The ASTM F3125 specification
The F3125 specification was introduced in 2016 to cover the six previous specifications that cover structural bolts. The six grades that now fall under the F3125 specification are A325, A325M, F1852, A490, A490M, and F2280.
Heavy hex structural bolts are designed for steel-to-steel structural connections. Since they have this specific application structural bolts are quite short, with a heavy hex head, and a full body diameter. Unless otherwise specified, F3125 structural bolts will have a Unified National Coarse thread pitch. The F3125 Grades A325 and A490 specifications are only intended for this use.
F3125 bolts have a diameter range of ½” to 1 ½” and are made from 4140 quenched and tempered steel. The steel is quenched and tempered to meet specific strength requirements which vary in each of the six grades that fall under the F3125 umbrella.
F3125 Bolt Types
Out of the six grades in F3125 A325 is the most commonly found. All of the grades also differ in strength and weathering properties.
The F3125 specification allows longer than standard thread lengths including fully threaded bolts at the customer’s request. The appropriate head marking for non-standard thread lengths is A325S or A490S respectively.
Structural Bolt Dimensions
| Bolt Diameter | D | F | C | H | T | |||||
| Body Diameter | Width Across Flats | Width Across Corners | Height | Thread Length | ||||||
| Max | Basic | Max | Min | Max | Min | Basic | Max | Min | Basic | |
| 1⁄2 | 0.515 | 7⁄8 | 0.875 | 0.85 | 1.01 | 0.969 | 5⁄16 | 0.323 | 0.302 | 1 |
| 5⁄8 | 0.642 | 11⁄16 | 1.062 | 1.031 | 1.227 | 1.175 | 25⁄64 | 0.403 | 0.378 | 1.25 |
| 3⁄4 | 0.768 | 11⁄4 | 1.25 | 1.212 | 1.443 | 1.383 | 15⁄32 | 0.483 | 0.455 | 1.38 |
| 7⁄8 | 0.895 | 17⁄16 | 1.438 | 1.394 | 1.66 | 1.589 | 35⁄64 | 0.563 | 0.531 | 1.5 |
| 1 | 1.022 | 15⁄8 | 1.625 | 1.575 | 1.876 | 1.796 | 39⁄64 | 0.627 | 0.591 | 1.75 |
| 11⁄8 | 1.149 | 113⁄16 | 1.812 | 1.756 | 2.093 | 2.002 | 11⁄16 | 0.718 | 0.658 | 2 |
| 11⁄4 | 1.277 | 2 | 2 | 1.938 | 2.309 | 2.209 | 25⁄32 | 0.813 | 0.749 | 2 |
| 13⁄8 | 1.404 | 23⁄16 | 2.188 | 2.119 | 2.526 | 2.416 | 27⁄32 | 0.878 | 0.81 | 2.25 |
| 11⁄2 | 1.531 | 23⁄8 | 2.375 | 2.3 | 2.742 | 2.622 | 15⁄16 | 0.974 | 0.902 | 2.25 |
Structural Bolt Length Tolerances
| Nominal Length, in. | Nominal Size, in. | |||
| 1/2 | 5/8 | ¾ thru 1 | 1-1/8 thru 1-1/2 | |
| Thru 6 in. | -0.12 | -0.12 | -0.19 | -0.25 |
| Over 6 in. | -0.19 | -0.25 | -0.25 | -0.25 |
Standard Sizes by Grade
| Grade | Diameter Range | Length Range |
| A325 | ½” to 1 ¼” | 2” to 7 1/2” |
| A325 -3 | ½” to 1 ¼” | 2” to 7 1/2” |
| F1852 (A325TC) | ¾” to 1 ¼” | 1 ¾” to 13 ½” |
| A490 | 5/8” to 1 ¼” | 2” to 7 1/2” |
| A490 – 3 | 5/8” to 1 ¼” | |
| F2280 (A490TC) | ¾” to 1 ¼” | 1 ¾” to 13 ½” |
Rotational Capacity Testing
| Bolt Length Required Rotation | Up to 4D | >4D to 8D | >8D to 12D |
| 120 ksi (830 MPa) min | 240° | 360° | 420° |
| 150 ksi (1040 MPa) min | 240° | 300° | 360° |
Additional Resources
ASTM: This website provides a very brief overview of the F3125 specification. If you are working with F3125 heavy hex structural bolts on a regular basis, we strongly recommend purchasing a copy of the ASTM F3125 specifications from ASTM International. ASTM International, formerly The American Society for Testing and Materials, is a standards organization covering a multitude of products including fasteners.
ASME: The American Society of Mechanical Engineers is an organization that develops dimensional standards for many fasteners including F3125 structural bolts. A copy of ASME B18.2.6 which covers heavy hex structural bolt dimensions and ASME B1.1 which provides dimensional criteria for Unified National Coarse can be purchased from Accuris.