Inch-Series Square Head Bolts for Anti-Rotation, Structural Timber, and Specialty Industrial Fastening.
ASME B18.2.1 Square Bolts are inch-series externally threaded fasteners featuring a square head that provides four flat wrench engagement surfaces and a naturally larger anti-rotation bearing footprint, making them particularly well-suited for structural timber connections, carriage-bolt style embedded anchor applications, agricultural equipment, railroad track hardware, and specialty industrial assemblies where the square head geometry locks against rotation when embedded in a square hole punched through the base material or restrained by a matching square washer in timber. The ASME B18.2.1 standard defines precise dimensional requirements for square head bolts including head height, width across flats, body diameter, thread length, width across corners, and fillet radius for nominal sizes ranging from 1/4 inch to 1-1/2 inches, with all associated manufacturing tolerances aligned to American National Standard requirements. When manufactured to ASTM A1082, the specification for high-strength stainless steel bolting material, these square bolts provide the corrosion resistance and structural performance required for outdoor timber structures, coastal construction, marine dock hardware, agricultural facility framing, and chemical plant structural steelwork , where the combination of a self-restraining square head geometry and stainless steel corrosion resistance provides a long-service-life fastening solution without periodic re-tightening. ASTM A1082 square bolts are also commonly used in custom embedded anchor applications where the square head acts as a positive mechanical anti-pull-through device in cast concrete or grout-filled anchor pockets. Full EN 10204 Type 3.1 certification is available for all ASTM A1082 square bolt procurement lots.
| 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 |
| 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 |
ASTM A1082/A1082M (2016) Table 1 lists 14 duplex (ferritic-austenitic) grades, S31100, S31260, S31803, S32101, S32202, S32205 (2205), S32304 (2304), S32506, S32550 (255), S32750 (2507), S32760, S32906, S32950 and S39277, and 5 precipitation-hardening grades, S15700 (Type 632), S17400 (Type 630), S17600 (Type 635), S17700 (Type 631) and S35500 (Type 634). Grade selection depends on design, service conditions and the mechanical properties required.
Dimensions follow ASME B18.2.1, tabulated on this page. ASTM A1082/A1082M sets chemistry (Table 1), heat treatment and mechanical properties (Tables 2 to 4) and marking (Table 5). ASTM A962/A962M general requirements also apply, and A1082 prevails where the two conflict (clause 4.1). Inch-pound units apply unless the order specifies A1082M (clause 1.4).
ISO 15156-3 Table A.24 accepts wrought duplex stainless steel for any equipment when it is solution-annealed and liquid-quenched or rapidly cooled, has 35 % to 65 % ferrite and has had no ageing heat treatment. With 30 ≤ PREN ≤ 40 and Mo ≥ 1.5 % the limit is 232 °C (450 °F) and 10 kPa (1.5 psi) H2S partial pressure; with 40 < PREN ≤ 45 it is 232 °C (450 °F) and 20 kPa (3 psi). Table A.27 accepts wrought 17-4 PH (S17400) at 33 HRC max after a double age at 620 °C (1150 °F) or a modified double age, for named wellhead, valve and subsurface components, up to 3.4 kPa (0.5 psi) H2S and pH 4.5 or above.
The grade symbol comes from Table 5 (clause 11.1), for example 32205 for S32205 or 174G for S17400 in H1150. With Supplementary Requirement S2 the marking goes on the heads of bolts of all sizes; if the area is too small, the grade symbol may go on one end and the manufacturer's identification on the other. Bolts under 1/4 in. [6 mm] are marked by agreement between purchaser and manufacturer.
Table 4: solution annealed at 1880 to 2060 °F [1025 to 1125 °C], water quenched. Up to and including 2 in.: tensile 116 ksi [800 MPa] and yield 80 ksi [550 MPa] min. Over 2 in.: 110 ksi [760 MPa] and 75 ksi [515 MPa] min. Elongation 15 % min and hardness 33 HRC or 310 HB max for both. The marking grade symbol is 32750 (Table 5).
