Specifications
Surface Treatments
Certifications
- ISO 9001 - 2015 Certified
- PED 2014/68/EC
- NACE MR0175/ISO 15156-2
- NORSOK M-650
- DFAR
- MERKBLATT AD 2000 W2/W7/W10
ASME B18.2.1 Square Bolts
Square head bolts providing exceptional wrench grip and resistance to rounding, traditionally used in railroad tracks and heavy timber structures.
ASME B18.2.1 Square Bolts feature a square-shaped head that offers four distinct flat surfaces for wrench engagement, providing superior torque transmission and significantly reducing the likelihood of head rounding compared to hex heads. This geometric advantage makes them particularly valuable in applications involving high torque, corroded conditions, or where the fastener must be held stationary from one side while the nut is tightened on the other. Historically and currently, square bolts are the standard for railroad track splicing, heavy timber framing, and agricultural machinery where durability and ease of maintenance are paramount. The square head can also be easily embedded into wood to prevent rotation during assembly. Manufactured to the rigorous dimensional standards of ASME B18.2.1, these bolts ensure consistent fit and performance in demanding structural and industrial environments.
ASME B18.2.1 Square Bolts Dimensions (mm)
| Nominal 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 | Nominal Thread Length, LT | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Max. | Min. | Basic | Max. | Min. | Max. | Min. | Basic | Max. | Min. | Max. | Min. | 152.4mm and Shorter | Over 152.4mm | ||
| 1/4 | 6.350 | 6.604 | 6.020 | 9.525 | 9.525 | 9.195 | 13.462 | 12.649 | 4.366 | 4.775 | 3.962 | 0.762 | 0.254 | 19.050 | 25.400 |
| 5/16 | 7.938 | 8.230 | 7.569 | 12.700 | 12.700 | 12.294 | 17.958 | 16.891 | 5.159 | 5.588 | 4.724 | 0.762 | 0.254 | 22.225 | 28.575 |
| 3/8 | 9.525 | 9.855 | 9.144 | 14.288 | 14.275 | 13.818 | 20.193 | 18.974 | 6.350 | 6.807 | 5.893 | 0.762 | 0.254 | 25.400 | 31.750 |
| 7/16 | 11.113 | 11.481 | 10.693 | 15.875 | 15.875 | 15.316 | 22.454 | 21.031 | 7.541 | 8.026 | 7.061 | 0.762 | 0.254 | 28.575 | 34.925 |
| 1/2 | 12.700 | 13.081 | 12.243 | 19.050 | 19.050 | 18.415 | 26.949 | 25.273 | 8.334 | 8.839 | 7.823 | 0.762 | 0.254 | 31.750 | 38.100 |
| 5/8 | 15.875 | 16.307 | 15.367 | 23.813 | 23.825 | 23.012 | 33.680 | 31.598 | 10.716 | 11.278 | 10.160 | 1.524 | 0.508 | 38.100 | 44.450 |
| 3/4 | 19.050 | 19.507 | 18.517 | 28.575 | 28.575 | 27.635 | 40.411 | 37.948 | 12.700 | 13.310 | 12.090 | 1.524 | 0.508 | 44.450 | 50.800 |
| 7/8 | 22.225 | 22.733 | 21.641 | 33.338 | 33.325 | 32.233 | 47.142 | 44.247 | 15.081 | 15.748 | 14.427 | 1.524 | 0.508 | 50.800 | 57.150 |
| 1 | 25.400 | 25.959 | 24.790 | 38.100 | 38.100 | 36.830 | 53.873 | 50.571 | 16.669 | 17.374 | 15.951 | 2.286 | 0.762 | 57.150 | 63.500 |
| 1 1/8 | 28.575 | 29.185 | 27.889 | 42.863 | 42.875 | 41.427 | 60.604 | 56.871 | 19.050 | 19.812 | 18.288 | 2.286 | 0.762 | 63.500 | 69.850 |
| 1 1/4 | 31.750 | 32.436 | 31.064 | 47.625 | 47.625 | 46.025 | 67.361 | 63.221 | 21.431 | 22.250 | 20.625 | 2.286 | 0.762 | 69.850 | 76.200 |
| 1 3/8 | 34.925 | 35.662 | 34.163 | 52.388 | 52.375 | 50.648 | 74.092 | 69.545 | 23.019 | 23.876 | 22.149 | 2.286 | 0.762 | 76.200 | 82.550 |
| 1 1/2 | 38.100 | 38.887 | 37.338 | 57.150 | 57.150 | 55.245 | 80.823 | 75.844 | 25.400 | 26.314 | 24.486 | 2.286 | 0.762 | 82.550 | 88.900 |
ASME B18.2.1 Square Bolts Length Tolerance (mm)
| Nominal Length | Nominal Size (inches) | |||||
|---|---|---|---|---|---|---|
| 1/4 to 3/8 | 7/16 and 1/2 | 9/16 to 3/4 | 7/8 and 1 | 1 1/8 to 1 1/2 | Over 1 1/2 | |
| Up to 25.4 mm, incl. | +0.508 0.762 |
+0.508 0.762 |
+0.508 0.762 |
... | ... | ... |
| Over 25.4 mm to 63.5 mm, incl. | +0.508 1.016 |
+1.016 1.524 |
+1.524 2.032 |
+2.032 2.54 |
+3.048 3.048 |
+4.572 4.572 |
| Over 63.5 mm to 101.6 mm, incl. | +1.016 1.524 |
+1.524 2.032 |
+2.032 2.54 |
+2.54 3.556 |
+4.064 4.064 |
+5.08 5.08 |
| Over 101.6 mm to 152.4 mm, incl. | +1.524 2.54 |
+2.032 2.54 |
+2.54 2.54 |
+3.048 4.064 |
+4.572 4.572 |
+5.588 5.588 |
| Longer than 152.4 mm | +2.54 4.572 |
+3.048 4.572 |
+3.556 4.572 |
+4.064 5.08 |
+5.588 5.588 |
+6.096 6.096 |
ASME B18.2.1 Square Bolts Dimensions (Inch)
| Nominal 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 | Nominal Thread Length for Bolt Lengths, LT | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Max. | Min. | Basic | Max. | Min. | Max. | Min. | Basic | Max. | Min. | Max. | Min. | 6 in. and Shorter | Over 6 in. | ||
| 1/4 | 0.2500 | 0.260 | 0.237 | 3/8 | 0.375 | 0.362 | 0.530 | 0.498 | 11/64 | 0.188 | 0.156 | 0.03 | 0.01 | 0.750 | 1.000 |
| 5/16 | 0.3125 | 0.324 | 0.298 | 1/2 | 0.500 | 0.484 | 0.707 | 0.665 | 13/64 | 0.220 | 0.186 | 0.03 | 0.01 | 0.875 | 1.125 |
| 3/8 | 0.3750 | 0.388 | 0.360 | 9/16 | 0.562 | 0.544 | 0.795 | 0.747 | 1/4 | 0.268 | 0.232 | 0.03 | 0.01 | 1.000 | 1.250 |
| 7/16 | 0.4375 | 0.452 | 0.421 | 5/8 | 0.625 | 0.603 | 0.884 | 0.828 | 19/64 | 0.316 | 0.278 | 0.03 | 0.01 | 1.125 | 1.375 |
| 1/2 | 0.5000 | 0.515 | 0.482 | 3/4 | 0.750 | 0.725 | 1.061 | 0.995 | 21/64 | 0.348 | 0.308 | 0.03 | 0.01 | 1.250 | 1.500 |
| 5/8 | 0.6250 | 0.642 | 0.605 | 15/16 | 0.938 | 0.906 | 1.326 | 1.244 | 27/64 | 0.444 | 0.400 | 0.06 | 0.02 | 1.500 | 1.750 |
| 3/4 | 0.7500 | 0.768 | 0.729 | 1 1/8 | 1.125 | 1.088 | 1.591 | 1.494 | 1/2 | 0.524 | 0.476 | 0.06 | 0.02 | 1.750 | 2.000 |
| 7/8 | 0.8750 | 0.895 | 0.852 | 1 5/16 | 1.312 | 1.269 | 1.856 | 1.742 | 19/32 | 0.620 | 0.568 | 0.06 | 0.02 | 2.000 | 2.250 |
| 1 | 1.0000 | 1.022 | 0.976 | 1 1/2 | 1.500 | 1.450 | 2.121 | 1.991 | 21/32 | 0.684 | 0.628 | 0.09 | 0.03 | 2.250 | 2.500 |
| 1 1/8 | 1.1250 | 1.149 | 1.098 | 1 11/16 | 1.688 | 1.631 | 2.386 | 2.239 | 3/4 | 0.780 | 0.720 | 0.09 | 0.03 | 2.500 | 2.750 |
| 1 1/4 | 1.2500 | 1.277 | 1.223 | 1 7/8 | 1.875 | 1.812 | 2.652 | 2.489 | 27/32 | 0.876 | 0.812 | 0.09 | 0.03 | 2.750 | 3.000 |
| 1 3/8 | 1.3750 | 1.404 | 1.345 | 2 1/16 | 2.062 | 1.994 | 2.917 | 2.738 | 29/32 | 0.940 | 0.872 | 0.09 | 0.03 | 3.000 | 3.250 |
| 1 1/2 | 1.5000 | 1.531 | 1.470 | 2 1/4 | 2.250 | 2.175 | 3.182 | 2.986 | 1 | 1.036 | 0.964 | 0.09 | 0.03 | 3.250 | 3.500 |
ASME B18.2.1 Square Bolts Length Tolerance (Inch)
| Nominal Length | Nominal Size | |||||
|---|---|---|---|---|---|---|
| 1/4 to 3/8 | 7/16 and 1/2 | 9/16 to 3/4 | 7/8 and 1 | 1 1/8 to 1 1/2 | Over 1 1/2 | |
| Up to 1 in., incl. | +0.02 0.03 |
+0.02 0.03 |
+0.02 0.03 |
... | ... | ... |
| Over 1 in. to 2 1/2 in., incl. | +0.02 0.04 |
+0.04 0.06 |
+0.06 0.08 |
+0.08 0.10 |
+0.12 0.12 |
+0.18 0.18 |
| Over 2 1/2 in. to 4 in., incl. | +0.04 0.06 |
+0.06 0.08 |
+0.08 0.10 |
+0.10 0.14 |
+0.16 0.16 |
+0.20 0.20 |
| Over 4 in. to 6 in., incl. | +0.06 0.10 |
+0.08 0.10 |
+0.10 0.10 |
+0.12 0.16 |
+0.18 0.18 |
+0.22 0.22 |
| Longer than 6 in. | +0.10 0.18 |
+0.12 0.18 |
+0.14 0.18 |
+0.16 0.20 |
+0.22 0.22 |
+0.24 0.24 |
Frequently Asked Questions
- What does ASME B18.2.1 square bolts cover?
- ASME B18.2.1 is the inch dimensional standard for square bolts. It defines head dimensions, body diameter, thread length, total length, drive recess, and tolerances so any compliant square bolts interchanges with standard nuts, washers and wrenches across manufacturers.
- What size range is available for square bolts under ASME B18.2.1?
- square bolts under ASME B18.2.1 covers 1/4 in to 1 1/2 in nominal diameter, traditional square head. TorqBolt manufactures the full range in ASTM A193 Grades B7, B7M, B8, B8M and the full ASTM A320 cryogenic counterpart on customer specification.
- What is the metric/inch counterpart of ASME B18.2.1 square bolts?
- The companion specification for square bolts is DIN 478/479 metric square. Both standards share equivalent functional dimensions but use the corresponding measurement system (inch vs metric ISO M-series).
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