.0

140

...

...

...

...

...

...

...

...

...

...

...

...

...

...

112.0 105.7 108.0 100.5 104.0 95.2 100.0 90.0

96.0 86.0

88.0

75.5

80.0

65.0

150

...

...

...

...

...

...

...

...

...

...

...

...

...

...

122.0 115.7 118.0 110.5 114.0 105.2 110.0 100.0 106.0 96.0

98.0

85.5

90.0

75.0

160

...

...

...

...

...

...

...

...

...

...

...

...

...

...

132.0 125.7 128.0 120.5 124.0 115.2 120.0 110.0 116.0 106.0 108.0

95.5 100.0

85.0

180

...

...

...

...

...

...

...

...

...

...

...

...

...

...

...

...

148.0 140.5 144.0 135.2 140.0 130.0 136.0 126.0 128.0 115.5 120.0 105.0

200

...

...

...

...

...

...

...

...

...

...

...

...

...

...

...

...

168.0 160.5 164.0 155.2 160.0 150.0 156.0 146.0 148.0 135.5 140.0 125.0

220

...

...

...

...

...

...

...

...

...

...

...

...

...

...

...

...

...

...

184.0 175.2 180.0 170.0 176.0 166.0 168.0 155.5 160.0 145.0

240

...

...

...

...

...

...

...

...

...

...

...

...

...

...

...

...

...

...

204.0 195.2 200.0 190.0 196.0 186.0 188.0 175.5 180.0 165.0

260

...

...

...

...

...

...

...

...

...

...

...

...

...

...

...

...

...

...

...

...

300

...

...

...

...

...

...

...

...

...

...

...

...

...

...

...

...

...

...

...

...

11.0

6.0

220.0 210.0 216.0 206.0 208.0 195.5 200.0 185.0
...

...

256.0 246.0 248.0 235.5 240.0 225.0

SOCKET HEAD CAP SCREWS (METRIC SERIES)
PER ASME/ANSI B18.3.1M-1986

39

METRIC SOCKET FLAT HEAD CAP SCREWS
Dimensions
Threads: ANSI B1.13M, ISO 262 (coarse series only)

NOTES

Applicable or Similar Specification: DIN ENISO10642

1. Material: ASTM F835M

General Note: Flat, countersunk head cap screws and
button head cap screws are designed and recommended for moderate fastening applications: machine guards,
hinges, covers, etc. They are not suggested for use in
critical high strength applications where socket head cap
screws should be used.

2. Dimensions: B18.3.5M
3. Property Class: 12.9
4. Hardness: Rc 38-43 (alloy steel)
5. Tensile Stress: 1040MPa
6. Shear Stress: 630 MPa
7. Yield Stress: 945 MPa
8. Sizes: For sizes up to and including M20, head angle
shall be 92°/90°. For larger sizes head angle shall be 62°/60°.
9. Thread Class: 4g 6g

L
T

J

APPROX 45°

D

THREAD
SIZE

A

S

LT

H

Head Angle See Note 8

LENGTH TOLERANCE
nominal screw diameter
M3 thru M24

nominal
screw length

tolerance on lgth., mm

Up to 16 mm, incl.
Over 16 to 60 mm, incl.
Over 60 mm

±0.3
±0.5
±0.8

DIMENSIONS

APPLICATION DATA
A

D

H

T

S

LT

J

nom.

nom.
thread
size

pitch

max.***

max.

ref.

min.

ref.

min.

M3
M4
M5

0.5
0.7
0.8

6.72
8.96
11.20

3
4
5

1.7
2.3
2.8

1.10
1.55
2.05

0.50
0.70
0.70

18
20
22

2
2.5
3

M6
M8
M10

1.0
1.25
1.50

13.44
17.92
22.40

6
8
10

3.3
4.4
5.5

2.25
3.20
3.80

0.85
1.20
1.50

24
28
32

4
5
6

M12
M16
M20
*M24

1.75
2.00
2.50
3.00

26.88
33.60
40.32
40.42

12
16
20
24

6.5
7.5
8.5
14.0

4.35
4.89
5.45
10.15

1.85
1.85
1.85
2.20

36
44
52
60

8
10
12
14

plain

All dimensions in millimeters.
*Non-stock Diameter
**Torque calculated to induce 420 MPa in the screw threads.
Torque values are for plain screws. (See Note, page 1.)

40

recommended
seating torque**

***Maximum to theoretical sharp corner

N-m

in-lbs.

1.2
2.8
5.5

11
25
50

9.5
24
47

85
210
415

82
205
400
640

725
1800
3550
5650

METRIC SOCKET BUTTON HEAD CAP SCREWS
Dimensions
Threads: ANSI B1.13M, ISO 262(coarse series only)

NOTES

Similar Specifications: ISO 7380

1. Material: ASTM F835M

General Note: Flat, countersunk head cap screws and
button head cap screws are designed and recommended for moderate fastening applications: machine guards,
hinges, covers, etc. They are not suggested for use in
critical high strength applications where socket head cap
screws should be used.

2. Dimensions: ANSI B18.3.4M
3. Property Class: 12.9
4. Hardness: Rc 38-43
5. Tensile Stress: 1040 MPa
6. Shear Stress: 630 MPa
7. Yield Stress: 945 MPa
8. Bearing surface of head square with body within 2°.
9. Thread Class: 4g 6g

H

L

J
T

APPROX 45°

S

THREAD
SIZE

A

R

LENGTH TOLERANCE
nominal screw diameter
M3 thru M16

nominal
screw length

tolerance on lgth., mm

Up to 16 mm, incl.
Over 16 to 60 mm, incl.
Over 60 mm

±0.3
±0.5
±0.8

DIMENSIONS

APPLICATION DATA
A

H

T

R

S

J

recommended
seating torque**

nom.
thread
size

pitch

max.

max.

min.

ref.

ref.

nom.

M3
M4
M5

0.5
0.7
0.8

5.70
7.60
9.50

1.65
2.20
2.75

1.05
1.35
1.92

2.95
4.10
5.20

.35
.35
.45

2.0
2.5
3.0

1.2
2.8
5.5

11
25
50

M6
M8
M10

1.0
1.285
1.50

10.50
14.00
18.00

3.30
4.40
5.50

2.08
2.75
3.35

5.60
7.50
10.00

.45
.45
.60

4.0
5.0
6.0

9.5
24.0
47.0

85
210
415

M12
*M16

1.75
2.0

21.00
28.00

6.60
8.60

4.16
5.20

11.00
15.00

.60
.60

8.0
10.0

82.0
205.0

725
1800

plain
N-m

in-lbs.

All dimensions in millimeters.
*Non-stock Diameter
**Torque calculated to induce 420 MPa in the screw threads.
Torque values are for plain screws. (See Note, page 1.)

41

METRIC SOCKET HEAD SHOULDER SCREWS
Threads: ANSI B 1.13 M, ISO 262

NOTES

Similar Specifications: ANSI B18.3.3M,
ISO 7379, DIN 9841

1. Material: ASTM A574M alloy steel
2. Hardness: Rc 36-43
3. Tensile Stress: 1100 MPa based on minimum thread neck area (G min.).
4. Shear Stress: 660 MPa
5. Concentricity: Body to head O.D. within 0.15 TIR when checked in a “V”
block.
Body to thread pitch diameter within 0.1 TIR when checked at a distance of
5mm from the shoulder at the threaded end.
Squareness, concentricity, parallelism, and bow of body to thread pitch
diameter shall be within 0.05 TIR per centimeter of body length with a maximum of 0.5 when seated against the shoulder in a threaded bushing and
checked on the body at a distance of 2.5 “D” from the underside of the head.
6. Squareness: The bearing surface of the head shall be perpendicular to
the axis of the body within a maximum deviation of 2°.
7. Thread Class: 4g 6g

+.25
-0.00

H

E

LENGTH

J
45¡

T

30¡

0.8

D

K

A

APPROX 45¡

THREAD
SIZE

G
J

30¡

F

I

APPLICATION
DATA

DIMENSIONS
nom.
size

thread
size

6
8
10

M5
M6
M8

12
16
20
24

M10
M12
M16
M20

A

T

pitch

max.

min.

max.

0.8
1.0
1.25

10.00
13.00
16.00

2.4
3.3
4.2

6.0
8.0
10.0

1.5
1.75
2.0
2.5

18.00
24.00
30.00
36.00

4.9
6.6
8.8
10.0

12.0
16.0
20.0
24.0

11.973
15.973
19.967
23.967

All dimensions in millimeters.
*Shoulder diameter tolerance h8 (ISO R 286)

42

D*

recommended
seating torque**

K

H

G

F

I

E

J

min.

min.

max.

min.

max.

max.

max.

nom.

N-m

in-lbs.

5.982
7.978
9.978

5.42
7.42
9.42

4.50
5.50
7.00

3.68
4.40
6.03

2.5
2.5
2.5

2.40
2.60
2.80

9.75
11.25
13.25

3
4
5

7
12
29

60
105
255

11.42
15.42
19.42
23.42

8.00
10.00
14.00
16.00

7.69
9.35
12.96
16.30

2.5
2.5
2.5
3.0

3.00
4.00
4.80
5.60

16.40
18.40
22.40
27.40

6
8
10
12

57
100
240
470

500
885
2125
4160

**See Note, page 1.

METRIC DOWEL PINS
Hardened and Ground 
 Dimensions

Applicable or Similar Specifications: ASME B18.8.5M,
ISO 8734 or DIN 6325.

NOTES
1. Material: ASME B18.8.5M-alloy steel
2. Hardness: Rockwell C60 minimum (surface)
Rockwell C 50-58 (core)

Installation warning: Dowel pins should not be installed
by striking or hammering. Wear safety glasses or shield
when pressing chamfered point end first.

3. Shear Stress: Calculated values based on 1050 MPa.
4. Surface Finish: 0.2 micrometer maximum

+0
L -0.5

A

C

0.2

B
10°–16°

DIMENSIONS

R

APPLICATION DATA
A
pin diameter

B
point diameter

C
R
crown height crown radius
max.
min.

calculated single
shear strength

nominal
size

max.

min.

max.

min.

3
4
5

3.008
4.009
5.009

3.003
4.004
5.004

2.9
3.9
4.9

2.6
3.6
4.6

0.8
0.9
1.0

0.3
0.4
0.4

7.4
13.2
20.6

6
8
10

6.010
8.012
10.012

6.004
8.006
10.006

5.8
7.8
9.8

5.4
7.4
9.4

1.1
1.3
1.4

0.4
0.5
0.6

12
16
20
25

12.013
16.013
20.014
25.014

12.007
16.007
20.008
25.008

11.8
15.8
19.8
24.8

11.4
15.3
19.3
24.3

1.6
1.8
2.0
2.3

0.6
0.8
0.8
1.0

kN

pounds

recommended
hole size
max.

min.

1,670
2,965
4,635

3.000
4.000
5.000

2.987
3.987
4.987

29.7
52.5
82.5

6,650
11,850
18,550

6.000
8.000
10.000

5.987
7.987
9.987

119.0
211.0
330.0
515.0

26,700
47,450
74,000
116,000

12.000
16.000
20.000
25.000

11.985
15.985
19.983
24.983

All dimensions in millimeters.

43

METRIC SOCKET SET SCREWS 
 Knurled Cup Point and Plain Cup Point 
 Dimensions
Threads: ANSI B 1.13M, ISO 261, ISO 262
(coarse series only)

NOTES

Grade: 45H

2. Hardness: Rockwell C45-53

Applicable or Similar Specifications: ANSI B 18.3.6M,
ISO 4029, DIN 916, DIN 915, DIN 914, DIN 913

3. Angle: The cup angle is 135 maximum for screw
lengths equal to or smaller than screw diameter. For
longer lengths, the cup angle will be 124 maximum

1. Material: ASTM F912M

4. Thread Class: 4g 6g

KNURLED CUP POINT

PLAIN CUP POINT

LENGTH TOLERANCE
nominal screw diameter
M1.6 thru M24

nominal
screw length

tolerance on lgth., mm

Up to 12 mm, incl.
Over 12 to 50 mm, incl.
Over 50 mm

±0.3
±0.5
±0.8

DIMENSIONS
nom.
thread
size

APPLICATION DATA
J max.

D
pitch

K

L
min. preferred

W

recommended*
seating torque

max.

plain cup

knurled cup

max.

plain cup

knurled cup

nom.

N-m

in-lbs.

1.0
1.32
1.75

0.80
1.00
1.25

–
–
–

–
–
–

2.0
2.5
3.0

–
–
–

0.7
0.9
1.3

0.09
0.21
0.57

0.8
1.8
5.0

0.92
2.2
4.0

8.0
19.0
35.0

MICROSIZE – Plain Cup Only
M1.6
M2
M2.5

0.35
0.40
0.45

STANDARD SIZE – Knurled Cup Point Supplied Unless Plain Cup Point Is Specified
M3
M4
M5

0.5
0.7
0.8

2.10
2.75
3.70

1.50
2.00
2.50

1.40
2.10
2.50

2.06
2.74
3.48

3.0
3.0
4.0

3.0
3.0
4.0

1.5
2.0
2.5

M6
M8
M10

1.0
1.25
1.5

4.35
6.00
7.40

3.00
5.00
6.00

3.30
5.00
6.00

4.14
5.62
7.12

4.0
5.0
6.0

5.0
6.0
8.0

3.0
4.0
5.0

7.2
17.0
33.0

M12
M16
M20
M24

1.75
2.0
2.5
3.0

8.60
12.35
16.00
18.95

8.00
10.00
14.00
16.00

8.00
10.00
14.00
16.00

8.58
11.86
14.83
17.80

8.0
12.0
16.0
20.0

10.0
14.0
18.0
20.0

6.0
8.0
10.0
12.0

54.0
134
237
440

All dimensions in millimeters.
*Not applicable to screws with a length equal to or less than the diameter. See Note, page 1.

44

64
150.0
290
480
1190
2100
3860

METRIC SOCKET SET SCREW

Flat Point, Cone Point, Dog Point Styles 
 Dimensions

REF. ISO 4026

FLAT POINT

REF. ISO 4027

CONE POINT

REF. ISO 4028
ISO 7435

DOG POINT

DIMENSIONS
flat point
nom.
thread
size

D

J

pitch

max.

M3
M4
M5

0.5
0.7
0.8

M6
M8
M10
M12
M16
M20
M24

cone point

max.

L
min.
preferred

2.10
2.75
3.70

2.0
2.5
3.5

1.00
1.25
1.50

4.25
6.00
7.40

1.75
2.00
2.50
3.00

8.60
12.35
16.00
18.95

J

dog point
H
nom.

max.

L
min.
preferred

long lgth.

L
min.
preferred

short lgth.

3.0
3.0
4.0

0.3
0.4
0.5

4.0
4.0
5.0

4.0
5.5
7.0

4.0
5.0
6.0

1.5
2.0
2.5

8.5
12.0
15.0
18.0

8.0
12.0
14.0
20.0

3.0
4.0
6.0
8.0

V
max.

0.75
1.00
1.25

1.5
2.0
2.5

5.0
5.0
6.0

2.00
2.50
3.50

6.0
6.0
8.0

1.50
2.00
2.50

3.0
4.0
5.0

6.0
8.0
8.0

4.00
5.50
7.00

10.0
14.0
18.0
20.0

3.00
4.00
5.00
6.00

6.0
8.0
10.0
12.0

12.0
16.0
20.0
22.0

8.50
12.00
15.00
18.00

45

METRIC LOW HEAD CAP SCREWS
Threads: ANSI B 1.13M, ISO 262
(coarse series only)

NOTES

Property Class: 10.9

2. Hardness: Rc 33-39

Similar Specifications: DIN 7984,
DIN 6912

3. Tensile Stress: 1040 MPa

1. Material: ASTM A574M-alloy steel

4. Yield Stress: 940 MPa
5. Thread Class: 4g 6g

L

H
G

J

APPROX 45°

T

D

THREAD
SIZE

A

LT

DIMENSIONS

APPLICATION DATA
A

D

G

T

H

LT

J

nom.
thread
size

pitch

max.

max.

min.

min.

max.

min.

nom.

N-m

in-lbs.

M4
M5
M6

0.7
0.8
1.0

7
8.5
10

4
5
6

1.06
1.39
1.65

1.48
1.85
2.09

2.8
3.5
4.0

20
22
24

3
4
5

4.5
8.5
14.5

40
75
130

M8
M10
M12

1.25
1.5
1.75

13
16
18

8
10
12

2.24
2.86
3.46

2.48
3.36
4.26

5.0
6.5
8.0

28
32
36

6
8
10

35
70
120

310
620
1060

M16
M20

2.0
2.5

24
30

16
20

4.91
6.10

4.76
6.07

10.0
12.5

44
52

12
14

300
575

2650
5100

plain

All dimensions in millimeters.
*Torque calculated to induce 620 MPa in the screw threads.
Torque values are for plain screws. (See Note, page 1.)

46

recommended*
seating torque

METRIC HEXAGON KEYS

Dimensions 
 Mechanical Properties 
 Socket Applications
These UNBRAKO keys are made to
higher requirements than ISO or DIN
keys, which may not properly torque
Class 12.9 cap screws. The strength
and dimensional requirements are
necessary to properly install the
products in this catalog.
Material: ANSI B18.3.2.M alloy steel
Dimensions: ANSI B18.3.2.M
Similar Specifications: DIN 911, ISO 2936

W

C
B

METRIC KEY APPLICATION CHART
socket cap screws
size
W

std. head
height

low
head

socket cap
screws

flat head
socket cap
screws

button head
shoulder
screws

0.7
0.9
1.3

socket set
screws
M1.6
M2
M2.5

1.5
2.0
2.5

M1.6/M2
M2.5
M3

3.0
4.0
5.0

M4
M5
M6

6.0
8.0
10.0

M3
M4
M5

M3
M4

M3
M4

M4
M5
M6

M5
M6
M8

M5
M6
M8

M6
M8
M10

M6
M8
M10

M8
M10
M12

M8
M10
M12

M10
M12
M16

M10
M12
M16

M12
M16
M20

M12
M16
M20

12.0
14.0
17.0

M14
M16
M20

M16
M20
M24

M20
M24

M24

M24

19.0
22.0
27.0

M24
M30
M36

32.0
36.0

M42
M48

DIMENSIONS

MECHANICAL PROPERTIES

key size W

C
nominal

B
mominal

torsional shear
strength minimum
N-m

torsional yield
strength minimum

max.

min.

short arm

long arm

0.711
0.889
1.270

0.698
0.876
1.244

5.5
9
13.5

31
31
42

*69
71
75

0.12
0.26
0.73

1.1
2.3
6.5

0.1
0.23
.63

1.500
2.000
2.500

1.470
1.970
2.470

14
16
18

45
50
56

78
83
90

1.19
2.9
5.4

10.5
26
48

1.02
2.4
4.4

3.000
4.000
5.000

2.960
3.960
4.960

20
25
28

63
70
80

100
106
118

6.000
8.000
10.000

5.960
7.950
9.950

32
36
40

90
100
112

140
160
170

74
183
345

655
1,620
3,050

64
158
296

566
1,400
2,620

12.000
14.000
17.000

11.950
13.930
16.930

45
55
60

125
140
160

212
236
250

634
945
1,690

5,610
8,360
15,000

546
813
1,450

4,830
7,200
12,800

19.000
22.000
24.000

18.930
21.930
23.930

70
80
90

180
*200
*224

280
*335
*375

2,360
3,670
4,140

20,900
32,500
36,600

2,030
3,160
3,560

18,000
28,000
31,500

27.000
32.000
36.000

26.820
31.820
35.820

100
125
140

*250
*315
*355

*500
*630
*710

5,870
8,320
11,800

51,900
73,600
104,000

5,050
7,150
10,200

44,700
63,300
90,300

9.3
22.2
42.7

In-lbs.

82
196
378

N-m

8.
18.8
36.8

In-lbs.
0.9
2.
5.6
9.
21
39
71
166
326

All dimensions in millimeters.
*Non-stock sizes

47

ISO TOLERANCES FOR METRIC FASTENERS
nominal
dimension
over

to

0

1

1

3

3

6

6

10

10

18

18

30

30

50

50

80

80

120

120
180
250
315
400

tolerance zone in mm (external measurements)
h6

h8

h10

h11

h13

h14

h15

h16

js14

tolerance zone in mm
js15

js16

js17

m6

H7

H8

H9

H11

H13

H14

+0.25
0
+0.30
0

0
0
0
0
–0.006 –0.014 –0.040 –0.060
0
0
0
0
–0.006 –0.014 –0.040 –0.060
0
0
0
0
–0.008 –0.018 –0.048 –0.075

0
–0.14
0
–0.14
0
–0.18

0
–0.25
0
–0.30

0
–0.40
0
–0.48

0
±0.125 ±0.20 ±0.30
–0.60
0
±0.15 ±0.24 ±0.375
–0.75

+0.002 +0.010 +0.0014 +0.025 +0.060 +0.14
+0.008
0
0
0
0
0
±0.50 +0.002 +0.010 +0.014 +0.025 +0.060 +0.14
+0.008
0
0
0
0
0
±0.60 +0.004 +0.012 +0.018 +0.030 +0.075 +0.18
+0.012
0
0
0
0
0

0
0
0
0
–0.009 –0.022 –0.058 –0.090
0
0
0
0
–0.011 –0.027 –0.070 –0.110
0
0
0
0
–0.030 –0.033 –0.084 –0.130

0
–0.22
0
–0.27
0
–0.33

0
–0.36
0
–0.43
0
–0.52

0
–0.58
0
–0.70
0
–0.84

0
±0.18 ±0.29
–0.90
0
±0.215 ±0.35
–1.10
0
± 0.26 ±0.42
–1.30

±.075 +0.006 +0.015 +0.022 +0.036 +0.090 +0.22
+0.0015
0
0
0
0
0
±0.90 +0.007 +0.018 +0.027 +0.043 +0.110 +0.27
+0.018
0
0
0
0
0
±1.05 +0.008 +0.021 +0.033 +0.052 +0.130 +0.33
+0.021
0
0
0
0
0

+0.36
0
+0.43
0
+0.52
0

0
–0.39
0
–0.46
0
–0.54

0
–0.62
0
–0.74
0
–0.87

0
–1.00
0
–1.20
0
–1.40

0
±0.31 ±0.50
–1.60
0
±0.37 ±0.60
–1.90
0
±0.435 ±0.70
–2.20

±0.80

±1.25

±0.95

±1.50

±1.10

±1.75

+0.62
0
+0.74
0
+0.87
0

180
250
315

±0.50 ±0.80 ±1.25
±0.575 ±0.925 ±1.45
±0.65 ±1.05 ±1.60

±2.00
±2.30
±2.60

400
500

±0.70 ±1.15
±0.775 ±1.25

±2.85
±3.15

±0.45
±0.55
±0.65

±1.80
±2.00

+0.39
0
+0.46
0
+0.54
0

ISO TOLERANCES FOR SOCKET SCREWS
nominal
dimension
over

3
6

tolerance zone in mm
to

C13

C14

D9

D10

D11

D12

EF8

E11

E12

Js9

K9

3

+0.20
+0.06

+0.31
+0.06

+0.045
+0.020

+0.060
+0.020

+0.080
+0.020

+0.12
+0.02

+0.024
+0.010

+0.074
+0.014

+0.100
+0.014

±0.0125

0
-0.025

+0.24
+0.06

+0.37
+0.07

+0.060
+0.030

+0.078
+0.030

+0.115
+0.030

+0.15
+0.03

+0.028
+0.014

+0.095
+0.020

+0.140
+0.020

±0.015

0
-0.030

+0.130
+0.040

+0.19
+0.40

+0.040
+0.018

+0.115
+0.025

+0.115
+0.025

±0.018

0
-0.036

+0.142
+0.032

+0.212
+0.032

6
10

10

18

+0.2
+0.05

18

30

+0.275
+0.065

30

50

+0.33
+0.08

50

80

+0.40
+0.10

References
ISO R 286
ISO 4759/I
ISO 4759/II
ISO 4759/III

48

Notes
ANSI standards allow slightly wider tolerances for
screw lengths than ISO and DIN.
The table is intended to assist in the design with metric
fasteners. For tolerances not listed here refer to the
complete standards.

ISO TOLERANCES
Tolerances for Metric Fasteners
The tolerances in the tables below are derived from ISO
standard: ISO 4759
The tables show tolerances on the most common metric
fasteners. However, occasionally some slight modifications
are made.

Item

DIN

Item

DIN

912

913
914
916

7991

915
966

Notes
Product grade A applies to sizes up to M24 and length not exceeding
10 x diameter or 150 mm, whatever is shorter.
Product grade B applies to the sizes above M24 and all sizes with lengths,
greater than 10 x diameter or 150 mm, whichever is shorter.

*Tolerance zones for socket set
screws

Feature
Hexagon Sockets

s
0.7
0.9
1.3
1.5
2
2.5
3
4
5
6
8
10
12
14
>14

Tolerance
*

**
EF8
JS9
K9

D9

D10

D10
D11

D11

**Tolerance zones for socket head
cap screws
Note: For S 0.7 to 1.3 the actual
allowance in the product standards
has been slightly modified for technical reasons.

E11

E11

E12

D12

49

CONVERSION CHART

SI UNITS & CONVERSIONS FOR CHARACTERISTICS OF MECHANICAL FASTENERS
conversion
property

unit

symbol

from

to

multiply by

approximate
equivalent

length

meter
centimeter
millimeter

m
cm
mm

inch
inch
foot

mm
cm
mm

25.4
2.54
304.8

25mm = 1 in.
300mm = 1ft.
1m = 39.37 in.

mass

kilogram
gram
tonne (megagram)

kg
g
t

once
pound
ton (2000 lb)

g
kg
kg

28.35
.4536
907.2

28g = 1 oz.
1kg = 2.2 lb. = 35 oz.
1t = 2200 lbs.

density

kilogram per
cubic meter

kg/m3

pounds per cu. ft.

kg/m

16.02

16kg/m = 1 lb/ft.3

temperature

deg. Celsius

°C

deg. Fahr.

°C

(°F – 32) x 5/9

0°C = 32°F
100°C = 212°F

area

square meter
square millimeter

m2
mm2

sq. in.
sq. ft.

mm2
m2

645.2
.0929

645mm2 = 1 in.2
1m2 = 11 ft.2

volume

cubic meter
cubic centimeter
cubic millimeter

m3
cm3
mm3

cu. in.
cu.ft.
cu. yd.

mm3
m3
m3

16387
.02832
.7645

16400mm3 = 1 in.3
1m3 = 35 ft.3
1m3 = 1.3 yd.3

force

newton
kilonewton
meganewton

N
kN
MN

ounce (Force)
pound (Force)
Kip

N
kN
MN

.278
.00445
.00445

1N = 3.6 ozf
4.4N = 1 lbf
1kN = 225 lbf

stress

megapascal
newtons/sq.m

MPa
N/m2

pound/in2 (psi)
Kip/in2 (ksi)

MPa
MPa

.0069
6.895

1MPa = 145 psi
7MPa = 1 ksi

torque

newton-meter

N•m

inch-ounce
inch-pound
foot-pound

N-m
N-m
N-m

.00706
.113
1.356

1N•m = 140 in. oz.
1N•m = 9 in. lb.
1N•m = .75 ft. lb.
1.4 N•m = 1 ft. lb.

50

TABLE OF CONTENTS
Technical Section
Page
Screw Fastener Theory and Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
Joint Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
The Torque-Tension Relationship. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
Stripping Strength of Tapped Holes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
High-Temperature Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
Corrosion In Threaded Fasteners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
Impact Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
Product Engineering Bulletin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
Metric Threads. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
Through-Hole Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
Drill and Counterbore Sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
Hardness-Tensile Conversion Chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
Thread Stress Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
Optional Part Numbering System. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 thru 89

IMPORTANT
The technical discussions represent typical applications only. The use of the information is at
the sole discretion of the reader. Because applications vary enormously, UNBRAKO does not
warrant the scenarios described are appropriate for any specific application. The reader must
consider all variables prior to using this information.

51

INSTALLATION CONTROL

Single Shear

Several factors should be considered in designing a joint
or selecting a fastener for a particular application.

JOINT DESIGN AND FASTENER SELECTION.
Joint Length
The longer the joint length, the greater the total elongation will occur in the bolt to produce the desired clamp
load or preload. In design, if the joint length is increased,
the potential loss of preload is decreased.

Double Shear

Joint Material
If the joint material is relatively stiff compared to the bolt
material, it will compress less and therefore provide a
less sensitive joint, less sensitive to loss of preload as a
result of brinelling, relaxation and even loosening.
Thread Stripping Strength
Considering the material in which the threads will be
tapped or the nut used, there must be sufficient engagement length to carry the load. Ideally, the length of
thread engagement should be sufficient to break the
fastener in tension. When a nut is used, the wall thickness of the nut as well as its length must be considered.
An estimate, a calculation or joint evaluation will be
required to determine the tension loads to which the bolt
and joint will be exposed. The size bolt and the number
necessary to carry the load expected, along with the
safety factor, must also be selected.
The safety factor selected will have to take into consideration the consequence of failure as well as the additional holes and fasteners. Safety factors, therefore, have
to be determined by the designer.

SHEAR APPLICATIONS
Shear Strength of Material
Not all applications apply a tensile load to the fastener. In
many cases, the load is perpendicular to the fastener in
shear. Shear loading may be single, double or multiple
loading.

OTHER DESIGN CONSIDERATIONS
Application Temperature
For elevated temperature, standard alloy steels are useful
to about 550°F–600°F. However, if plating is used, the
maximum temperature may be less (eg. cadmium
should not be used over 450°F.
Austenitic stainless steels (300 Series) may be useful
to 800°F. They can maintain strength above 800°F but will
begin to oxidize on the surface.
Corrosion Environment
A plating may be selected for mild atmospheres or salts.
If plating is unsatisfactory, a corrosion resistant fastener
may be specified. The proper selection will be based
upon the severity of the corrosive environment.

FATIGUE STRENGTH
S/N Curve
Most comparative fatigue testing and specification
fatigue test requirements are plotted on an S/N curve.
In this curve, the test stress is shown on the ordinate
(y-axis) and the number of cycles is shown on the
abscissa (x-axis) in a lograthmic scale. On this type
curve, the high load to low load ratio must be shown.
This is usually R =.1, which means the low load in all
tests will be 10% of the high load.

There is a relationship between the tensile strength
of a material and its shear strength. For alloy steel, the
shear strength is 60% of its tensile strength. Corrosion
resistant steels (e.g. 300-Series stainless steels) have a
lower tensile/shear relationship and it is usually 50-55%

Single shear strength is exactly one-half the double shear
value. Shear strength listed in pounds per square inch
(psi) is the shear load in pounds multiplied by the cross
sectional area in square inches.

SPS

Curve represents 90% probability of survival

R=0.1

100,000
90,000
80,000
Maximum Stress (psi)

Single/Double Shear

Typical Unbrako Socket Head Cap Screws
S-N Curve for Finite Fatigue Life

70,000
60,000
50,000
40,000
30,000
20,000
10,000
0
104

105

106

107

Cycles to Failure

Effect of Preload
Increasing the R to .2, .3 or higher will change the curve
shape. At some point in this curve, the number of cycles
will reach 10 million cycles. This is considered the

52

SCREW FASTENER THEORY & APPLICATIONS
endurance limit or the stress at which infinite life might
be expected.
Modified Goodman/ Haigh Soderberg Curve
The S/N curve and the information it supplies will not
provide the information needed to determine how an
individual fastener will perform in an actual application.
In application, the preload should be higher than any of
the preloads on the S/N curve.
Therefore, for application information, the modified
Goodman Diagram and/or the Haigh Soderberg Curve
are more useful. These curves will show what fatigue
performance can be expected when the parts are properly
preloaded.
MODIFIED GOODMAN DIAGRAM
UNBRAKO TYPICAL SHCS

SPS

5 x 106 Cycles Run-Out
90% Probability of Survival
#8–32

3/8–24

3/8–16

5/8–11

1/4–20 (2 x 106 cycles)

180
VDI 2230 Prediction for #8 RTBHT (99% PS)
VDI 2230 Prediction for 5/8 RTBHT (99% PS)
160

Since stress/strain is a constant relationship for any
given material, we can use that relationship just as the
elongation change measurements were used previously.
Now, however, the strain can be detected from strain
gages applied directly to the outside surface of the bolt
or by having a hole drilled in the center of the bolt and
the strain gage installed internally. The output from these
gages need instrumentation to convert the gage electrical
measurement method. It is, however, an expensive
method and not always practical.
Turn of the Nut
The nut turn method also utilizes change in bolt length.
In theory, one bolt revolution (360° rotation) should
increase the bolt length by the thread pitch. There are
at least two variables, however, which influence this
relationship. First, until a snug joint is obtained, no bolt
elongation can be measured. The snugging produces a
large variation in preload. Second, joint compression is
also taking place so the relative stiffnesses of the joint
and bolt influences the load obtained.

VARIABLES IN TORQUE

140

Stress (ksi)

Strain

120

Coefficient of Friction

100

Since the torque applied to a fastener must overcome
all friction before any loading takes place, the amount of
friction present is important.

80

In a standard unlubricated assembly, the friction to
be overcome is the head bearing area and the thread-tothread friction. Approximately 50% of the torque applied
will be used to overcome this head-bearing friction and
approximately 35% to overcome the thread friction. So
85% of the torque is overcoming friction and only 15% is
available to produce bolt load.

60

40

20

0
0

20

40

60

80

100

120

140

160

180

Mean Stress (ksi)

METHODS OF PRELOADING
Elongation
The modulus for steel of 30,000,000 (thirty million) psi
means that a fastener will elongate .001 in/in of length
for every 30,000 psi in applied stress. Therefore, if 90,000
psi is the desired preload, the bolt must be stretched .003
inches for every inch of length in the joint.
This method of preloading is very accurate but it
requires that the ends of the bolts be properly prepared
and also that all measurements be very carefully made.
In addition, direct measurements are only possible where
both ends of the fastener are available for measurement
after installation. Other methods of measuring length
changes are ultrasonic, strain gages and turn of the nut.
Torque
By far, the most popular method of preloading is by
torque. Fastener manufacturers usually have recommended seating torques for each size and material
fastener. The only requirement is the proper size
torque wrench, a conscientious operator and the
proper torque requirement.

If these interfaces are lubricated (cadmium plate,
molybdenum disulfide, anti-seize compounds, etc.), the
friction is reduced and thus greater preload is produced
with the same torque.
The change in the coefficient of friction for different
conditions can have a very significant effect on the
slope of the torque tension curve. If this is not taken
into consideration, the proper torque specified for a
plain unlubricated bolt may be sufficient to yield or
break a lubricated fastener.
Thread Pitch
The thread pitch must be considered when a given stress
is to be applied, since the cross-sectional area used for
stress calculations is the thread tensile stress area and is
different for coarse and fine threads. The torque recommendations, therefore, are slightly higher for fine threads
than for coarse threads to achieve the same stress.
Differences between coarse and fine threads.
Coarse Threads are…

 more readily available in industrial fasteners.

 easier to assemble because of larger helix angle.

 require fewer turns and reduce cross threading.

 higher thread stripping strength per given length.

 less critical of tap drill size.

 not as easily damaged in handling.
53

Their disadvantages are…

 lower tensile strength.

 reduced vibrational resistance.

 coarse adjustment.
Fine Threads provide...

 higher tensile strength.

 greater vibrational resistance.

 finer adjustment.
Their disadvantages are…

 easier cross threaded.

 threads damaged more easily by handling.

 tap drill size slightly more critical.

 slightly lower thread stripping strength.

Some materials, such as stainless steel, are warm
forged at temperatures up to 1000°F. The heating results
in two benefits, lower forging pressures due to lower
yield strength and reduced work hardening rates.
Machining is the oldest method and is used for very
large diameters or small production runs.
The disadvantage is that machining cuts the metal
grain flow, thus creating planes of weakness at the
critical head-to-shank fillet area. This can reduce tension
fatigue performance by providing fracture planes.

Other Design Guidelines

Fillets

In addition to the joint design factors discussed, the
following considerations are important to the proper use
of high-strength fasteners.

The head-to-shank transition (fillet) represents a sizable
change in cross section at a critical area of bolt performance. It is important that this notch effect be minimized. A generous radius in the fillet reduces the notch
effect. However, a compromise is necessary because too
large a radius will reduce load-bearing area under the
head.


 Adequate thread engagement should be guaranteed
by use of the proper mating nut height for the system.
Minimum length of engagement recommended in a
tapped hole depends on the strength of the material,
but in all cases should be adequate to prevent stripping.

 Specify nut of proper strength level. The bolt and nut
should be selected as a system.

 Specify compatible mating female threads. 2B tapped
holes or 3B nuts are possibilities.

Composite radii such as elliptical fillets, maximize
curvature on the shank side of the fillet and minimize it
on the head side to reduce loss of bearing area on the
load-bearing surface.


 Corrosion, in general, is a problem of the joint, and not
just of the bolt alone. This can be a matter of galvanic
action between dissimilar metals. Corrosion of the
fastener material surrounding the bolt head or nut can
be critical with high-strength bolting. Care must be
exercised in the compatibility of joint materials and/or
coatings to protect dissimilar metals.

Critical Fastener Features

PROCESSING CONTROL

Threads

The quality of the raw material and the processing
control will largely affect the mechanical properties of
the finished parts.

Threads can be produced by grinding, cutting or rolling.

Head-Shank-Fillet: This area on the bolt must not be
restricted or bound by the joint hole. A sufficient chamfer
or radius on the edge of the hole will prevent interference that could seriously reduce fatigue life. Also, if the
bolt should seat on an unchamfered edge, there might
be serious loss of preload if the edge breaks under load.

MATERIAL SELECTION

In a rolled thread, the material is caused to flow into
the thread die contour, which is ground into the surface
during the manufacture of the die. Machines with two or
three circular dies or two flat dies are most common.

The selection of the type of material will depend on its
end use. However, the control of the analysis and quality
is a critical factor in fastener performance. The material
must yield reliable parts with few hidden defects such as
cracks, seams, decarburization and internal flaws.

Thread cutting requires the least tooling costs and is
by far the most popular for producing internal threads. It
is the most practical method for producing thin wall
parts and the only technique available for producing
large diameter parts (over 3 inches in diameter).

FABRICATION METHOD

Thread grinding yields high dimensional precision
and affords good control of form and finish. It is the only
practical method for producing thread plug gages.

Head
There are two general methods of making bolt heads,
forging and machining. The economy and grain flow
resulting from forging make it the preferred method.
The temperature of forging can vary from room
temperature to 2000°F. By far, the greatest number of
parts are cold upset on forging machines known as
headers or boltmakers. For materials that do not have
enough formability for cold forging, hot forging is used.
Hot forging is also used for bolts too large for cold upsetting due to machine capacity. The largest cold forging

54

machines can make bolts up to 1-1/2 inch diameter. For
large quantities of bolts, hot forging is more expensive
then cold forging.

Both machining and grinding have the disadvantage
of cutting material fibers at the most critical point of
performance.
The shape or contour of the thread has a great effect
on the resulting fatigue life. The thread root should be
large and well rounded without sharp corners or stress
risers. Threads with larger roots should always be used
for harder materials.
In addition to the benefits of grain flow and controlled shape in thread rolling, added fatigue life can
result when the rolling is performed after heat treatment.

SCREW FASTENER THEORY & APPLICATIONS
This is the accepted practice for high fatigue performance bolts such as those used in aircraft and space
applications.
FASTENER POINT END

Fatigue tests on threaded fasteners are usually alternating tension-tension loading. Most testing is done at
more severe strain than its designed service load but
ususally below the material yield strength.
Shear testing, as previously mentioned, consists
of loading a fastener perpendicular to its axis. All shear
testing should be accomplished on the unthreaded
portion of the fastener.
Checking hardness of parts is an indirect method for
testing tensile strength. Over the years, a correlation of
tensile strength to hardness has been obtained for most
materials. See page 83 for more detailed information.
Since hardness is a relatively easy and inexpensive test,
it makes a good inspection check. In hardness checking,
it is very important that the specimen be properly prepared and the proper test applied.

0
1
2
3
4
RELATIVE INTERNAL STRESS
AT FIRST ENGAGED THREAD
FASTENER HEAD END

EVALUATING PERFORMANCE
Mechanical Testing
In the fastener industy, a system of tests and examinations has evolved which yields reliable parts with proven
performance.
Some tests are conducted on the raw material; some
on the finished product.
There always seems to be some confusion regarding
mechanical versus metallurgical properties. Mechanical
properties are those associated with elastic or inelastic
reaction when force is applied, or that involve the relationship between stress and strain. Tensile testing stresses the fastener in the axial direction. The force at which
the fastener breaks is called the breaking load or ultimate
tensile strength. Load is designated in pounds, stress in
pounds per square inch and strain in inches per inch.
When a smooth tensile specimen is tested, the chart
obtained is called a Stress-Strain Curve. From this curve,
we can obtain other useful data such as yield strength.
The method of determining yield is known as the offset
method and consists of drawing a straight line parallel to
the stress strain curve but offset from the zero point by a
specified amount. This value is usually 0.2% on the strain
ordinate. The yield point is the intersection of the stressstrain curve and the straight line. This method is not
applicable to fasteners because of the variables introduced by their geomety.
When a fastener tensile test is plotted, a load/
elongation curve can be obtained. From this curve, a
yield determination known as Johnson‘s 2/3 approximate
method for determination of yield strength is used to
establish fastener yield, which will be acceptable for
design purposes. It is not recommended for quality
control or specification requirements.
Torque-tension testing is conducted to correlate the
required torque necessary to induce a given load in a
mechanically fastened joint. It can be performed by hand
or machine. The load may be measured by a tensile
machine, a load cell, a hydraulic tensile indicator or by
a strain gage.

Stress durability is used to test parts which have
been subjected to any processing which may have an
embrittling effect. It requires loading the parts to a value
higher than the expected service load and maintaining
that load for a specified time after which the load is
removed and the fastener examined for the presence of
cracks.
Impact testing has been useful in determining the
ductile brittle transformation point for many materials.
However, because the impact loading direction is
transverse to a fastener's normal longitude loading, its
usefulness for fastener testing is minimal. It has been
shown that many fastener tension impact strengths do
not follow the same pattern or relationship of Charpy
or Izod impact strength.
Metallurgical Testing
Metallurgical testing includes chemical composition,
microstructure, grain size, carburization and decarburization, and heat treat response.
The chemical composition is established when the
material is melted. Nothing subsequent to that process
will influence the basic composition.
The microstructure and grain size can be influenced
by heat treatment. Carburization is the addition of carbon
to the surface which increases hardness. It can occur if
heat treat furnace atmospheres are not adequately controlled. Decarburization is the loss of carbon from the
surface, making it softer. Partial decarburization is preferable to carburization, and most industrial standards allow
it within limits.
In summary, in order to prevent service failures, many
things must be considered:
The Application Requirements
Strength Needed – Safety Factors

 Tension/Shear/Fatigue

 Temperature

 Corrosion

 Proper Preload
The Fastener Requirements

 Material

 Fabrication Controls

 Performance Evaluations

55

AN EXPLANATION OF JOINT DIAGRAMS
When bolted joints are subjected to external tensile
loads, what forces and elastic deformation really exist?
The majority of engineers in both the fastener manufacturing and user industries still are uncertain. Several
papers, articles, and books, reflecting various stages of
research into the problem have been published and the
volume of this material is one reason for confusion.
The purpose of this article is to clarify the various
explanations that have been offered and to state the
fundamental concepts which apply to forces and elastic
deformations in concentrically loaded joints. The article
concludes with general design formulae that take into
account variations in tightening, preload loss during
service, and the relation between preloads, external
loads and bolt loads.

load must then be applied to the bolt. If the external load
is alternating, the increased stress levels on the bolt produce a greatly shortened fatigue life.
When seating requires a certain minimum force or
when transverse loads are to be transformed by friction,
the minimum clamping load FJ min is important.
FJ min = FB max – Fe

The Joint Diagram
Forces less than proof load cause elastic strains.
Conversely, changes in elastic strains produce force
variations. For bolted joints this concept is usually
demonstrated by joint diagrams.
The most important deformations within a joint are
elastic bolt elongation and elastic joint compression in
the axial direction. If the bolted joint in Fig. 1 is subjected
to the preload Fi the bolt elongates as shown by the line
OB in Fig. 2A and the joint compresses as shown by
the line OJ. These two lines, representing the spring
characteristics of the bolt and joint, are combined into
one diagram in Fig. 2B to show total elastic deformation.
If a concentric external load Fe is applied under the
bolt head and nut in Fig. 1, the bolt elongates an additional amount while the compressed joint members
partially relax. These changes in deformation with
external loading are the key to the interaction of forces
in bolted joints.
In Fig. 3A the external load Fe is added to the joint
diagram Fe is located on the diagram by applying the
upper end to an extension of OB and moving it in until
the lower end contacts OJ. Since the total amount of
elastic deformation (bolt plus joint) remains constant for
a given preload, the external load changes the total bolt
elongation to ∆lB + λ and the total joint compression to
∆lJ – λ.
In Fig. 3B the external load Fe is divided into an additional bolt load FeB and the joint load FeJ, which unloads
the compressed joint members. The maximum bolt load
is the sum of the load preload and the additional bolt
load:
FB max = Fi + FeB
If the external load Fe is an alternating load, FeB is that
part of Fe working as an alternating bolt load, as shown
in Fig. 3B. This joint diagram also illustrates that the joint
absorbs more of the external load than the bolt subjected
to an alternating external load.
The importance of adequate preload is shown in Fig.
3C. Comparing Fig. 3B and Fig. 3C, it can be seen that FeB
will remain relatively small as long as the preload Fi is
greater than FeJ. Fig. 3C represents a joint with insufficient
preload. Under this condition, the amount of external
load that the joint can absorb is limited, and the excess

56

Fig. 1 (above) Joint
components

Fig. 2 Joint diagram is obtained by combining load vs.
deformation diagrams of bolt and joints.
Fig. 3 The complete simple
joint diagrams show
external load Fe added (A),
and external load divided
into an additional bolt
load FeB and reduction in
joint compression FeJ (B).
Joint diagram (C) shows
how insufficient preload Fi
causes excessive additional
bolt load FeB.

JOINT DIAGRAMS
Spring Constants
To construct a joint diagram, it is necessary to determine
the spring rates of both bolt and joint. In general, spring
rate is defined as:
K= F
∆l

When the outside diameter of the joint is smaller than or
equal to the bolt head diameter, i.e.,as in a thin bushing,
the normal cross sectioned area is computed:
As = π (Dc2 – Dh2)
4
where

From Hook’s law:

Dc = OD of cylinder or bushing and

∆l = lF
EA

Dh = hole diameter

Therefore:
K = EA
l
To calculate the spring rate of bolts with different
cross sections, the reciprocal spring rates, or compliances, of each section are added:
1
KB

=

1
K1

1
K2

+

+

....

+

When the outside diameter of the joint is larger than
head or washer diameter DH, the stress distribution is in
the shape of a barrel, Fig 5. A series of investigations
proved that the areas of the following substitute cylinders
are close approximations for calculating the spring contents of concentrically loaded joints.
When the joint diameter DJ is greater than DH but less
than 3DH;

1
Kn

Thus, for the bolt shown in Fig. 4:
1 =
KB

1
E

(

0.4d + l1 + l2 + l3 + 0.4d
A1
A1
A2
Am
Am

)

0
-20
-40

20

where
40

d = the minor thread diameter and

-60

Am = the area of the minor thread diameter
This formula considers the elastic deformation of the
head and the engaged thread with a length of 0.4d each.
Calculation of the spring rate of the compressed
joint members is more difficult because it is not always
obvious which parts of the joint are deformed and which
are not. In general, the spring rate of a clamped part is:

60
-40
-35

80

100

-30

KJ = EAS
lJ

-25

where As is the area of a substitute cylinder to be
determined.

-20

100

-15

-10

-5

-25

-30

0.4d
I1

-35
-40

I2

I3
0.4d

d

Ij

Fig. 4 Analysis
of bolt lengths
contributing to
the bolt spring
rate.

Fig. 5 Lines of equal axial stresses in a bolted joint
obtained by the axisymmetric finite element method are
shown for a 9/16–18 bolt preloaded to 100 KSI. Positive
numbers are tensile stresses in KSI; negative numbers
are compressive stresses in KSI.
57

As = π (DH2 – Dh2)
4
π
+
8

(

DJ
– 1
DH

)(

Effect of Loading Planes

)

DHlJ
l2
+ J
5
100

When the joint diameter DJ is equal to or greater than 3DH:
As = π [(DH + 0.1 lJ)2 – Dh2]
4
These formulae have been verified in laboratories by
finite element method and by experiments.
Fig. 6 shows joint diagrams for springy bolt and
stiff joint and for a stiff bolt and springy joint. These
diagrams demonstrate the desirability of designing with
springy bolt and a stiff joint to obtain a low additional
bolt load FeB and thus a low alternating stress.
The Force Ratio
Due to the geometry of the joint diagram, Fig. 7,
Fe KB
FeB =
KB + KJ
KB
Defining Φ =
KB + KJ
FeB = FeΦ and
F
Φ, called the Force Ratio, = eB
Fe
For complete derivation of Φ, see Fig. 7.
To assure adequate fatigue strength of the selected
fastener the fatigue stress amplitude of the bolt resulting
from an external load Fe is computed as follows:
σB = ± FeB/2 or
Am
Φ
Fe
σB = ±
2 Am

The joint diagram in Fig 3, 6 and 7 is applicable only
when the external load Fe is applied at the same loading
planes as the preloaded Fi, under the bolt head and the
nut. However, this is a rare case, because the external
load usually affects the joint somewhere between the
center of the joint and the head and the nut.
When a preloaded joint is subjected to an external
load Fe at loading planes 2 and 3 in Fig. 8, Fe relieves the
compression load of the joint parts between planes 2
and 3. The remainder of the system, the bolt and the
joint parts between planes 1-2 and 3-4, feel additional
load due to Fe applied planes 2 and 3, the joint material
between planes 2 and 3 is the clamped part and all other
joint members, fastener and remaining joint material,
are clamping parts. Because of the location of the loading planes, the joint diagram changes from black line
to the blue line. Consequently, both the additional bolt
load FB max decrease significantly when the loading planes
of Fe shift from under the bolt head and nut toward the
joint center.
Determination of the length of the clamped parts is,
however, not that simple. First, it is assumed that the
external load is applied at a plane perpendicular to the
bolt axis. Second, the distance of the loading planes
from each other has to be estimated. This distance may
be expressed as the ratio of the length of clamped parts
to the total joint length. Fig. 9 shows the effect of two
different loading planes on the bolt load, both joints
having the same preload Fi and the same external load
Fe . The lengths of the clamped parts are estimated to
be 0.75lJ for joint A, and 0.25lJ for joint B.
In general, the external bolt load is somewhere
between FeB = 1ΦFe for loading planes under head and
nut and FeB = 0ΦFe = 0 when loading planes are in the
joint center, as shown in Fig. 10. To consider the loading
planes in calculations, the formula:

Fe
2

Fe
2

Fe
2

Fe
2

Fe
2

Fe
2

Fe
2

Fe
2

Fig. 6 Joint diagram of a springy bolt in a stiff joint (A), is compared to a diagram of a stiff bolt in a springy joint (B).
Preload Fi and external load Fe are the same but diagrams show that alternating bolt stresses are significantly lower
with a spring bolt in a stiff joint.
58

JOINT DIAGRAMS

1

Fe
2

Fe
2

2
nlj

Ij

3
4

Fe
2

Fe
2

Fig. 7 Analysis of external load Fe and derivation of
Force Ratio Φ.
Fi
Fi
= KB and tan β =
= KJ
∆lB
∆lJ
FeB
FeJ
FeB
F
λ=
=
=
= eJ
tan α
tan β
KB
KJ
FeJ = λ tan β and FeB = λ tan α
Since Fe = FeB + FeJ
Fe = FeB + λ tan β
FeB
Substituting
for λ produces:
tan α
tan α =

Fe = FeB +

or

FeB tan β
tan α

Multiplying both sides by tan α:
Fe tan α = FeB (tan α + tan β) and
Fe tan α
FeB =
tan α tan β
Substituting KB for tan α and KJ for tan β
FB
FeB = Fe
KB + KJ
Defining Φ =
FeB = Φ Fe
F
Φ = eB
Fe

Fig. 8 Joint diagram shows effect of loading planes of Fe
on bolt loads FeB and FB max . Black diagram shows FeB
and FB max resulting from Fe applied in planes 1 and 4.
Orange diagram shows reduced bolt loads when Fe is
applied in planes 2 and 3.

A

Estimated:

Fe

Fe

KB
KB + KJ
and it becomes obvious why Φ
is called force ratio.

B
Fe

Fe

Fig. 9 When external load is applied relatively near bolt
head, joint diagram shows resulting alternating stress αB
(A). When same value external load is applied relatively
near joint center, lower alternating stress results (B).
59

F1

F1

Fig. 10 Force diagrams show the effect of the loading planes of the external load on the bolt load.

Fig. 11 Modified joint diagram
shows nonlinear compression
of joint at low preloads.

60

JOINT DIAGRAMS
FeB = Φ Fe must be modified to :
FeB = n Φ Fe
where n equals the ratio of the length of the clamped
parts due to Fe to the joint length lj. The value of n can
range from 1, when Fe is applied under the head and nut,
to O, when Fe is applies at the joint center. Consequently
the stress amplitude:
σB = ± Φ Fe
becomes
2 Am
σB = ± n Φ Fe
2 Am

where ∆Fi is the amount of preload loss to be
expected. For a properly designed joint, a preload loss
∆Fi = – (0.005 to 0.10) Fi should be expected.
The fluctuation in bolt load that results from tightening is expressed by the ratio:
a = Fi max
Fi min
where a varies between 1.25 and 3.0 depending on the
tightening method.
Considering a the general design formulae are:
Fi nom = FJ min = (1 – Φ) Fe

General Design Formulae

Fi max = a [ Fj min + (1 – Φ) Fe + ∆Fi ]

Hitherto, construction of the joint diagram has assumed
linear resilience of both bolt and joint members. However,
recent investigations have shown that this assumption is
not quite true for compressed parts.

FB max = a [ Fj min + (1 – Φ) Fe + ∆Fi ] + ΦFe

Taking these investigations into account, the joint
diagram is modified to Fig. 11. The lower portion of the
joint spring rate is nonlinear, and the length of the linear
portion depends on the preload level Fi. The higher Fi the
longer the linear portion. By choosing a sufficiently high
minimum load, Fmin>2Fe, the non-linear range of the joint
spring rate is avoided and a linear relationship between
FeB and Fe is maintained.
Also from Fig. 11 this formula is derived:
Fi min = FJ min + ( 1 – Φ) Fe + ∆Fi

Conclusion
The three requirements of concentrically loaded joints
that must be met for an integral bolted joint are:
1. The maximum bolt load FB max must be less than
the bolt yield strength.
2. If the external load is alternating, the alternating
stress must be less than the bolt endurance limit to avoid
fatigue failures.
3. The joint will not lose any preload due to permanent set or vibration greater than the value assumed for
∆Fi .

SYMBOLS
A
Am
As
Ax
d
Dc
DH
Dh
DJ
E
F
Fe
FeB
FeJ
Fi
∆Fi
Fi min
Fi max
Fj nom

Area (in.2)
Area of minor thread diameter (in.2)
Area of substitute cylinder (in.2)
Area of bolt part 1x (in.2)
Diameter of minor thread (in.)
Outside diameter of bushing (cylinder) (in.)
Diameter of Bolt head (in.)
Diameter of hole (in.)
Diameter of Joint
Modulus of Elasticity (psi)
Load (lb)
External load (lb.)
Additinal Bolt Load due to external load (lb)
Reduced Joint load due to external load (lb)
Preload on Bolt and Joint (lb)
Preload loss (–lb)
Minimum preload (lb)
Maximum preload (lb)
Nominal preload (lb)

FB max Maximum Bolt load (lb)
FJ min Minimum Joint load (lb)
K
Spring rate (lb/in.)
KB
Spring rate of Bolt (lb/in.)
KJ
Spring rate of Joint (lb/in.)
Kx
Spring rate of Bolt part lx (lb/in.)
l
Length (in.)
∆l
Change in length (in.)
lB
Length of Bolt (in.)
∆lB Bolt elongation due to Fi (in.)
lJ
Length of Joint (in.)
∆lJ Joint compression to Fi (in.)
lx
Length of Bolt part x (in.)
Length of clamped parts
n
Total Joint Length
α
Tightening factor
Φ
Force ratio
λ
Bolt and Joint elongation due to Fe (in.)
σB
Bolt stress amplitude (± psi)

61

TIGHTENING TORQUES AND THE
TORQUE-TENSION RELATIONSHIP
All of the analysis and design work done in advance will
have little meaning if the proper preload is not achieved.
Several discussions in this technical section stress the
importance of preload to maintaining joint integrity.
There are many methods for measuring preload (see
Table 12). However, one of the least expensive techniques that provides a reasonable level of accuracy
versus cost is by measuring torque. The fundamental
characteristic required is to know the relationship
between torque and tension for any particular bolted
joint. Once the desired design preload must be identified
and specified first, then the torque required to induce
that preload is determined.
Within the elastic range, before permanent stretch
is induced, the relationship between torque and tension
is essentially linear (see figure 13). Some studies have
found up to 75 variables have an effect on this relationship: materials, temperature, rate of installation, thread
helix angle, coefficients of friction, etc. One way that has
been developed to reduce the complexity is to depend
on empirical test results. That is, to perform experiments
under the application conditions by measuring the
induced torque and recording the resulting tension. This
can be done with relatively simple, calibrated hydraulic
pressure sensors, electric strain gages, or piezoelectric
load cells. Once the data is gathered and plotted on a
chart, the slope of the curve can be used to calculate a
correlation factor. This technique has created an accepted
formula for relating torque to tension.
T=KDP
T = torque, lbf.-in.
D = fastener nominal diameter, inches
P = preload, lbf.
K = “nut factor,” “tightening factor,” or “k-value”
If the preload and fastener diameter are selected in
the design process, and the K-value for the application
conditions is known, then the necessary torque can be
calculated. It is noted that even with a specified torque,
actual conditions at the time of installation can result in
variations in the actual preload achieved (see Table 12).
One of the most critical criteria is the selection of the
K-value. Accepted nominal values for many industrial
applications are:

fastener underhead, full friction reduction will not be
achieved. Similarly, if the material against which the
fastener is bearing, e.g. aluminum, is different than the
internal thread material, e.g. cast iron, the effective
friction may be difficult to predict, These examples
illustrate the importance and the value of identifying
the torque-tension relationship. It is a recommend
practice to contact the lubricant manufacturer for
K-value information if a lubricant will be used.
The recommended seating torques for Unbrako
headed socket screws are based on inducing preloads
reasonably expected in practice for each type. The
values for Unbrako metric fasteners are calculated
using VDI2230, a complex method utilized extensively
in Europe. All values assume use in the received condition in steel holes. It is understandable the designer
may need preloads higher than those listed. The
following discussion is presented for those cases.

TORSION-TENSION YIELD AND TENSION
CAPABILITY AFTER TORQUING
Once a headed fastener has been seated against a bearing surface, the inducement of torque will be translated
into both torsion and tension stresses. These stresses
combine to induce twist. If torque continues to be induced,
the stress along the angle of twist will be the largest
stress while the bolt is being torqued. Consequently, the
stress along the bolt axis (axial tension) will be something
less. This is why a bolt can fail at a lower tensile stress
during installation than when it is pulled in straight tension
alone, eg . a tensile test. Research has indicated the axial
tension can range from 135,000 to 145,000 PSI for industry
socket head cap screws at torsion-tension yield, depending on diameter. Including the preload variation that can
occur with various installation techniques, eg. up to 25%,
it can be understood why some recommended torques
induce preload reasonably lower than the yield point.
Figure 13 also illustrates the effect of straight tension
applied after installation has stopped. Immediately after
stopping the installation procedure there will be some
relaxation, and the torsion component will drop toward
zero. This leaves only the axial tension, which keeps
the joint clamped together. Once the torsion is relieved,
the axial tension yield value and ultimate value for the
fastener will be appropriate.

K = 0.20 for as-received steel bolts into steel holes
K = 0.15 steel bolts with cadmium plating, which acts like
a lubricant,
K = 0.28 steel bolts with zinc plating.
The K-value is not the coefficient of the friction (µ); it is
an empirically derived correlation factor.

Preload Measuring
Method

It is readily apparent that if the torque intended for a
zinc plated fastener is used for cadmium plated fastener,
the preload will be almost two times that intended; it
may actually cause the bolt to break.

Feel (operator’s judgement)
Torque wrench
Turn of the nut
Load-indicating washers
Fastener elongation
Strain gages

Another influence is where friction occurs. For steel
bolts holes, approximately 50% of the installation torque
is consumed by friction under the head, 35% by thread
friction, and only the remaining 15% inducing preload
tension. Therefore, if lubricant is applied just on the

62

Table 12
Industrial Fasteners Institute’s
Torque-Measuring Method
Accuracy
Percent

Relative
Cost

±35
±25
±15
±10
±3 to 5
±1

1
1.5
3
7
15
20

THE TORQUE-TENSION RELATIONSHIP
Fig. 13 Torque/Tension Relationship

Fig. 14
TORQUE VS. INDUCED LOAD
UNBRAKO SOCKET HEAD CAP SCREW

Straight tension
Bolt tension (lb.)

TYPICAL

Straight tension
after torquing
to preload
Torque-induced
tension

Elongation (in.)

Fig. 15 Recommended Seating Torques (Inch-Lb.) for Application in Various Materials
UNBRAKO pHd (1960 Series) Socket Head Cap Screws
mild steel Rb 87
cast iron Rb 83 note 1
screw
size

brass Rb 72
note 2

aluminum Rb 72
(2024-T4) note 3

UNC

UNF

UNC

UNF

UNC

UNF

plain

plain

plain

plain

plain

plain

#0
#1
#2

–
*3.8
*6.3

*2.1
*4.1
*6.8

–
*3.8
*6.3

*2.1
*4.1
*6.8

–
*3.8
*6.3

*2.1
*4.1
*6.8

#3
#4
#5

*9.6
*13.5
*20

*10.3
*14.8
*21

*9.6
*13.5
*20

*10.3
*14.8
*21

*9.6
*13.5
*20

*10.3
*14.8
*21

#6
#8
#10

*25
*46
*67

*28
*48
*76

*25
*46
*67

*28
*48
*76

*25
*46
*67

*28
*48
*76

1/4
5/16
3/8

*158
*326
*580

*180
*360
635

136
228
476

136
228
476

113
190
397

113
190
397

7/16
1/2
9/16

*930
*1,420
*2,040

*1,040
*1,590
2,250

680
1,230
1,690

680
1,230
1,690

570
1,030
1,410

570
1,030
1,410

5/8
3/4
7/8

*2,820
*5,000
*8,060

3,120
5,340
8,370

2,340
4,000
6,280

2,340
4,000
6,280

1,950
3,340
5,230

1,950
3,340
5,230

1
1 1/8
1 1/4

*12,100
*13,800
*19,200

12,800
*15,400
*21,600

9,600
13,700
18,900

9,600
13,700
18,900

8,000
11,400
15,800

8,000
11,400
15,800

1 3/8
1 1/2

*25,200
*33,600

*28,800
*36,100

24,200
32,900

24,200
32,900

20,100
27,400

20,100
27,400

NOTES:
1. Torques based on 80,000 psi bearing stress under head of screw.
2. Torques based on 60,000 psi bearing stress under head of screw.
3. Torques based on 50,000 psi bearing stress under head of screw.

*Denotes torques based on 100,000 psi tensile stress in screw
threads up to 1" dia., and 80,000 psi for sizes 1 1/8" dia. and larger.
To convert inch-pounds to inch-ounces – multiply by 16.
To convert inch-pounds to foot-pounds – divide by 12.

63

STRIPPING STRENGTH OF TAPPED HOLES
Charts and sample problems for obtaining minimum
thread engagement based on applied load, material,
type of thread and bolt diameter.
Knowledge of the thread stripping strength of
tapped holes is necessary to develop full tensile strength
of the bolt or, for that matter, the minimum engagement
needed for any lesser load.
Conversely, if only limited length of engagement is
available, the data help determine the maximum load
that can be safely applied without stripping the threads
of the tapped hole.
Attempts to compute lengths of engagement and
related factors by formula have not been entirely satisfactory-mainly because of subtle differences between
various materials. Therefore, strength data has been
empirically developed from a series of tensile tests of
tapped specimens for seven commonly used metals
including steel, aluminum, brass and cast iron.
The design data is summarized in the six accompanying charts, (Charts E504-E509), and covers a range
of screw thread sizes from #0 to one inch in diameter
for both coarse and fine threads. Though developed
from tests of Unbrako socket head cap screws having
minimum ultimate tensile strengths (depending on the
diameter) from 190,000 to 180,000 psi , these stripping
strength values are valid for all other screws or bolts of
equal or lower strength having a standard thread form.
Data are based on static loading only.
In the test program, bolts threaded into tapped specimens of the metal under study were stressed in tension
until the threads stripped. Load at which stripping
occurred and the length of engagement of the specimen
were noted. Conditions of the tests, all of which are met
in a majority of industrial bolt applications, were:

 Tapped holes had a basic thread depth within the range
of 65 to 80 per cent. Threads of tapped holes were
Class 2B fit or better.

 Minimum amount of metal surrounding the tapped
hole was 2 1/2 times the major diameter.

 Test loads were applied slowly in tension to screws
having standard Class 3A threads. (Data, though, will
be equally applicable to Class 2A external threads as
well.)

 Study of the test results revealed certain factors that
greatly simplified the compilation of thread stripping
strength data:

 Stripping strengths are almost identical for loads
applied either by pure tension or by screw torsion.
Thus data are equally valid for either condition of
application.

64


 Stripping strength values vary with diameter of screw.
For a given load and material, larger diameter bolts
required greater engagement.

 Minimum length of engagement (as a percent of
screw diameter) is a straight line function of load.
This permits easy interpolation of test data for any
intermediate load condition.

 When engagement is plotted as a percentage of bolt
diameter, it is apparent that stripping strengths for a
wide range of screw sizes are close enough to be
grouped in a single curve. Thus , in the accompanying
charts, data for sizes #0 through #12 have been represented by a single set of curves.
With these curves, it becomes a simple matter to
determine stripping strengths and lengths of engagement for any condition of application. A few examples
are given below:
Example 1. Calculate length of thread engagement
necessary to develop the minimum ultimate tensile
strength (190,000 psi) of a 1/2–13 (National Coarse)
Unbrako cap screw in cast iron having an ultimate shear
strength of 30,000 psi. E505 is for screw sizes from #0
through #10; E506 and E507 for sizes from 1/4 in. through
5/8 in.; E508 and E509 for sizes from 3/4 in. through 1 in.
Using E506 a value 1.40D is obtained. Multiplying nominal bolt diameter (0.500 in.) by 1.40 gives a minimum
length of engagement of 0.700 in.
Example 2. Calculate the length of engagement for
the above conditions if only 140,000 psi is to be applied.
(This is the same as using a bolt with a maximum tensile
strength of 140,000psi.) From E506 obtain value of 1.06D
Minimum length of engagement = (0.500) (1.06) = 0.530.
Example 3. Suppose in Example 1 that minimum
length of engagement to develop full tensile strength
was not available because the thickness of metal allowed
a tapped hole of only 0.600 in. Hole depth in terms of
bolt dia. = 0.600/0.500 = 1.20D. By working backwards in
Chart E506, maximum load that can be carried is approximately 159,000 psi.
Example 4. Suppose that the hole in Example 1 is
to be tapped in steel having an ultimate shear strength
65,000 psi. There is no curve for this steel in E506 but a
design value can be obtained by taking a point midway
between curves for the 80,000 psi and 50,000 psi steels
that are listed. Under the conditions of the example, a
length of engagement of 0.825D or 0.413 in. will be
obtained.

STRIPPING STRENGTH OF TAPPED HOLES
THREAD STRIPPING STRENGTH
IN VARIOUS MATERIALS
FOR UNBRAKO SOCKET HEAD CAP SCREWS
SIZES #0 THROUGH #10 COARSE AND FINE THREADS
TYPICAL

THREAD STRIPPING STRENGTH
IN VARIOUS MATERIALS
FOR UNBRAKO SOCKET HEAD CAP SCREWS
SIZES 1/4" THRU 5/8" DIAMETER COARSE THREADS
TYPICAL

65

THREAD STRIPPING STRENGTH
IN VARIOUS MATERIALS
FOR UNBRAKO SOCKET HEAD CAP SCREWS
SIZES 1/4" THRU 5/8" DIAMETER FINE THREADS
TYPICAL

THREAD STRIPPING STRENGTH
IN VARIOUS MATERIALS
FOR UNBRAKO SOCKET HEAD CAP SCREWS
SIZES 3/4" THRU 1" DIAMETER COARSE THREADS
TYPICAL

66

STRIPPING STRENGTH OF TAPPED HOLES
THREAD STRIPPING STRENGTH
IN VARIOUS MATERIALS
FOR UNBRAKO SOCKET HEAD CAP SCREWS
SIZES 3/4" THRU 1" DIAMETER FINE THREADS
TYPICAL

THREAD STRIPPING STRENGTH
IN VARIOUS MATERIALS
FOR UNBRAKO SOCKET HEAD CAP
SCREWS
SIZES OVER 1"
TYPICAL

67

HIGH-TEMPERATURE JOINTS
Bolted joints subjected to cyclic loading perform best if
an initial preload is applied. The induced stress minimizes the external load sensed by the bolt, and reduces
the chance of fatigue failure. At high temperature, the
induced load will change, and this can adeversely affect
the fastener performance. It is therefore necessary to
compensate for high-temperature conditions when
assembling the joint at room temperature. This article
describes the factors which must be considered and illustrates how a high-temperature bolted joint is designed.
In high-temperature joints, adequate clamping force
or preload must be maintained in spite of temperatureinduced dimensional changes of the fastener relative to
the joint members. the change in preload at any given
temperature for a given time can be calculated, and the
affect compensated for by proper fastener selection and
initial preload.
Three principal factors tend to alter the initial
clamping force in a joint at elevated temperatures,
provided that the fastener material retains requisite
strength at the elevated temperature. These factors are:
Modulus of elasticity, coefficient of thermal expansion,
and relaxation.
Modulus Of Elasticity: As temperature increases,
less stress or load is needed to impart a given amount
of elongation or strain to a material than at lower temperatures. This means that a fastener stretched a certain
amount at room temperature to develop a given preload
will exert a lower clamping force at higher temperature
if there is no change in bolt elongation.
Coefficient of Expansion: With most materials, the
size of the part increases as the temperature increases.
In a joint, both the structure and the fastener grow with
an increase in temperature, and this can result,depending on the materials, in an increase or decrease in the
clamping force. Thus, matching of materials in joint
design can assure sufficient clamping force at both
room and elevated temperatures. Table 16 lists mean
coefficient of thermal expansion of certain fastener
alloys at several temperatures.
Relaxation: At elevated temperatures, a material
subjected to constant stress below its yield strength will
flow plastically and permanently change size. This phenomenon is called creep. In a joint at elevated temperature, a fastener with a fixed distance between the bearing
surface of the head and nut will produce less and less
clamping force with time. This characteristic is called
relaxation. It differs from creep in that stress changes
while elongation or strain remains constant. Such
elements as material, temperature, initial stress,
manufacturing method, and design affect the rate of
relaxation.
Relaxation is the most important of the three factors.
It is also the most critical consideration in design of
elevated-temperature fasteners. A bolted joint at 1200°F
can lose as much as 35 per cent of preload. Failure to
compensate for this could lead to fatigue failure through
a loose joint even though the bolt was properly tightened
initially.

68

If the coefficient of expansion of the bolt is greater
than that of the joined material, a predictable amount of
clamping force will be lost as temperature increases.
Conversely, if the coefficient of the joined material is
greater, the bolt may be stressed beyond its yield or even
fracture strength. Or, cyclic thermal stressing may lead to
thermal fatigue failure.
Changes in the modulus of elasticity of metals with
increasing temperature must be anticipated, calculated,
and compensated for in joint design. Unlike the coefficient of expansion, the effect of change in modulus is to
reduce clamping force whether or not bolt and structure
are the same material, and is strictly a function of the
bolt metal.
Since the temperature environment and the materials of the structure are normally “fixed,” the design
objective is to select a bolt material that will give the
desired clamping force at all critical points in the operating range of the joint. To do this, it is necessary to
balance out the three factors-relaxation, thermal expansion, and modulus-with a fourth, the amount of initial
tightening or clamping force.
In actual joint design the determination of clamping
force must be considered with other design factors such
as ultimate tensile, shear, and fatigue strength of the fastener at elevated temperature. As temperature increases
the inherent strength of the material decreases.
Therefore, it is important to select a fastener material
which has sufficient strength at maximum service
temperature.
Example
The design approach to the problem of maintaining satisfactory elevated-temperature clamping force in a joint
can be illustrated by an example. The example chosen is
complex but typical. A cut-and-try process is used to
select the right bolt material and size for a given design
load under a fixed set of operating loads and environmental conditions, Fig.17.
The first step is to determine the change in thickness, ∆t, of the structure from room to maximum
operating temperature.
For the AISI 4340 material:
∆t1 = t1(T2 – T1)α
∆t1 = (0.50)(800 – 70) (7.4 x 10–6)
∆t1 = 0.002701 in.
For the AMS 6304 material:
∆t2 = (0.75)(800 – 70)(7.6  10–6)
∆t2 = 0.004161 in.
The total increase in thickness for the joint members
is 0.00686 in.
The total effective bolt length equals the total joint
thickness plus one-third of the threads engaged by the
nut. If it is assumed that the smallest diameter bolt
should be used for weight saving, then a 1/4-in. bolt
should be tried. Thread engagement is approximately
one diameter, and the effective bolt length is:

HIGH-TEMPERATURE JOINTS
Fc

Fc

AISI 4340

Fw

AMS 6304

T2 = 0.75 in.

Fc

Fc

Fw

d = Bolt diam, in.

T1 = Room temperature= 70°F

E = Modulus of elasticity, psi

T2 = Maximum operatng
temperature for
1000 hr=800°F

Fb = Bolt preload, lb
Fc = Clamping force, lb
(Fb=Fc)
Fw = Working load=1500 lb
static + 100 lb cyclic
L

150

T1 = 0.50 in.
Fb

Fw

200

t

= Panel thickness, in.

a

= Coefficient of thermal
expansion

= Effective bolt length, inc.

Stress (1000 psi)

Fw

ss

tre

100

um

S

m
xi

a

M

50

um

s

es

r
St

m
ni

i

M

44,000 psi
21,000 psi
150
50
100
Mean Stress (1000 psi)

200

Fig. 18 – Goodman diagram of maximum and minimum
operating limits for H-11 fastener at 800°F. Bolts stressed
within these limits will give infinite fatigue life.

Fig. 17 – Parameters for joint operating at 800°F.
L = t1 + t2 + (1/3 d)
L = 0.50 + 0.75 +(1/3 x 0.25)
L = 1.333 in.
The ideal coefficient of thermal expansion of the
bolt material is found by dividing the total change in
joint thickness by the bolt length times the change in
temperature.
αb =
α=

∆t
L  ∆t

.00686
= 7.05  10–6 in./in./deg. F
(1.333)(800 – 70)

The material, with the nearest coefficient of expansion is with a value of 9,600,000 at 800°F.
To determine if the bolt material has sufficient
strength and resistance to fatigue, it is necessary to calculate the stress in the fastener at maximum and minimum load. The bolt load plus the cyclic load divided by
the tensile stress of the threads will give the maximum
stress. For a 1/4-28 bolt, tensile stress area,from thread
handbook H 28, is 0.03637 sq. in. The maximum stress is
Smax =

Bolt load = 1500 + 100
Stress area
0.03637

Because of relaxation, it is necessary to determine
the initial preload required to insure 1500-lb. clamping
force in the joint after 1000 hr at 800°F.
When relaxation is considered, it is necessary to
calculate the maximum stress to which the fastener is
subjected. Because this stress is not constant in dynamic
joints, the resultant values tend to be conservative.
Therefore, a maximum stress of 44,000 psi should be
considered although the necessary stress at 800°F need
be only 41,200 psi. Relaxation at 44,000 psi can be interpolated from the figure, although an actual curve could
be constructed from tests made on the fastener at the
specific conditions.
The initial stress required to insure a clamping stress
of 44,000 psi after 1000 hr at 800°F can be calculated by
interpolation.
x = 61,000 – 44,000 = 17,000
y = 61,000 – 34,000 = 27,000
B = 80,000 – 50,000 = 30,000
A = 80,000 – C
x =
y

A
B

17,000 = 80,000 – C
27,000
30,000

C = 61,100 psi

Smax = 44,000 psi
and the minimum bolt stress is 41,200 psi.
H-11 has a yield strength of 175,000 psi at 800°F,
Table 3, and therefore should be adequate for the working loads.
A Goodman diagram, Fig. 18, shows the extremes
of stress within which the H-11 fastener will not fail by
fatigue. At the maximum calculated load of 44,000 psi,
the fastener will withstand a minimum cyclic loading at
800°F of about 21,000 psi without fatigue failure.

The bolt elongation required at this temperature is
calculated by dividing the stress by the modulus at temperature and multiplying by the effective length of the
bolt. That is: (61,000  1.333)/24.6  106 = 0.0033
Since the joint must be constructed at room temperature, it is necessary to determine the stresses at
this state. Because the modulus of the fastener material
changes with temperature, the clamping force at room
temperature will not be the same as at 800°F. To deter-

69

mine the clamping stress at assembly conditions, the
elongation should be multiplied by the modulus of
elasticity at room temperature.
.0033  30.6  106 = 101,145 psi
The assembly conditions will be affected by the
difference between th ideal and actual coefficients of
expansion of the joint. The ideal coeffienct for the fastener material was calculated to be 7.05 but the closest
material – H-11 – has a coefficient of 7.1. Since this
material has a greater expansion than calculated, there
will be a reduction in clamping force resulting from the
increase in temperature. This amount equals the difference between the ideal and the actual coefficients
multiplied by the change in temperature, the length of
the fastener, and the modulus of elasticity at 70°F.

used to apply preload (the most common and simplest
method available), a plus or minus 25 per cent variation
in induced load can result. Therefore, the maximum load
which could be expected in this case would be 1.5 times
the minimum, or:
(1.5)(102,635) = 153,950 psi
This value does not exceed the room-temperature
yield strength for H-11 given in Table 19.
Since there is a decrease in the clamping force with
an increase in temperature and since the stress at operating temperature can be higher than originally calculated
because of variations in induced load, it is necessary to
ascertain if yield strength at 800°F will be exceeded
(max stress at 70°F + change in stress)  E at 800°F
E at 70°F

[(7.1 – 7.05)  10 ][800 – 70][1.333] 
–6

[153,950 + (–1490)]  24.6  106 = 122,565
30.6  106

[30.6  106] = 1,490 psi
The result must be added to the initial calculated
stresses to establish the minimum required clamping
stress needed for assembling the joint at room
temperature.
101,145 + 1,490 = 102,635 psi
Finally, the method of determining the clamping
force or preload will affect the final stress in the joint at
operating conditions. For example, if a torque wrench is

This value is less than the yield strength for H-11 at
800°F, Table 19. Therefore, a 1/4-28 H-11 bolt stressed
between 102,635 psi and 153,950 psi at room temperature will maintain a clamping load 1500 lb at 800°F after
1000 hr of operation. A cyclic loading of 100 lb, which
results in a bolt loading between 1500 and 1600 lb will
not cause fatigue failure at the operating conditions.

Table 16

PHYSICAL PROPERTIES OF MATERIALS USED TO MANUFACTURE ALLOY STEEL SHCS’S
Coefficient of Thermal Expansion, µm/m/°K1
20°C to
68°F to

100
212

200
392

300
572

400
752

Table 19 - Yield Strength at Various Temperatures
500
932

600
1112

Alloy

–––––––– Temperature (F) ––––––––
70
800
1000
1200

Material
5137M,
51B37M2

–

12.6

13.4

13.9

14.3

14.6

41373

11.2

11.8

12.4

13.0

13.6

–

3

4140

12.3

12.7

–

13.7

–

14.5

43403

–

12.4

–

13.6

–

14.5

3

8735

11.7

12.2

12.8

13.5

–

14.1

87403

11.6

12.2

12.8

13.5

–

14.1

Modulus of Elongation (Young’s Modulus)
E = 30,000,000 PSI/in/in

Stainless Steels
Type 302
Type 403
PH 15-7 Mo

High Strength Iron-Base Alloys
AISI 4340
200,000 130,000 75,000
H-11 (AMS 6485) 215,000 175,000 155,000
AMS 6340
160,000 100,000 75,000
Nickel-Base Alloys
Iconel X
115,000
Waspaloy
115,000

1. Developed from ASM, Metals HDBK, 9th Edition, Vol. 1 (°C = °K for values listed)
3. AISI
4. Multiply values in table by .556 for µin/in/°F.

70

35,000 34,000
110,000 95,000
149,000 101,000

30,000
38,000
–

High Strength Iron-Base Stainless Alloys
A 286
95,000
95,000 90,000 85,000
AMS 5616
113,000
80,000 60,000 40,000
Unitemp 212
150,000 140,000 135,000 130,000

NOTES:
2. ASME SA574

35,000
145,000
220,000

-

–
–
–

98,000
106,000 100,000

CORROSION IN THREADED FASTENERS
All fastened joints are, to some extent, subjected to corrosion of some form during normal service life. Design
of a joint to prevent premature failure due to corrosion
must include considerations of the environment, conditions of loading , and the various methods of protecting
the fastener and joint from corrosion.

tions. For example, stainless steel and aluminum resist
corrosion only so long as their protective oxide film
remains unbroken. Alloy steel is almost never used, even
under mildly corrosive conditions, without some sort of
protective coating. Of course, the presence of a specific
corrosive medium requires a specific corrosion-resistant
fastener material, provided that design factors such as
tensile and fatigue strength can be satisfied.

Three ways to protect against corrosion are:
1. Select corrosion-resistant material for the fastener.

Protective Coating

2. Specify protective coatings for fastener, joint interfaces, or both.

A number of factors influence the choice of a corrosionresistant coating for a threaded fastener. Frequently, the
corrosion resistance of the coating is not a principal
consideration. At times it is a case of economics. Often,
less-costly fastener material will perform satisfactorily in
a corrosive environment if given the proper protective
coating.

3. Design the joint to minimize corrosion.
The solution to a specific corrosion problem may
require using one or all of these methods. Economics
often necessitate a compromise solution.
Fastener Material

Factors which affect coating choice are:

The use of a suitably corrosion-resistant material is
often the first line of defense against corrosion. In fastener
design, however, material choice may be only one of
several important considerations. For example, the most
corrosion-resistant material for a particular environment
may just not make a suitable fastener.


 Corrosion resistance

 Temperature limitations

 Embrittlement of base metal

 Effect on fatigue life

Basic factors affecting the choice of corrosion resistant threaded fasteners are:


 Effect on locking torque


 Tensile and fatigue strength.


 Dimensional changes


 Compatibility with adjacent material


 Position on the galvanic series scale of the fastener and
materials to be joined.


 Thickness and distribution

 Adhesion characteristics


 Special design considerations: Need for minimum
weight or the tendency for some materials to gall.

Conversion Coatings: Where cost is a factor and corrosion is not severe, certain conversion-type coatings are
effective. These include a black-oxide finish for alloy-steel
screws and various phosphate base coatings for carbon
and alloy-steel fasteners. Frequently, a rust-preventing oil
is applied over a conversion coating.


 Susceptibility of the fastener material to other types
of less obvious corrosion. For example, a selected
material may minimize direct attack of a corrosive
environment only to be vulnerable to fretting or stress
corrosion.
Some of the more widely used corrosion-resistant
materials, along with approximate fastener tensile
strength ratings at room temperature and other pertinent
properties, are listed in Table 1. Sometimes the nature of
corrosion properties provided by these fastener materials
is subject to change with application and other condi-

Paint: Because of its thickness, paint is normally not
considered for protective coatings for mating threaded
fasteners. However, it is sometimes applied as a supplemental treatment at installation. In special cases, a fastener may be painted and installed wet, or the entire joint
may be sealed with a coat of paint after installation.

TABLE 1 – TYPICAL PROPERTIES OF CORROSION RESISTANT FASTENER MATERIALS
Materials
Stainless Steels
303, passive
303, passive, cold worked
410, passive
431, passive
17-4 PH
17-7 PH
AM 350
15-7 Mo
A-286
A-286, cold worked

Tensile
Strength
(1000 psi)

Yield Strength
at 0.2% offset
(1000 psi)

Maximum
Service
Temp (F)

Mean Coefficient
of Thermal Expan.
(in./in./deg F)

Density
(lbs/cu in.)

Base Cost
Index

Position
on Galvanic
Scale

80
125
170
180
200
200
200
200
150
220

40
80
110
140
180
185
162
155
85
170

800
800
400
400
600
600
800
600
1200
1200

10.2
10.3
5.6
6.7
6.3
6.7
7.2
–
9.72
–

0.286
0.286
0.278
0.280
0.282
0.276
0.282
0.277
0.286
0.286

Medium
Medium
Low
Medium
Medium
Medium
Medium
Medium
Medium
High

8
9
15
16
11
14
13
12
6
7

71

Electroplating: Two broad classes of protective
electroplating are: 1. The barrier type-such as chrome
plating-which sets up an impervious layer or film that is
more noble and therefore more corrosion resistant than
the base metal. 2. The sacrificial type, zinc for example,
where the metal of the coating is less noble than the
base metal of the fastener. This kind of plating corrodes
sacrificially and protects the fastener.
Noble-metal coatings are generally not suitable for
threaded fasteners-especially where a close-tolerance fit
is involved. To be effective, a noble-metal coating must
be at least 0.001 in. thick. Because of screw-thread geometry, however, such plating thickness will usually exceed
the tolerance allowances on many classes of fit for
screws.

Corrosion resistance can be increased by using a
conversion coating such as black oxide or a phosphatebase treatment. Alternatively, a sacrificial coating such
as zinc plating is effective.
For an inexpensive protective coating, lacquer or
paint can be used where conditions permit.
For Galvanic Corrosion: If the condition is severe,
electrically insulate the bolt and joint from each other..

Because of dimensional necessity, threaded fastener
coatings, since they operate on a different principle, are
effective in layers as thin as 0.0001 to 0.0002 in.

The fastener may be painted with zinc chromate
primer prior to installation, or the entire joint can be
coated with lacquer or paint.

The most widely used sacrificial platings for threaded
fasteners are cadmium, zinc, and tin. Frequently, the
cadmium and zinc are rendered even more corrosion
resistant by a post-plating chromate-type conversion
treatment. Cadmium plating can be used at temperatures
to 450°F. Above this limit, a nickel cadmium or nickel-zinc
alloy plating is recommended. This consists of alternate
deposits of the two metals which are heat-diffused into a
uniform alloy coating that can be used for applications to
900°F. The alloy may also be deposited directly from the
plating bath.

Another protective measure is to use a bolt that is
cathodic to the joint material and close to it in the galvanic
series. When the joint material is anodic, corrosion will
spread over the greater area of the fastened materials.
Conversely, if the bolt is anodic, galvanic action is most
severe.

Fastener materials for use in the 900 to 1200°F
range (stainless steel, A-286), and in the 1200° to 1800°F
range (high-nickel-base super alloys) are highly corrosion
resistant and normally do not require protective coatings,
except under special environment conditions.
Silver plating is frequently used in the higher temperature ranges for lubrication to prevent galling and
seizing, particularly on stainless steel. This plating can
cause a galvanic corrosion problem, however, because
of the high nobility of the silver.
Hydrogen Embrittlement: A serious problem, known
as hydrogen embrittlement, can develop in plated alloy
steel fasteners. Hydrogen generated during plating can
diffuse into the steel and embrittle the bolt. The result is
often a delayed and total mechanical failure, at tensile
levels far below the theoretical strength, high-hardness
structural parts are particularly susceptible to this condition. The problem can be controlled by careful selection
of plating formulation, proper plating procedure, and
sufficient baking to drive off any residual hydrogen.
Another form of hydrogen embrittlement, which is
more difficult to control, may occur after installation.
Since electrolytic cell action liberates hydrogen at the
cathode, it is possible for either galvanic or concentration-cell corrosion to lead to embrittling of the bolt
material.
Joint Design
Certain precautions and design procedures can be
followed to prevent, or at least minimize, each of the
various types of corrosion likely to attack a threaded
joint. The most important of these are:

72

For Direct Attack: Choose the right corrosionresistant material. Usually a material can be found that
will provide the needed corrosion resistance without
sacrifice of other important design requirements. Be
sure that the fastener material is compatible with the
materials being joined.

Steel

Copper

Insulation
washer
Insulation
gasket

Steel

FIG. 1.1 – A method of electrically insulating a bolted
joint to prevent galvanic corrosion.

For Concentration-Cell Corrosion: Keep surfaces
smooth and minimize or eliminate lap joints, crevices,
and seams. Surfaces should be clean and free of organic
material and dirt. Air trapped under a speck of dirt on the
surface of the metal may form an oxygen concentration
cell and start pitting.
For maximum protection, bolts and nuts should have
smooth surfaces, especially in the seating areas. Flushhead bolts should be used where possible. Further,
joints can be sealed with paint or other sealant material.
For Fretting Corrosion: Apply a lubricant (usually oil)
to mating surfaces. Where fretting corrosion is likely to
occur: 1. Specify materials of maximum practicable hardness. 2. Use fasteners that have residual compressive
stresses on the surfaces that may be under attack. 3.
Specify maximum preload in the joint. A higher clamping
force results in a more rigid joint with less relative movement possible between mating services.

CORROSION IN THREADED FASTENERS
For Stress Corrosion: Choose a fastener material that
resists stress corrosion in the service environment.
Reduce fastener hardness (if reduced strength can be
tolerated), since this seems to be a factor in stress
corrosion.
Minimize crevices and stress risers in the bolted joint
and compensate for thermal stresses. Residual stresses
resulting from sudden changes in temperature accelerate
stress corrosion.
If possible, induce residual compressive stresses into
the surface of the fastener by shot-peening or pressure
rolling.
For Corrosion Fatigue: In general, design the joint
for high fatigue life, since the principal effect of this form
of corrosion is reduced fatigue performance. Factors
extending fatigue performance are: 1. Application and
maintenance of a high preload. 2. Proper alignment to
avoid bending stresses.
If the environment is severe, periodic inspection is
recommended so that partial failures may be detected
before the structure is endangered.
As with stress and fretting corrosion, compressive
stresses induced on the fastener surfaces by thread
rolling, fillet rolling, or shot peening will reduce corrosion
fatigue. Further protection is provided by surface coating.

TYPES OF CORROSION
Direct Attack…most common form of corrosion affecting
all metals and structural forms. It is a direct and general
chemical reaction of the metal with a corrosive mediumliquid, gas, or even a solid.
Galvanic Corrosion…occurs with dissimilar metals contact.
Presence of an electrolyte, which may be nothing more
than an individual atmosphere, causes corrosive action
in the galvanic couple. The anodic, or less noble material,
is the sacrificial element. Hence, in a joint of stainless
steel and titanium, the stainless steel corrodes. One of
the worst galvanic joints would consist of magnesium
and titanium in contact.
Concentration Cell Corrosion…takes place with metals in
close proximity and, unlike galvanic corrosion, does not
require dissimilar metals. When two or more areas on
the surface of a metal are exposed to different concentrations of the same solution, a difference in electrical
potential results, and corrosion takes place.

If the solution consists of salts of the metal itself, a metalion cell is formed, and corrosion takes place on the surfaces in close contact. The corrosive solution between
the two surfaces is relatively more stagnant (and thus
has a higher concentration of metal ions in solution) than
the corrosive solution immediately outside the crevice.
A variation of the concentration cell is the oxygen cell in
which a corrosive medium, such as moist air, contains
different amounts of dissolved oxygen at different points.
Accelerated corrosion takes place between hidden
surfaces (either under the bolt head or nut, or between
bolted materials) and is likely to advance without
detection.
Fretting…corrosive attack or deterioration occurring
between containing, highly-loaded metal surfaces subjected to very slight (vibratory) motion. Although the
mechanism is not completely understood, it is probably
a highly accelerated form of oxidation under heat and
stress. In threaded joints, fretting can occur between
mating threads, at the bearing surfaces under the head
of the screw, or under the nut. It is most likely to occur in
high tensile, high-frequency, dynamic-load applications.
There need be no special environment to induce this
form of corrosion...merely the presence of air plus vibratory rubbing. It can even occur when only one of the
materials in contact is metal.
Stress Corrosion Cracking…occurs over a period of time
in high-stressed, high-strength joints. Although not fully
understood, stress corrosion cracking is believed to be
caused by the combined and mutually accelerating
effects of static tensile stress and corrosive environment.
Initial pitting somehow tales place which, in turn, further
increases stress build-up. The effect is cumulative and, in
a highly stressed joint, can result in sudden failure.
Corrosion Fatigue…accelerated fatigue failure occurring
in the presence of a corrosive medium. It differs from
stress corrosion cracking in that dynamic alternating
stress, rather than static tensile stress, is the contributing
agent.
Corrosion fatigue affects the normal endurance limit of
the bolt. The conventional fatigue curve of a normal bolt
joint levels off at its endurance limit, or maximum
dynamic load that can be sustained indefinitely without
fatigue failure. Under conditions of corrosion fatigue,
however, the curve does not level off but continues
downward to a point of failure at a finite number of
stress cycles.

73

GALVANIC CORROSION
Magnesium

M

M

M

M

M

M

M

M

M

M

M

M

M

M

M

M

M

M

Cadmium and Zinc Plate, Galvanized Steel,
Beryllium, Clad Aluminum

B

M

M

B

T

N

T

N

N

N

N

T

N

N

B

N

N

N

Aluminum, 1100, 3003, 5052, 6063,
6061, 356

M

M

B

B

N

B

T

N

N

N

T

N

T

B

N

N

N

M

M

M

M

M

M

M

M

M

M

M

M

M

M

M

M

M

M

M

M

M

M

M

M

M

M

M

M

M

M

M

B

B

B

B

B

N

N

N

N

N

B

N

N

N

B

B

B

B

B

N

T

N

T

B

N

N

N

B

B

B

B

B

B

N

T

B

N

N

N

B

B

B

B

B

B

B

B

B

B

B

B

B

B

B

B

B

B

N

N

N

B

B

B

B

B

B

N

N

N

Commercial yellow Brass and Bronze;
QQ-B-611 Brass

B

B

B

B

B

B

N

N

Copper, Bronze, Brass, Copper Alloys per
QQ-C-551, QQ-B-671, MIL-C-20159;
Silver Solder per QQ-S-561

B

B

B

B

B

B

N

Steel, AISI 301, 302, 303, 304, 316, 321,
347, A 286

B

B

B

B

B

N

Steel, (except corrosion-resistant types)
Aluminum, 2024, 2014, 7075
Lead, Lead-Tin Solder
Tin, Indium, Tin-Lead Solder
Steel, AISI 410, 416, 420
Chromium Plate, Tungsten, Molybdenum

Steel, AISI 431, 440; AM 355; PH Steels
Leaded Brass, Naval Brass, Leaded Bronze

Nickel-Copper Alloys per QQ-N-281,
QQ-N-286, and MIL-N-20184
Nickel, Monel, Cobalt, High-Nickel and
High Cobalt Alloys

KEY:

B

B

B

B

B

B

B

B

B

B

B

B

M

B

M

B

B

B

B

N

N

N
B

Titanium

Silver, High-Silver Alloys
Rhodium, Graphite, Palladium

B

Gold, Platinum, Gold-Platinum Alloys

FIG. 19 – Metals compatibility chart

74

M

LEGEND:
N – Not compatible
B – Compatible
T – Compatible if not exposed within two
miles of salt water
M – Compatible when finished with at
least one coat of primer

B

IMPACT PERFORMANCE
THE IMPACT PERFORMANCE OF
THREADED FASTENERS
Much has been written regarding the significance of the
notched bar impact testing of steels and other metallic
materials. The Charpy and Izod type test relate notch
behavior (brittleness versus ductility) by applying a single
overload of stress. The results of these tests provide
quantitive comparisons but are not convertible to energy
values useful for engineering design calculations. The
results of an individual test are related to that particular
specimen size, notch geometry and testing conditions
and cannot be generalized to other sizes of specimens
and conditions.
The results of these tests are useful in determining
the susceptibility of a material to brittle behavior when
the applied stress is perpendicular to the major stress.
In externally threaded fasteners, however, the loading
usually is applied in a longitudinal direction. The impact
test, therefore, which should be applicable would be one
where the applied impact stress supplements the major
stress. Only in shear loading on fasteners is the major
stress in the transverse direction.
Considerable testing has been conducted in an effort
to determine if a relationship exists between the Charpy
V notch properties of a material and the tension properties of an externally threaded fastener manufactured
from the same material.
Some conclusions which can be drawn from the
extensive impact testing are as follows:

1. The tension impact properties of externally threaded
fasteners do not follow the Charpy V notch impact pattern.
2. Some of the variables which effect the tension impact
properties are:
A. The number of exposed threads
B. The length of the fastener
C. The relationship of the fastener shank diameter to
the thread area.
D. The hardness or fastener ultimate tensile strength
Following are charts showing tension impact versus
Charpy impact properties, the effect of strength and
diameter on tension impact properties and the effect of
test temperature.
Please note from figure 21 that while the Charpy
impact strength of socket head cap screw materials are
decreasing at sub-zero temperatures, the tension impact
strength of the same screws is increasing. This compares
favorable with the effect of cryogenic temperatures on
the tensile strength of the screws. Note the similar
increase in tensile strength shown in figure 22.
It is recommended, therefore, that less importance be
attached to Charpy impact properties of materials which
are intended to be given to impact properties for threaded
fasteners. If any consideration is to be given to impact
properties of bolts or screws, it is advisable to investigate
the tension impact properties of full size fasteners since
this more closely approximates the actual application.

75

TABLE 20
LOW-TEMPERATURE IMPACT PROPERTIES OF SELECTED ALLOY STEELS
heat temperature*

impact energy,
ft.-lb

composition, %

76

quenching tempering
temp.
temp.
Hardness
F+
F
Rc

AISI no.

C

Mn

Ni

Cr

Mo

–300°F

–200°F

4340

0.38

0.77

1.65

0.93

0.21

1550

400
600
800
1000
1200

52
48
44
38
30

11
10
9
15
15

15
14
13
18
28

4360

0.57

0.87

1.62

1.08

0.22

1475

800
1000
1200

48
40
30

5
9
12

4380

0.76

0.91

1.67

1.11

0.21

1450

800
1000
1200

49
42
31

4620

0.20

0.67

1.85

0.30

0.18

1650

300
800
1000
1200

4640

0.43

0.69

1.78

0.29

0.20

1550

4680

0.74

0.77

1.81

0.30

0.21

8620

0.20

0.89

0.60

0.68

8630

0.34

0.77

0.66

8640

0.45

0.78

8660

0.56

0.81

–100°F

transition
temp.
(50%
brittle)
°F

O°F

100°F

20
15
16
28
55

21
15
21
36
55

21
16
25
36
55

–
–
–
–130
–185

6
10
15

10
13
25

11
18
42

14
23
43

–
–10
–110

4
8
5

5
8
11

8
10
19

9
12
33

10
15
38

–
60
–50

42
34
29
19

14
11
16
17

20
16
34
48

28
33
55
103

35
55
78
115

35
55
78
117

–
–
–
–

800
1000
1200

42
37
29

16
17
17

17
22
30

20
35
55

25
39
97

27
69
67

–
–190
–180

1450

800
1000
1200

46
41
31

5
11
11

8
12
13

13
15
17

15
19
39

16
22
43

–
–
–

0.20

1650

300
800
1000
1200

43
36
29
21

11
8
25
10

16
13
33
85

23
20
65
107

35
35
76
115

35
45
76
117

–
–20
–150
–195

0.62

0.22

1575

800
1000
1200

41
34
27

7
11
18

12
20
28

17
43
74

25
53
80

31
54
82

0
–155
–165

0.65

0.61

0.20

1550

800
1000
1200

46
38
30

5
11
18

10
15
22

14
24
49

20
40
63

23
40
66

–
–110
–140

0.70

0.56

0.25

1475

800
1000
1200

47
41
30

4
10
16

6
12
18

10
15
25

13
20
54

16
30
60

–
–10
–90

IMPACT PERFORMANCE

TYPICAL TENSION IMPACT AND CHARPY IMPACT
STANDARD UNBRAKO SOCKET HEAD CAP SCREWS
UNBRAKO ENGINEERING

TENSION ± 3/8" SIZE SCREWS
TESTED FULL SIZE

Chart No.
Date:

180
TENSION IMPACT
FASTENER

TENSION IMPACT LBF.-FT.

160

140

120

100

80

60

40
CHARPY V NOTCH
SPECIMEN
20

±300

±200

±100

0

100

200

TEMPERATURE, F
FIG. 21
77

TYPICAL TENSION IMPACT STRENGTH,
EFFECT OF FASTENER STRENGTH AND DIAMETER
UNBRAKO ENGINEERING

ROOM TEMPERATURE
Chart No.
Date:

180

TENSION IMPACT LBF.-FT.

160

140
3/8
120

100

80
5/16
60

40

20
1/4

120

140

160

180

200

220

FASTENER RATED ULTIMATE TENSILE STRENGTH – KSI
FIG. 22
78

PRODUCT ENGINEERING BULLETIN
UNBRAKO PRODUCT ENGINEERING BULLETIN

Standard Inch Socket Head Cap Screws
Are Not Grade 8 Fasteners
There is a common, yet reasonable, misconception
that standard, inch, alloy steel socket head cap
screws are “Grade 8”. This is not true. The misconception is reasonable because “Grade 8” is a term
generally associated with “high strength” fasteners.
A person desiring a “high strength” SHCS may
request a “Grade 8 SHCS”. This is technically
incorrect for standard SHCSs. The term Grade 8
defines specific fastener characteristics which must

be met to be called “Grade 8”. Three of the most
important characteristics are not consistent with
requirements for industry standard SHCSs: tensile
strength, hardness, and head marking. Some basic
differences between several fastener classifications
are listed below. The list is not comprehensive but
intended to provide a general understanding.
SHCSs can be manufactured to meet Grade 8
requirements on a special order basis.

Fastener
Designation

Grade 2

Grade 5

Grade 8

Industry
SHCS

Unbrako
SHCS

Applicable
Standard

SAE
J429

SAE
J429

SAE
J429

ASTM
A574

ASTM
A574
SPS-B-271

Strength
Level, UTS
KSI, min.

74
(1/4-3/4)
60
(7/8 - 1 1/2)

120
(1/4 - 1)
105
(1 1/8 - 1 1/2)

150
(1/4 - 11/2)

180
(≤1/2)
170
(> 1/2)

190
(≤ 1/2)
180
(> 1/2)

Hardness,
Rockwell

B80-B100
B70-B100

C25-C34
C19-C30

C33-C39

C39-C45
C37-C45

C39-C43
C38-C43

General
Material Type

Low or Medium
Carbon Steel

Medium
Carbon Steel

Medium
Carbon Alloy
Steel

Medium
Carbon Alloy
Steel

Medium
Carbon Alloy
Steel

Identification
Requirement

None

Three Radial
Lines

Six Radial
Lines

SHCS
Configuration

Mfr’s ID

Typical
Fasteners

Bolts Screws
Studs Hex Heads

Bolts Screws
Studs Hex Heads

Bolts Screws
Studs Hex Heads

Socket Head
Cap Screws

Socket Head
Cap Screws

79

THREADS IN BOTH SYSTEMS
Thread forms and designations have been the subject of
many long and arduous battles through the years.
Standardization in the inch series has come through
many channels, but the present unified thread form could
be considered to be the standard for many threaded
products, particularly high strength ones such as socket
head cap screws, etc. In common usage in U.S.A.,
Canada and United Kingdom are the Unified National
Radius Coarse series, designated UNRC, Unified National
Radius Fine series, designated UNRF, and several special
series of various types, designated UNS. This thread,
UNRC or UNRF, is designated by specifying the diameter
and threads per inch along with the suffix indicating the
thread series, such as 1/4 - 28 UNRF. For threads in Metric
units, a similar approach is used, but with some slight
variations. A diameter and pitch are used to designate
the series, as in the Inch system, with modifications as
follows: For coarse threads, only the prefix M and the
diameter are necessary, but for fine threads, the pitch is
shown as a suffix. For example, M16 is a coarse thread
designation representing a diameter of 16 mm with a
pitch of 2 mm understood. A similar fine thread part
would be M16 x 1.5 or 16 mm diameter with a pitch of
1.5 mm.

For someone who has been using the Inch system, there
are a couple of differences that can be a little confusing.
In the Inch series, while we refer to threads per inch as
pitch; actually the number of threads is 1/pitch. Fine
threads are referenced by a larger number than coarse
threads because they “fit” more threads per inch.
In Metric series, the diameters are in millimeters, but the
pitch is really the pitch. Consequently the coarse thread
has the large number. The most common metric thread
is the coarse thread and falls generally between the inch
coarse and fine series for a comparable diameter.
Also to be considered in defining threads is the tolerance
and class of fit to which they are made. The International
Standards Organization (ISO) metric system provides
for this designation by adding letters and numbers in a
certain sequence to the callout. For instance, a thread
designated as M5 x 0.8 4g6g would define a thread of
5 mm diameter, 0.8 mm pitch, with a pitch diameter
tolerance grade 6 and allowance “g”. These tolerances
and fields are defined as shown below, similar to the
Federal Standard H28 handbook, which defines all of the
dimensions and tolerances for a thread in the inch series.
The callout above is similar to a designation class 3A fit,
and has a like connotation.

COMPLETE DESIGNATIONS
Metric Thread Designation
Nominal Size
Pitch
Tolerance Class Designation
M5 X 0.8 – 4g6g

80

Tolerance Position
(Allowance)
Tolerance Grade

)
)
)

Crest Diameter Tolerance Symbol

Tolerance Position
(Allowance)
Tolerance Grade

)
)
)

Pitch Diameter Tolerance Symbol

METRIC THREADS
Example of thread tolerance positions and magnitudes.
Comparision 5/16 UNC and M8. Medium tolerance grades – Pitch diameter.

µm
+200

DEVIATIONS

NUT THREAD
5/16 UNC

M8

+150
+100
+50

2B

external

internal

basic clearance

h
g
e

H
G

none
small
large

6H

0
Allowance

Allowance = 0

–50
2A

6g

6h

–100
–160
–200
µm

5/16 UNC

Plain

After
plating

BOLT THREAD

NOTE:
Lower case letters = external threads
Capital letters = internal threads

81

THROUGH-HOLE PREPARATION
DRILL AND COUNTERBORE SIZES FOR INCH SOCKET HEAD CAP SCREWS
Note 1

Note 3

Close Fit: Normally limited to holes for those lengths of
screws threaded to the head in assemblies in which: (1)
only one screw is used; or (2) two or more screws are
used and the mating holes are produced at assembly or
by matched and coordinated tooling.

Chamfering: It is considered good practice to chamfer or
break the edges of holes that are smaller than “F” maximum in parts in which hardness approaches, equals or
exceeds the screw hardness. If holes are not chamfered,
the heads may not seat properly or the sharp edges may
deform the fillets on the screws, making them susceptible to fatigue in applications that involve dynamic loading. The chamfers, however, should not be larger than
needed to ensure that the heads seat properly or that the
fillet on the screw is not deformed. Normally, the chamfers do not need to exceed “F” maximum. Chamfers
exceeding these values reduce the effective bearing area
and introduce the possibility of indentation when the
parts fastened are softer than screws, or the possiblity of
brinnelling of the heads of the screws when the parts are
harder than the screws. (See “F” page 6).

Note 2
Normal Fit: Intended for: (1) screws of relatively long
length; or (2) assemblies that involve two or more screws
and where the mating holes are produced by conventional tolerancing methods. It provides for the maximum
allowable eccentricty of the longest standard screws and
for certain deviations in the parts being fastened, such as
deviations in hole straightness; angularity between the
axis of the tapped hole and that of the hole for the shank;
differneces in center distances of the mating holes and
other deviations.

A

X

C
countersink
diameter D
Max. + 2F(Max.)

UNRC

drill size for hole A
nominal
size

basic
screw
diameter

nom.

dec.

nom.

dec.

counterbore
diameter

0
1
2

0.0600
0.0730
0.0860

51*
46*
3/32

0.0670
0.0810
0.0937

49*
43*
36*

0.0730
0.0890
0.1065

1/8
5/32
3/16

0.074
0.087
0.102

3
4
5

0.0990
0.1120
0.1250

36*
1/8
9/64

0.1065
0.1250
0.1406

31*
29*
23*

0.1200
0.1360
0.1540

7/32
7/32
1/4

6
8
10

0.1380
0.1640
0.1900

23*
15*
5*

0.1540
0.1800
0.2055

18*
10
2*

0.1695
0.1935
0.2210

1/4
5/16
3/8

0.2500
0.3125
0.0375

17/64
21/64
25/64

0.2656
0.3281
0.3906

9/23
11/32
13/32

7/16
1/2
5/8

0.4375
0.5000
0.6250

29/64
33/64
41/64

0.4531
0.5156
0.6406

3/4
7/8
1

0.7500
0.8750
1.0000

49/64
57/64
1-1/64

1-1/4
1-1/2

1.2500
1.5000

1-9/32
1-17/32

tap drill size
UNRF

**body
drill
size

counterbore
size

–
1.5mm
#50

3/64
#53
#50

#51
#46
3/32

1/8
5/32
3/16

0.115
0.130
0.145

#47
#43
#38

#45
#42
#38

#36
1/8
9/64

7/32
7/32
1/4

9/32
5/16
3/8

0.158
0.188
0.218

#36
#29
#25

#33
#29
#21

#23
#15
#5

9/32
5/16
3/8

0.2812
0.3437
0.4062

7/16
17/32
5/8

0.278
0.346
0.415

#7
F
5/16

#3
I
Q

17/64
21/64
25/64

7/16
17/32
5/8

15/32
17/32
21/32

0.4687
0.5312
0.6562

23/32
13/16
1

0.483
0.552
0.689

U
27/64
35/64

25/64
29/64
14.5mm

29/64
33/64
41/64

23/32
13/16
1

0.7656
0.8906
1.0156

25/32
29/32
1-1/32

0.7812
0.9062
1.0312

1-3/16
1-3/8
1-5/8

0.828
0.963
1.100

21/32
49/64
7/8

11/16
20.5mm
59/64

49/64
57/64
1-1/64

1-3/16
1-3/8
1-5/8

1.2812
1.5312

1-5/16
1-9/16

1.3125
1.5625

2
2-3/8

1.370
1.640

1-7/64
34mm

1-11/64
36mm

1-9/32
1-17/32

2
2-3/8

close fit

normal fit

** Break edge of body drill hole to clear screw fillet.
82

hole dimensions

DRILL AND COUNTERBORE SIZES
DRILL AND COUNTERBORE SIZES FOR METRIC SOCKET HEAD CAP SCREWS
Note 1

Note 3

Close Fit: Normally limited to holes for those lengths of
screws threaded to the head in assemblies in which: (1)
only one screw is used; or (2) two or more screws are
used and the mating holes are produced at assembly or
by matched and coordinated tooling.

Chamfering: It is considered good practice to chamfer or
break the edges of holes that are smaller than “B” maximum in parts in which hardness approaches, equals or
exceeds the screw hardness. If holes are not chamfered,
the heads may not seat properly or the sharp edges may
deform the fillets on the screws, making them susceptible to fatigue in applications that involve dynamic loading. The chamfers, however, should not be larger than
needed to ensure that the heads seat properly or that the
fillet on the screw is not deformed. Normally, the chamfers do not need to exceed “B” maximum. Chamfers
exceeding these values reduce the effective bearing area
and introduce the possibility of indentation when the
parts fastened are softer than screws, or the possiblity of
brinnelling of the heads of the screws when the parts are
harder than the screws.

Note 2
Normal Fit: Intended for: (1) screws of relatively long
length; or (2) assemblies that involve two or more screws
and where the mating holes are produced by conventional tolerancing methods. It provides for the maximum
allowable eccentricty of the longest standard screws and
for certain deviations in the parts being fastened, such as
deviations in hole straightness; angularity between the
axis of the tapped hole and that of the hole for the shank;
differneces in center distances of the mating holes and
other deviations.

A

X

Y

B

Counterbore
Diameter

Countersink
Diameter
[Note (3)]

Transition
Diameter, Max.

Nominal Drill Size
Nominal Size
or Basic
Screw Diameter

Close Fit
[Note (1)]

Normal Fit
[Note (2)]

M1.6
M2
M2.5
M3
M4

1.80
2.20
2.70
3.40
4.40

1.95
2.40
3.00
3.70
4.80

3.50
4.40
5.40
6.50
8.25

2.0
2.6
3.1
3.6
4.7

2.0
2.6
3.1
3.6
4.7

M5
M6
M8
M10
M12

5.40
6.40
8.40
10.50
12.50

5.80
6.80
8.80
10.80
12.80

9.75
11.25
14.25
17.25
19.25

5.7
6.8
9.2
11.2
14.2

5.7
6.8
9.2
11.2
14.2

M14
M16
M20
M24

14.50
16.50
20.50
24.50

14.75
16.75
20.75
24.75

22.25
25.50
31.50
37.50

16.2
18.2
22.4
26.4

16.2
18.2
22.4
26.4

M30
M36
M42
M48

30.75
37.00
43.00
49.00

31.75
37.50
44.0
50.00

47.50
56.50
66.00
75.00

33.4
39.4
45.6
52.6

33.4
39.4
45.6
52.6

83

HARDNESS – TENSILE CONVERSION
INCH
ROCKWELL – BRINELL – TENSILE CONVERSION
Rockwell
“C”
scale

Brinell
hardness
number

tensile
strength
approx.
1000 psi

Rockwell
“C”
scale

Brinell
hardness
number

tensile
strength
approx.
1000 psi

Rockwell
Brinell
hardness
number

tensile
strength
approx.
1000 psi

60
59
58

654
634
615

336
328
319

43
42
41

408
398
387

200
194
188

26
25
24

259
253
247

123
120
118

57
56
55

595
577
560

310
301
292

40
39
38

377
367
357

181
176
170

23
22
21

241
235
230

115
112
110

54
53
52

543
524
512

283
274
265

37
36
35

347
337
327

165
160
155

20
(19)
(18)

225
220
215

107
104
103

51
50
49

500
488
476

257
249
241

34
33
32

318
309
301

150
147
142

(17)
(16)
(15)

96

210
206
201

102
100
99

48
47
46

464
453
442

233
225
219

31
30
29

294
285
279

139
136
132

(14)
(13)
(12)

95
94
93

197
193
190

97
96
93

45
44

430
419

212
206

28
27

272
265

129
126

(11)
(10)

92

186
183

91
90

Brinell
hardness
number

tensile
strength
approx.
MPa

259
253
247

848
827
814

241
235
230

793
772
758

225
220
215

738
717
710

“C”
“B”
scale scale

100
99
98
97

METRIC
ROCKWELL – BRINELL – TENSILE CONVERSION

84

Rockwell
“C”
scale

Brinell
hardness
number

tensile
strength
approx.
MPa

Rockwell
“C”
scale

Brinell
hardness
number

tensile
strength
approx.
MPa

Rockwell

60
59
58

654
634
615

2,317
2,261
2,199

43
42
41

408
398
387

1,379
1,338
1,296

26
25
24

57
56
55

595
577
560

2,137
2,075
2,013

40
39
38

377
367
357

1,248
1,213
1,172

23
22
21

54
53
52

543
524
512

1,951
1,889
1,827

37
36
35

347
337
327

1,138
1,103
1,069

20
(19)
(18)

51
50
49

500
488
476

1,772
1,717
1,662

34
33
32

318
309
301

1,034
1,014
979

(17)
(16)
(15)

96

210
206
201

703
690
683

48
47
46

464
453
442

1,606
1,551
1,510

31
30
29

294
285
279

958
938
910

(14)
(13)
(12)

95
94
93

197
193
190

669
662
641

45
44

430
419

1,462
1,420

28
27

272
265

889
869

(11)
(10)

92

186
183

627
621

“C”
“B”
scale scale

100
99
98
97

THREAD STRESS AREAS
Inch and Metric
STRESS AREAS FOR THREADED FASTENERS – INCH
Square Inches

Threads Per in.

Tensile Stress Area Per H-28
Diameter (in.)

Diameter (mm)

UNRC

UNRF

UNRC

UNRF

Nominal Shank

#0
#1
#2

0.06
0.07
0.09

1.52
1.85
2.18

–
64
56

80
72
64

–
0.00263
0.00370

0.00180
0.00278
0.00394

0.002827
0.004185
0.005809

#3
#4
#5

0.10
0.11
0.13

2.51
2.84
3.18

48
40
40

56
48
44

0.00487
0.00604
0.00796

0.00523
0.00661
0.00830

0.007698
0.009852
0.012272

#6
#8
#10

0.14
0.16
0.19

3.51
4.17
4.83

32
32
24

40
36
32

0.00909
0.0140
0.0175

0.01015
0.01474
0.0200

0.014957
0.021124
0.028353

1/4
5/16
3/8

0.25
0.31
0.38

6.35
7.94
9.53

20
18
16

28
24
24

0.0318
0.0524
0.0775

0.0364
0.0580
0.0878

0.049087
0.076699
0.11045

7/16
1/2
9/16

0.44
0.50
0.56

11.11
12.70
14.29

14
13
12

20
20
18

0.1063
0.1419
0.182

0.1187
0.1599
0.203

0.15033
0.19635
0.25

5/8
3/4
7/8

0.63
0.75
0.88

15.88
19.05
22.23

11
10
9

18
16
14

0.226
0.334
0.462

0.256
0.373
0.509

0.31
0.44179
0.60132

1
1-1/8
1-1/4

1.00
1.13
1.25

25.40
28.58
31.75

8
7
7

12
12
12

0.606
0.763
0.969

0.663
0.856
1.073

0.79
0.99402
1.2272

1-3/8
1-1/2
1-3/4

1.38
1.50
1.75

34.93
38.10
44.45

6
6
5

12
12
12

1.155
1.405
1.90

1.315
1.581
2.19

1.4849
1.7671
2.4053

2
2-1/4
2-1/2

2.00
2.25
2.50

50.80
57.15
63.50

4-1/2
4-1/2
4

12
12
12

2.50
3.25
4.00

2.89
3.69
4.60

3.1416
3.9761
4.9088

2-3/4
3

2.75
3.00

69.85
76.20

4
4

12
12

4.93
5.97

5.59
6.69

5.9396
7.0686

STRESS AREAS FOR THREADED FASTENERS – METRIC
Nominal Dia. Thread
and Pitch
(mm)

Thread Tensile
Stress Area
(mm2)

Nominal
Shank Area
(mm2)

Nominal Dia. Thread
and Pitch
(mm)

Thread Tensile
Stress Area
(mm2)

Nominal
Shank Area
(mm2)

1.6 x 0.35
2.0 x 0.4
2.5 x 0.45

1.27
2.07
3.39

2.01
3.14
4.91

18 x 2.5
20 x 2.5
22 x 2.5

192
245
303

254
314
380

3.0 x 0.5
4.0 x 0.7
5.0 x 0.8

5.03
8.78
14.2

7.07
12.6
19.6

24 x 3
27 x 3
30 x 3.5

353
459
561

452
573
707

6.0 x 1
8.0 x 1.25
10 x 1.5

20.1
36.6
58.00

28.3
50.3
78.5

33 x 3.5
36 x 4
42 x 4.5
48 x 5

694
817
1120
1470

855
1018
1385
1810

12 x 1.75
14 x 2
16 x 2

84.3
115
157

113
154
201

85

ENGINEERING PART NUMBERS – INCH
Unbrako provides a stock number for every standard, stocked item in its price list. However, there may be particular
sizes or optional features the user may desire. The following part numbering system allows the engineer or designer to
record a particular description for ordering.

Alloy
Steel

Drilled
Head (3)

#4

UNRC

1 1/2"

Cadmium
Plate

20097

H3

-94

C

-24

C

FINISH
B – Chemical Black Oxide
C – Cadmium Plate – Silver
D – Cadmium Plate – Yellow

S – Silver Plate
U – Zinc Plate – Silver
Z – Zinc Plate – Yellow

No letter indicates standard black finish (Thermal Oxide)
for alloy steel and passivation for stainless steel.
LENGTH in 16ths
THREAD TYPE C – coarse, F – fine
DIAMETER*
DIA.

#0

#1

#2

#3

#4

#5

#6

#8

#10

1/4

5/16

3/8

7/16

1/2

9/16

DASH NO.

90

91

92

93

94

95

96

98

3

4

5

6

7

8

9

DIA.

5/8

3/4

7/8

1

DASH NO.

10

12

14

16

1 1/8 1 1/4 1 3/8 1 1/2 1 3/4
18

20

22

OPTIONAL FEATURES
Cross Drilled Heads:
H1 – 1 Hole Thru
H2 – 2 Hole2 Thru
H3 – 3 Holes Thru

24

28

2
32

2 1/4 2 1/2 2 3/4
36

40

44

Self-Locking:
E – LOC-WEL to MIL-DTL18240
L – LOC-WEL (Commercial)
P – Nylon Plug
TF – TRU-FLEX
K – Nylon Plug to MIL-DTL18240

BASE NUMBER
20097 – socket head cap screw – alloy steel
20098 – socket head cap screw – stainless steel
72531 – low head cap screw
12705 – shoulder screw
16990 – flat head cap screw – alloy steel
16991 – flat head cap screw – stainless steel
38030 – button head cap screw – alloy
38031 – button head cap screw – stainless steel
05455 – square head cap screw – knurled cup
05456 – square head cap screw – half dog
Set Screws
Alloy
Steel
28700
28701
28704
28702
28705
28706

* Shoulder screws are designated by shoulder diameter

86

Stainless
Steel
28707
28708
28709
28710
28711
28713

3
48

flat point
cup point
knurled cup point
cone point
oval point
half dog point

OPTIONAL PART NUMBERING SYSTEM

PRESSURE PLUG PART NUMBERS
Basic Part No. Material
29466

A

1/4"

FINISH

Finish
-4

C

B – Chemical Black Oxide
C – Cadmium Plate-Silver
D – Cadmium Plate-Yellow
S – Silver Plate
U – Zinc Plate – Silver
Z – Zinc Plate – Yellow

NOMINAL SIZE IN 16ths
OPTIONAL FEATURES
BASIC PART NUMBER
** Standard stock available in austenitic stainless steel, brass, and alloy only
** Standard stock available in austenitic stainless steel, and alloy only

DOWEL PINS PART NUMBERS
dowel pin

1/4"

.001 oversize

1/2"

28420

–250

B

–8

A – Austenitic Stainless
D – Aluminum
E – Brass
No letter – alloy steel

29466 – dry seal
*38194 – LEVEL-SEAL
**69188 – PTFE/TEFLON coated

The Part number consists of (1) a basic part number describing
the item; (2) a dash number and letter designating diameter and
oversize dimension; (3) a dash number designating length.
LENGTH in 16ths
OVERSIZE A-.0002, B-.001, C-.002 (see below)
DIAMETER in thousandths
BASIC PART NUMBER

HEX KEYS PART NUMBERS
long arm

1/4"

05854

–13

28420 – Standard Dowel Pins
69382 – Pull-Out Dowel Pins

The Part number consists of (1) a basic part number describing
the item; (2) a dash number designating size and a letter denoting
finish.
FINISH Standard Black Finish (Thermal Oxide)

See dash number in dimension table page 32
BASIC PART NUMBER

05853 – short arm wrench
05854 – long arm wrench
78950-6" – long arm wrench

87

ENGINEERING PART NUMBERS – METRIC

Alloy
Steel

Drilled
Head (3)

4MM
Dia.

Thread
Pitch

Length

Cadmium
Plate

76000

H3

-M4

-0.7

-12

C

FINISH
B – Chemical Black Oxide
C – Cadmium Plate – Silver
D – Cadmium Plate – Yellow

S – Silver Plate
U – Zinc Plate – Silver
Z – Zinc Plate – Yellow

No letter indicates standard black finish (Thermal Oxide)
for alloy steel and passivation for stainless steel.
LENGTH in mm
THREAD TYPE STATE THREAD PITCH
DIAMETER in mm*
OPTIONAL FEATURES
Cross Drilled Heads:
H1 – 1 Hole Thru
H2 – 2 Hole2 Thru
H3 – 3 Holes Thru

Self-Locking:
E – LOC-WEL to MIL-DTL-18240
L – LOC-WEL (Commercial)
P – Nylon Plug
TF – TRU-FLEX
K – Nylon Plug to MIL-DTL-18240

BASE NUMBER
76000 – metric socket head cap screw – alloy steel
76001 – metric socket head cap screw – stainless steel
76002 – metric low head cap screw – alloy
76032 – metric low head cap screw – stainless steel
76005 – metric flat head cap screw – alloy steel
76006 – metric flat head cap screw – stainless steel
76003 – metric button head cap screw – alloy
76004 – metric button head cap screw – stainless steel
76007 – metric shoulder screw – alloy
Metric Set Screws
Alloy
Stainless
Steel
Steel
76010
76016
76011
76017
76012
76018
76013
76019
76014
76020
76015
76021

* Shoulder screws are designated by shoulder diameter

88

flat point
cup point
knurled cup point
cone point
oval point
half dog point

METRIC

HEX KEYS PART NUMBERS (METRIC)
long arm

5mm

76023

5

The Part number consists of (1) a basic part number describing
the item; (2) a dash number designating size.

FINISH Standard Black Finish (Thermal Oxide)

Key size in mm
BASIC PART NUMBER

DOWEL PINS PART NUMBERS (METRIC)
dowel pin

6mm

.0275 oversize

8mm

76024

–6

B

–8

76022 – short arm wrench
76023 – long arm wrench

The Part number consists of (1) a basic part number describing
the item; (2) a dash number and letter designating diameter and
oversize dimension; (3) a dash number designating length.
LENGTH in mm
OVERSIZE A-.0055, B-.0275mm
DIAMETER in mm
BASIC PART NUMBER

76024 – Standard Dowel Pins
76035 – Pull-Out Dowel Pins

89

Advantages built into every detail.
THE UNBRAKO DIFFERENCE
Your application demands a
fastener which outperforms
all others. We build our
products for life, to help
you build your products
for life.
What’s holding your
product together?

HIGHER MIN ULT TENSILE
10,000 PSI stronger than
industry standard
COMPOUND FILLET RADIUS
Doubles fatigue life at critical
head-shank juncture
WIDE RADIUS THREADS
Maximizes fatigue resistance
where it’s needed most
3R (RADIUSED ROOT RUNOUT)
THREAD
Increases fatigue life up to 300%
E CODE “LOT CODE” MARKINGS
The ultimate in fastener
traceability

CALL FOR A
SAMPLE
AND EXPERIENCE
THE UNBRAKO
DIFFERENCE FOR
YOURSELF.
SPS Technologies Limited
Unbrako Division
Grovelands Industrial Estate
Exhall, Coventry CV7 9ND, England
Phone: 44-247-658-5050 Fax: 44-247-658-5055
FORM 5519 REV. D-15M-08-04

BUILT FOR LIFE.
SPS International Ltd.
Shannon Industrial Estate
Shannon, County Clare, Ireland
Phone: 353-61-716-500 Fax: 353-61-716-584
Email: unbrako.europe@spstech.com

Unbrako Pty. Limited
Norcal Road, Nunawading
Victoria 3131 Australia
Phone: 61-3-9894-0026 Fax: 61-3-9894-0038
Email: info@spstech.com.au

Unbrako North America
SPS Technologies
4444 Lee Road, Cleveland, Ohio 44128-2902
Phone: 216-581-3000 Fax: 800-225-5777
Email: unbrakosales@spstech.com
Printed in USA



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