steel bolts
length of engagement

When a bolt fails because too great a load has been applied, either the threads will be stripped or the bolt will fracture. In general, it is better if the bolt fractures, especially if the internal threads are in a casting or workpiece, since if a Heli-Coil™ or similar repair cannot be made, the piece is ruined.

The minimum length of engagement is chosen so that at least as many threads are engaged as are needed to withstand a pull that will fracture the bolt.

The tables below apply only when the tensile strengths of the bolt and of the nut or other internally-threaded part are approximately equal. NOTE that these are minimums; see the rule-of-thumb at the bottom of the page.

UNC and UNCR, class 2A/2B
Size Minimum Length
of Engagement,
inches
Minimum
Thickness of
Machine Screw
Nuts¹
Minimum
Thickness of Hex Nuts
Am. Nat'l Standard
 & Unified Standard²
1-64 0.063 0.043
2-56 0.073 0.057
3-48 0.083 0.057
4-40 0.088 0.087
5-40 0.099 0.102
6-32 0.101 0.102
8-32 0.124 0.117
10-24 0.133 0.117
12-24 0.155 0.148
1/4″-20 0.173 0.178 0.212
5/16″-18 0.223 0.208 0.258
3/8″-16 0.269 0.239 0.320
7/16″-14 0.31 0.365
1/2″-13 0.36 0.427
9/16″-12 0.41 0.473
5/8″-11 0.45 0.535
3/4″-10 0.55 0.617
7/8″-9 0.64 0.724
1″-8 0.73 0.831
UNF and UNFR, class 2A/2B
Size Minimum Length
of Engagement,
Inches
Minimum
Thickness of
Machine Screw
Nuts¹
 Minimum
Thickness of Hex Nuts
Am. Nat'l Standard
& Unified Standard²
1-72 0.067 0.043
2-64 0.078 0.057
3-56 0.088 0.057
4-48 0.094 0.087
5-44 0.103 0.102
6-40 0.112 0.102
8-36 0.130 0.117
10-32 0.146 0.117
12-28 0.163 0.148
1/4″-28 0.195 0.178 0.212
5/16″-24 0.242 0.208 0.258
3/8″-24 0.304 0.239 0.320
7/16″-20 0.347 0.365
1/2″-20 0.400 0.427
9/16″-18 0.450 0.473
5/8″-18 0.513 0.535
3/4″-16 0.605 0.617
7/8″-14 0.709 0.724
1″-12 0.797 0.831

An old rule-of-thumb³ is that in a threaded hole in steel, the length of engagement should be at least 1.5 times the diameter of the screw, and in cast iron or brass, 1.8 to 2 times the diameter. Blind holes should be at least half a diameter deeper than the length of engagement.

1. ANSI B18.6.3-1972(R1991)

2. ANSI/ASME B18.2.2-1987(R1999)

3. Frederick A. Halsey.
Handbook for Machine Designers, Shop Men and Draftsmen. 2nd Ed.
New York: McGraw-Hill, 1916.

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