ASME B18.2.1 is a standard developed by the American Society of Mechanical Engineers (ASME) that covers the general and dimensional requirements for inch-series square and hex bolts, screws, and studs. These fasteners are commonly referred to as "hex bolts," "hex screws," or "hex cap screws."
The standard covers both imperial (inch-based) and metric sizes and includes provisions for both finished and semi-finished products. It specifies the minimum mechanical properties, dimensional tolerances, and other requirements for these fasteners, as well as the methods for testing and inspection.
ASME B18.2.1 also includes provisions for different material grades, including low-carbon steel, medium-carbon steel, alloy steel, and stainless steel. It is important to note that the standard does not cover bolts with a tensile strength greater than 170,000 psi or screws or studs with a tensile strength greater than 120,000 psi.
ASME B18.2.1 is widely used in the manufacturing and construction industries and is an important reference for engineers, designers, and procurement professionals responsible for specifying and purchasing these fasteners. It is regularly updated to keep pace with advances in fastener technology and manufacturing processes.
| Nomianal Size or Basic product Diameter | Full-Size Body Diameter, E | Width Across Flats, F | Width Across Corners, G | Head Height, H | Radius of Fillet, R | Thread Length for Bolt Lengths, LT | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 6 in. & Shorter | Over 6 in. | |||||||||||||
| Max. | Min. | Basic | Max. | Min. | Max. | Min. | Basic | Max. | Min. | Max. | Min. | Nom. | Nom. | |
| 1/4 0.2500 |
0.260 | 0.237 | 7/16 | 0.438 | 0.425 | 0.505 | 0.484 | 11/64 | 0.188 | 0.150 | 0.03 | 0.01 | 0.750 | 1.000 |
| 5/16 0.3125 |
0.324 | 0.298 | 1/2 | 0.500 | 0.484 | 0.577 | 0.552 | 7/32 | 0.235 | 0.195 | 0.03 | 0.01 | 0.875 | 1.125 |
| 3/8 0.3750 |
0.388 | 0.360 | 9/16 | 0.562 | 0.544 | 0.650 | 0.620 | 1/4 | 0.268 | 0.226 | 0.03 | 0.01 | 1.000 | 1.250 |
| 7/16 0.4375 |
0.452 | 0.421 | 5/8 | 0.625 | 0.603 | 0.722 | 0.687 | 19/64 | 0.316 | 0.272 | 0.03 | 0.01 | 1.125 | 1.375 |
| 1/2 0.5000 |
0.515 | 0.482 | 3/4 | 0.750 | 0.725 | 0.866 | 0.826 | 11/32 | 0.364 | 0.302 | 0.03 | 0.01 | 1.250 | 1.500 |
| 5/8 0.6250 |
0.642 | 0.605 | 15/16 | 0.938 | 0.906 | 1.083 | 1.033 | 27/64 | 0.444 | 0.378 | 0.06 | 0.02 | 1.500 | 1.750 |
| 3/4 0.7500 |
0.768 | 0.729 | 1 1/8 | 1.125 | 1.088 | 1.299 | 1.240 | 1/2 | 0.524 | 0.455 | 0.06 | 0.02 | 1.750 | 2.000 |
| 7/8 0.8750 |
0.895 | 0.852 | 1 5/16 | 1.312 | 1.269 | 1.516 | 1.447 | 37/64 | 0.604 | 0.531 | 0.06 | 0.02 | 2.000 | 2.250 |
| 1 1.0000 |
1.022 | 0.976 | 1 1/2 | 1.500 | 1.450 | 1.732 | 1.653 | 43/64 | 0.700 | 0.591 | 0.09 | 0.03 | 2.250 | 2.500 |
| 1 1/8 1.1250 |
1.149 | 1.098 | 1 11/16 | 1.688 | 1.631 | 1.949 | 1.859 | 3/4 | 0.780 | 0.658 | 0.09 | 0.03 | 2.500 | 2.750 |
| 1 1/4 1.2500 |
1.277 | 1.223 | 1 7/8 | 1.875 | 1.812 | 2.165 | 2.066 | 27/32 | 0.876 | 0.749 | 0.09 | 0.03 | 2.750 | 3.000 |
| 1 3/8 1.3750 |
1.404 | 1.345 | 2 1/16 | 2.062 | 1.994 | 2.382 | 2.273 | 29/32 | 0.940 | 0.810 | 0.09 | 0.03 | 3.000 | 3.250 |
| 1 1/2 1.5000 |
1.531 | 1.470 | 2 1/4 | 2.250 | 2.175 | 2.598 | 2.480 | 1/ | 1.036 | 0.902 | 0.09 | 0.03 | 3.250 | 3.500 |
| 1 5/8 1.6250 |
1.658 | 1.591 | 2 7/16 | 2.438 | 2.356 | 2.815 | 2.616 | 1 3/32 | 1.116 | 0.978 | 0.09 | 0.03 | 3.500 | 3.750 |
| 1 3/4 1.7500 |
1.785 | 1.716 | 2 5/8 | 2.625 | 2.538 | 3.031 | 2.893 | 1 5/32 | 1.196 | 1.054 | 0.12 | 0.04 | 3.750 | 4.000 |
| 1 7/8 1.8750 |
1.912 | 1.839 | 2 13/16 | 2.812 | 2.719 | 3.248 | 3.099 | 1 1/4 | 1.276 | 1.130 | 0.12 | 0.04 | 4.000 | 4.250 |
| 2 2.000 |
2.039 | 1.964 | 3 | 3.000 | 2.9000 | 3.464 | 3.306 | 1 11/32 | 1.388 | 1.175 | 0.12 | 0.04 | 4.250 | 4.500 |
| 2 1/4 2.2500 |
2.305 | 2.214 | 3 3/8 | 3.375 | 3.262 | 3.897 | 3.719 | 1 1/2 | 1.548 | 1.327 | 0.19 | 0.06 | 4.750 | 5.000 |
| 2 1/2 2.5000 |
2.559 | 2.461 | 3 3/4 | 3.750 | 3.625 | 4.330 | 4.133 | 1 21/32 | 1.708 | 1.479 | 0.19 | 0.06 | 5.250 | 5.500 |
| 2 3/4 2.7500 |
2.827 | 2.711 | 4 1/8 | 4.125 | 3.988 | 4.763 | 4.546 | 1 13/16 | 1.869 | 1.632 | 0.19 | 0.06 | 5.750 | 6.000 |
| 3 3.000 |
3.081 | 2.961 | 4 1/2 | 4.500 | 4.350 | 5.196 | 4.959 | 2 | 2.060 | 1.815 | 0.19 | 0.06 | 6.250 | 6.500 |
| 3 1/4 3.2500 |
3.335 | 3.210 | 4 7/8 | 4.875 | 4.712 | 5.629 | 5.372 | 2 3/16 | 2.251 | 1.936 | 0.19 | 0.06 | 6.750 | 7.000 |
| 3 1/2 3.5000 |
3.589 | 3.461 | 5 1/4 | 5.250 | 5.075 | 6.062 | 5.786 | 2 5/16 | 2.380 | 2.057 | 0.19 | 0.06 | 7.250 | 7.500 |
| 3 3/4 3.7500 |
3.858 | 3.726 | 5 5/8 | 5.625 | 5.437 | 6.495 | 6.198 | 2 1/2 | 2.572 | 2.241 | 0.19 | 0.06 | 7.750 | 8.000 |
| 4 4.000 |
4.111 | 3.975 | 6 | 6.000 | 5.800 | 6.928 | 6.612 | 2 11/16 | 2.764 | 2.424 | 0.19 | 0.06 | 8.250 | 8.500 |
