The document discusses bolt thread shear stripping and provides guidelines for determining minimum thread engagement to prevent stripping. It defines the loading mechanism in threads and provides design rules for thread engagement. Equations are given for calculating the shear area of internal and external threads based on tolerance class. Tables show thread tensile/shear areas and bolt/material shear strength ratios. An example calculation demonstrates how to determine the minimum thread engagement required to prevent stripping for a given bolt-material combination. Minimum engagements are provided for various sizes, materials and property classes.

1. 1. BOLT THREAD SHEAR STRIPPING
It is good practice for the thread engagement to be sufficient so that if the bolt is overloaded, a bolt would break
rather than multiple threads stripping simultaneously, since it is a more visible failure to identify in a maintenance
programme.
For information, the loading mechanism for external in internal threads is: the first thread experiences the highest
load, plastically deforms and sheds load to the next thread, which plastically deforms to shed load to the next
thread and so forth.
A rough design rule states that a full thread engagement in steel should be a minimum 1.5 times the bolt outside
diameter for coarse-pitch threads; however this rule does not reduce the importance of performing a calculation.
Figure 1 – Bolt Thread Stripping Example
If the internal thread is stronger than external, the thread failure will occur by stripping the external thread. The
external thread will shear at the diameter defined by the minor diameter of the internal thread). The unit shear area
of the external thread at this diameter is the following:
AS =
π D
P
P
2
+ 0.57735 d − D
Where:
ASs External Thread Shear Area (mm
2
per mm of thread engagement)
D1 max Maximum Minor Diameter of Internal Thread (mm)
d2 min Minimum Pitch Diameter of External Thread (mm)
P Thread Pitch (mm)
If the external thread is stronger than internal, the thread failure will occur by stripping the internal thread. The
internal thread will shear at the diameter defined by the major diameter of the external thread. The unit shear area
of the internal thread at this diameter is the following:
AS =
π d
P
P
2
+ 0.57735 d − D
Where:
ASn Internal Thread Shear Area (mm
2
per mm of thread engagement)
D2 max Maximum Pitch Diameter of Internal Thread (mm)
d min Minimum Major Diameter of External Thread (mm)
P Thread Pitch (mm)

2. 1.1 Tolerance Class 6H/6g
The internal and external thread shear areas for 6H/6g tolerance class are the following:
Nominal
Diameter
Thread
Pitch
Tensile
Stress
Area
Tensile
Thread
Root
Area
External Thread Internal Thread
Tolerance Class: 6g Tolerance Class: 6H
Major
Diameter
Pitch
Diameter
Thread
Shear Area
Pitch
Diameter
Minor
Diameter
Thread
Shear Area
d P As A3 d min d2 min Ass D2 max D1 max Asn
mm mm mm
2
mm
2
mm mm
mm
2
per
mm
mm mm
mm
2
per
mm
1.6 0.35 1.27 1.08 1.496 1.291 1.9 1.458 1.321 2.6
2 0.4 2.07 1.79 1.886 1.654 2.4 1.830 1.679 3.4
2.5 0.45 3.39 2.98 2.380 2.117 3.2 2.303 2.138 4.5
3 0.5 5.03 4.47 2.874 2.580 3.9 2.775 2.599 5.5
4 0.7 8.78 7.75 3.838 3.433 5.5 3.663 3.422 7.8
5 0.8 14.2 12.7 4.826 4.361 7.1 4.605 4.334 10.0
6 1 20.1 17.9 5.794 5.212 8.6 5.500 5.153 12.2
8 1.25 36.6 32.8 7.760 7.042 12.2 7.348 6.912 16.8
10 1.5 58.0 52.3 9.732 8.862 15.6 9.206 8.676 21.5
12 1.75 84.3 76.2 11.701 10.679 19.0 11.063 10.441 26.1
14 2 115.4 104.7 13.682 12.503 22.4 12.913 12.210 31.0
16 2 157 144 15.682 14.503 26.1 14.913 14.210 35.6
18 2.5 192 175 17.623 16.164 29.5 16.600 15.744 40.8
20 2.5 245 225 19.623 18.164 33.3 18.600 17.744 45.4
22 2.5 303 282 21.623 20.164 37.0 20.600 19.744 50.0
24 3 353 324 23.577 21.803 40.5 22.316 21.252 55.0
27 3 459 427 26.577 24.803 46.2 25.316 24.252 62.0
30 3.5 561 519 29.522 27.462 51.6 28.007 26.771 69.6
33 3.5 694 647 32.522 30.462 57.4 31.007 29.771 76.6
36 4 817 759 35.465 33.118 63.1 33.702 32.270 84.1
39 4 976 913 38.465 36.118 69.0 36.702 35.270 91.2
42 4.5 1121 1045 41.437 38.778 74.3 39.392 37.799 99.2
45 4.5 1306 1224 44.437 41.778 80.2 42.392 40.799 106.4
48 5 1473 1377 47.399 44.431 85.8 45.087 43.297 114.2
52 5 1758 1652 51.399 48.431 93.8 49.087 47.297 123.8
56 5.5 2030 1905 55.365 52.088 101.4 52.783 50.796 134.1
60 5.5 2362 2227 59.365 56.088 109.4 56.783 54.796 143.8
64 6 2676 2520 63.320 59.743 116.9 60.478 58.305 153.9
Notes:
• The thread shear area is represented as mm
2
per mm of thread engagement.
Table 1 – Thread Tensile and Shear Stripping Areas – Coarse Series – 6H/6g Tolerance Class

3. 1.2 Tolerance Class 6AZ/6g
The internal and external thread shear areas for 6AZ/6g tolerance class (used with hot-dip galvanized or
sherardized coating) are the following:
Nominal
Diameter
Thread
Pitch
Tensile
Stress
Area
Tensile
Thread
Root
Area
External Thread Internal Thread
Tolerance Class: 6g Tolerance Class: 6AZ
Major
Diameter
Pitch
Diameter
Thread
Shear Area
Pitch
Diameter
Minor
Diameter
Thread
Shear Area
d P As A3 d min d2 min Ass D2 max D1 max Asn
mm mm mm
2
mm
2
mm mm
mm
2
per
mm
mm mm
mm
2
per
mm
10 1.5 58.0 52.3 9.732 8.862 12.6 9.536 9.006 17.6
12 1.75 84.3 76.2 11.701 10.679 15.8 11.398 10.776 22.1
14 2 115.4 104.7 13,682 12.503 19.2 13.253 12.550 26.8
16 2 157 144 15.682 14.503 22.2 15.253 14.550 30.7
18 2.5 192 175 17.623 16.164 26.1 16.950 16.094 36.3
20 2.5 245 225 19.623 18.164 29.3 18.950 18.094 40.4
22 2.5 303 282 21.623 20.164 32.6 20.950 20.094 44.5
24 3 353 324 23.577 21.803 36.4 22.676 21.612 49.9
27 3 459 427 26.577 24.803 41.5 25.676 24.612 56.2
30 3.5 561 519 29.522 27.462 47.1 28.377 27.141 63.9
33 3.5 694 647 32.522 30.462 52.4 31.377 30.141 70.4
36 4 817 759 35.465 33.118 58.2 34.082 32.650 77.9
39 4 976 913 38.465 36.118 63.6 37.082 35.650 84.5
42 4.5 1121 1045 41.437 38.778 69.1 39.782 38.189 92.7
45 4.5 1306 1224 44.437 41.778 74.5 42.782 41.189 99.4
48 5 1473 1377 47.399 44.431 80.3 45.487 43.697 107.3
52 5 1758 1652 51.399 48.431 87.6 49.487 47.697 116.4
56 5.5 2030 1905 55.365 52.088 95.3 53.193 51.206 126.6
60 5.5 2362 2227 59.365 56.088 102.8 57.193 55.206 135.8
64 6 2676 2520 63.320 59.743 110.3 60.898 58.725 145.8
Notes:
• The thread shear area is represented as mm
2
per mm of thread engagement.
Table 2 – Thread Tensile and Shear Stripping Areas – Coarse Series – 6AZ/6g Tolerance Class

4. 1.3 Bolt Shear Strength
The shear strength of bolts is not normally listed in standards since the majority of bolts are used as clamps and
not as shear pins, the following table has been directly taken from VDI 2230 and shows typical values for the ratio
of the shear to ultimate tensile strength of the bolt.
If the bolt material is not listed below, it might help to know that most of the common fastener materials with
hardness up to 40 HRC have shear strengths in the region of 60% of the ultimate tensile strength. The reference
values for the shear strength to ultimate tensile strength ratio of the bolts are given below.
Property Class Shear Strength Ratio
8.8 0.65
10.9 0.62
12.9 0.60
A4-70 0.72
A4-80 0.68
Table 3 – Bolt Shear Strength Ratios (VDI 2230)
1.4 Material Shear Strength
The shear strength of materials is not normally specified in standards. The reference values for ratios of shear
strength to ultimate tensile strength of different types of material are given below.
Material Type Shear Strength Ratio
Structural Steel 0.80
Heat-Treatable Steel 0.65
Case-Hardening Steel 0.85
Cast Iron 0.90
Stainless Steel (Austenite) 0.80
Table 4 – Material Shear Strength Ratios (VDI 2230)

5. 2. WORKED EXAMPLE – THREAD STRIPPING
The main design rule is to make sure that the length of thread engagement between external and internal threads
is long enough and capable of developing a full strength of the bolt stress diameter. In other words, we want the
bolt to fail before the threads strip simply because a broken bolt is more an obvious failure and easier to detect
than a stripped thread.
Figure 1 – Minimum Thread Engagement
The following example calculates the minimum thread engagement required and is the following:
Nominal Diameter 20 mm
Thread Pitch 2.5 mm
Property Class 10.9
Tensile Stress Area 245 mm
2
Ultimate Tensile Strength 1040 MPa
Shear Strength Ratio 0.62
Table 5 – Bolt Specification
Parent Material S355
Ultimate Tensile Strength 470 MPa
Shear Strength Ratio 0.80
Table 6 – Parent Material Specification
Tolerance Class
External Thread 6g
Internal Thread 6H
Thread Stripping Area
External Thread 33.3 mm
2
per mm of thread engagement
Internal Thread 45.4 mm
2
per mm of thread engagement
Table 7 – External and Internal Thread Specification

6. Step 1
The minimum bolt shear strength and nominal parent material shear strength is the following:
τ = 0.62 ∗ σ". = 0.62 ∗ 1040 = %&' ()*
τ+,-./ = 0.80 ∗ σ".+,-./ = 0.80 ∗ 470 = 12% ()*
Step 2
The minimum force to break the bolt is the following:
F". = σ". ∗ A 4-.,5 = 1040 ∗ 245 = 6'' 78
Step 3
The unit force to strip the external and internal engaged threads per mm of thread engagement is the following:
F9 -:+. = AS ∗ τ = 33.3 ∗ 645 = 6; 78/==
F9 -:+.+,-./ = AS+,-./ ∗ τ+,-./ = 45.4 ∗ 376 = ;2 78/==
Step 4
The minimum effective thread engagement is the following:
L.??.@ :A.. 4-.,5../B,B.C./ =
F".
MIN F9 -:+. , F9 -:+.+,-./
=
255
17
= ;' ==
Step 5
An additional 2 times thread pitch shall be added to take into account for the chamfer on the bolt thread. An
additional 50% shall also be added for additional safety as UTSmax of the bolt is approximately 30% larger than
UTSmin specified in ISO 898-1: 2013.
The minimum thread engagement is the following:
LC:/. 4-.,5../B,B.C./ = L.??.@ :A.. 4-.,5../B,B.C./ + 0.50 ∗ L.??.@ :A.. 4-.,5../B,B.C./ + 2 PITCH = 62 ==
Conclusion
The minimum length of thread engagement for M20 bolt is equal to 27mm (or approximately 1.4 times bolt
diameter.

7. 3. MINIMUM THREAD ENGAGEMENT
The minimum thread engagements based on external and internal threads shear strength are the following:
Size Pitch
Property
Class
Minimum Thread Engagement
S355 S460 S690 304 / 316L 431S29
17/4 PH
(H1150D)
mm mm - mm mm mm mm mm mm
M10 1.5
8.8 12 12 12 12 12 12
10.9 14 13 12 14 12 12
12.9 16 14 12 15 12 12
A4-70 11 11 11 11 11 11
A4-80 12 11 11 11 11 11
M12 1.75
8.8 14 14 14 14 14 14
10.9 17 15 14 16 14 14
12.9 19 17 15 18 15 15
A4-70 13 13 13 13 13 13
A4-80 14 13 13 13 13 13
M14 2
8.8 16 16 16 16 16 16
10.9 19 17 16 19 16 16
12.9 22 20 17 21 17 17
A4-70 15 15 15 15 15 15
A4-80 16 15 15 15 15 15
M16 2
8.8 18 18 18 18 18 18
10.9 22 20 19 21 19 19
12.9 25 23 19 24 19 19
A4-70 17 17 17 17 17 17
A4-80 18 17 17 17 17 17
M18 2.5
8.8 21 20 20 20 20 20
10.9 25 22 21 23 21 21
12.9 28 25 21 27 21 21
A4-70 19 19 19 19 19 19
A4-80 20 19 19 19 19 19
M20 2.5
8.8 23 22 22 22 22 22
10.9 27 24 23 26 23 23
12.9 31 28 23 30 23 23
A4-70 20 20 20 20 20 20
A4-80 22 21 21 21 21 21
M22 2.5
8.8 25 24 24 24 24 24
10.9 30 27 25 29 25 25
12.9 35 31 25 33 25 25
A4-70 22 22 22 22 22 22
A4-80 24 23 23 23 23 23
M24 3
8.8 27 26 26 26 26 26
10.9 33 29 27 31 27 27
12.9 37 33 28 35 28 28
A4-70 24 24 24 24 24 24
A4-80 26 25 25 25 25 25
Note:
• The table is based on the assumption, that the shear strength to ultimate tensile strength ratio of the parent material is equal to 0.8.
• An additional 2 times thread pitch of the thread engagement has been added to take into account the chamfer on the bolt thread.
• An additional 50% of the thread engagement has been added as an additional safety factor. See worked example below for the formula.
From Bolt Science Handbook the bolt UTSmax is approximately 30% larger than bolt UTSmin specified in ISO 898-1: 2013. The UTSmax is
based on the maximum hardness.
Table 8 – Minimum Thread Engagement for Thread Tolerance Class 6H/6g

8. 4. MINIMUM THREAD ENGAGEMENT WITH RESPECT TO VARIOUS MATERIAL
4.1 Property Class 8.8
The minimum thread engagements based on external and internal threads shear strength are the following:
Size Pitch Property
Class
Minimum Thread Engagement
S355 S460 S690 304 / 316L 431S29
17/4 PH
(H1150D)mm mm
M10 1.5
8.8 1.2d 1.2d 1.2d 1.2d 1.2d 1.2d
M12 1.75
M14 2
M16 2
M18 2.5
M20 2.5
M22 2.5
M24 3
M27 3
M30 3.5
M33 3.5
M36 4
Note:
• The table is based on the assumption, that the shear strength to ultimate tensile strength ratio of the parent material is equal to 0.8.
• An additional 2 times thread pitch of the thread engagement has been added to take into account the chamfer on the bolt thread.
• An additional 50% of the thread engagement has been added as an additional safety factor. See worked example below for the formula.
From Bolt Science Handbook the bolt UTSmax is approximately 30% larger than bolt UTSmin specified in ISO 898-1: 2013. The UTSmax is
based on the maximum hardness.
Table 9 – Minimum Thread Engagement for Thread Tolerance Class 6H/6g
4.2 Property Class 10.9
The minimum thread engagements based on external and internal threads shear strength are the following:
Size Pitch Property
Class
Minimum Thread Engagement
S355 S460 S690 304 / 316L 431S29
17/4 PH
(H1150D)mm mm
M10 1.5
10.9 1.4d 1.3d 1.2d 1.4d 1.2d 1.2d
M12 1.75
M14 2
M16 2
M18 2.5
M20 2.5
M22 2.5
M24 3
M27 3
M30 3.5
M33 3.5
M36 4
Note:
• The table is based on the assumption, that the shear strength to ultimate tensile strength ratio of the parent material is equal to 0.8.
• An additional 2 times thread pitch of the thread engagement has been added to take into account the chamfer on the bolt thread.
• An additional 50% of the thread engagement has been added as an additional safety factor. See worked example below for the formula.
From Bolt Science Handbook the bolt UTSmax is approximately 30% larger than bolt UTSmin specified in ISO 898-1: 2013. The UTSmax is
based on the maximum hardness.
Table 10 – Minimum Thread Engagement for Thread Tolerance Class 6H/6g

9. 4.3 Property Class 12.9
The minimum thread engagements based on external and internal threads shear strength are the following:
Size Pitch Property
Class
Minimum Thread Engagement
S355 S460 S690 304 / 316L 431S29
17/4 PH
(H1150D)mm mm
M10 1.5
12.9 1.6d 1.4d 1.2d 1.5d 1.2d 1.2d
M12 1.75
M14 2
M16 2
M18 2.5
M20 2.5
M22 2.5
M24 3
M27 3
M30 3.5
M33 3.5
M36 4
Note:
• The table is based on the assumption, that the shear strength to ultimate tensile strength ratio of the parent material is equal to 0.8.
• An additional 2 times thread pitch of the thread engagement has been added to take into account the chamfer on the bolt thread.
• An additional 50% of the thread engagement has been added as an additional safety factor. See worked example below for the formula.
From Bolt Science Handbook the bolt UTSmax is approximately 30% larger than bolt UTSmin specified in ISO 898-1: 2013. The UTSmax is
based on the maximum hardness.
Table 11 – Minimum Thread Engagement for Thread Tolerance Class 6H/6g
4.4 Property Class A4-70
The minimum thread engagements based on external and internal threads shear strength are the following:
Size Pitch Property
Class
Minimum Thread Engagement
S355 S460 S690 304 / 316L 431S29
17/4 PH
(H1150D)mm mm
M10 1.5
A4-70 1.1d 1.1d 1.1d 1.1d 1.1d 1.1d
M12 1.75
M14 2
M16 2
M18 2.5
M20 2.5
M22 2.5
M24 3
M27 3
M30 3.5
M33 3.5
M36 4
Note:
• The table is based on the assumption, that the shear strength to ultimate tensile strength ratio of the parent material is equal to 0.8.
• An additional 2 times thread pitch of the thread engagement has been added to take into account the chamfer on the bolt thread.
• An additional 50% of the thread engagement has been added as an additional safety factor. See worked example below for the formula.
From Bolt Science Handbook the bolt UTSmax is approximately 30% larger than bolt UTSmin specified in ISO 898-1: 2013. The UTSmax is
based on the maximum hardness.
Table 12 – Minimum Thread Engagement for Thread Tolerance Class 6H/6g

10. 4.5 Property Class A4-80
The minimum thread engagements based on external and internal threads shear strength are the following:
Size Pitch Property
Class
Minimum Thread Engagement
S355 S460 S690 304 / 316L 431S29
17/4 PH
(H1150D)mm mm
M10 1.5
A4-80 1.2d 1.1d 1.1d 1.1d 1.1d 1.1d
M12 1.75
M14 2
M16 2
M18 2.5
M20 2.5
M22 2.5
M24 3
M27 3
M30 3.5
M33 3.5
M36 4
Note:
• The table is based on the assumption, that the shear strength to ultimate tensile strength ratio of the parent material is equal to 0.8.
• An additional 2 times thread pitch of the thread engagement has been added to take into account the chamfer on the bolt thread.
• An additional 50% of the thread engagement has been added as an additional safety factor. See worked example below for the formula.
From Bolt Science Handbook the bolt UTSmax is approximately 30% larger than bolt UTSmin specified in ISO 898-1: 2013. The UTSmax is
based on the maximum hardness.
Table 13 – Minimum Thread Engagement for Thread Tolerance Class 6H/6g