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 Lag Screws
Heavy-duty square head wood screws designed for high-torque installation in timber framing and industrial machinery mounting.
ASME B18.2.1 Square Lag Screws are robust fasteners featuring a square head and coarse, widely spaced threads specifically engineered for gripping into wood, timber, and other soft materials. The square head design allows for maximum torque application using a standard wrench, providing superior driving power compared to hex heads in situations where space permits, while minimizing the risk of head rounding under extreme load. These lag screws are commonly used in heavy timber construction, railroad track fastening, utility pole attachments, and securing heavy machinery to wooden foundations. Manufactured to strict dimensional tolerances defined in the ASME B18.2.1 standard, they are available in various finishes including hot-dip galvanized and zinc-plated to ensure longevity in outdoor and corrosive environments.
ASME B18.2.1 Square Lag Screws Dimensions (mm)
| Nominal Size or Basic Product Diameter | Body or Shoulder Diameter, E | Width Across Flats, F | Width Across Corners, G | Head Height, H | Minimum Shoulder Length, S | Radius of Fillet, R | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Max. | Min. | Basic | Max. | Min. | Max. | Min. | Basic | Max. | Min. | Max. | Min. | |||
| #10 | 5.055 | 4.521 | 7.144 | 7.137 | 6.883 | 10.109 | 9.449 | 3.175 | 3.556 | 2.794 | 2.388 | 0.762 | 0.254 | |
| 1/4 | 0.2500 | 6.604 | 6.020 | 9.525 | 9.525 | 9.195 | 13.462 | 12.649 | 4.366 | 4.775 | 3.962 | 2.388 | 0.762 | 0.254 |
| 5/16 | 0.3125 | 8.230 | 7.569 | 12.700 | 12.700 | 12.294 | 17.958 | 16.891 | 5.159 | 5.588 | 4.724 | 3.175 | 0.762 | 0.254 |
| 3/8 | 0.3750 | 9.855 | 9.144 | 14.288 | 14.275 | 13.818 | 20.193 | 18.974 | 6.350 | 6.807 | 5.893 | 3.175 | 0.762 | 0.254 |
| 7/16 | 0.4375 | 11.481 | 10.693 | 15.875 | 15.875 | 15.316 | 22.454 | 21.031 | 7.541 | 8.026 | 7.061 | 3.962 | 0.762 | 0.254 |
| 1/2 | 0.5000 | 13.081 | 12.243 | 19.050 | 19.050 | 18.415 | 26.949 | 25.273 | 8.334 | 8.839 | 7.823 | 3.962 | 0.762 | 0.254 |
| 5/8 | 0.6250 | 16.307 | 15.367 | 23.813 | 23.825 | 23.012 | 33.680 | 31.598 | 10.716 | 11.278 | 10.160 | 7.925 | 1.524 | 0.508 |
| 3/4 | 0.7500 | 19.507 | 18.517 | 28.575 | 28.575 | 27.635 | 40.411 | 37.948 | 12.700 | 13.310 | 12.090 | 9.525 | 1.524 | 0.508 |
| 7/8 | 0.8750 | 22.733 | 21.641 | 33.338 | 33.325 | 32.233 | 47.142 | 44.247 | 15.081 | 15.748 | 14.427 | 9.525 | 1.524 | 0.508 |
| 1 | 1.0000 | 25.959 | 24.790 | 38.100 | 38.100 | 36.830 | 53.873 | 50.571 | 16.669 | 17.374 | 15.951 | 15.875 | 2.286 | 0.762 |
| 1 1/8 | 1.1250 | 29.185 | 27.889 | 42.863 | 42.875 | 41.427 | 60.604 | 56.871 | 19.050 | 19.812 | 18.288 | 15.875 | 2.286 | 0.762 |
| 1 1/4 | 1.2500 | 32.436 | 31.064 | 47.625 | 47.625 | 46.025 | 67.361 | 63.221 | 21.431 | 22.250 | 20.625 | 15.875 | 2.286 | 0.762 |
ASME B18.2.1 Square Lag Screws 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. | +0.000 0.762 |
+0.000 0.762 |
+0.000 0.762 |
... | ... | ... |
| Over 25.4 mm to 63.5 mm, incl. | +0.000 1.016 |
+0.000 1.524 |
+0.000 2.032 |
+0.000 2.540 |
+0.000 3.048 |
+0.000 4.572 |
| Over 63.5 mm to 101.6 mm, incl. | +0.000 1.524 |
+0.000 2.032 |
+0.000 2.540 |
+0.000 3.556 |
+0.000 4.064 |
+0.000 5.080 |
| Over 101.6 mm to 152.4 mm, incl. | +0.000 2.540 |
+0.000 2.540 |
+0.000 2.540 |
+0.000 4.064 |
+0.000 4.572 |
+0.000 5.588 |
| Longer than 152.4 mm | +0.000 4.572 |
+0.000 4.572 |
+0.000 4.572 |
+0.000 5.080 |
+0.000 5.588 |
+0.000 6.096 |
ASME B18.2.1 Square Lag Screws Dimensions (Inch)
| Nominal Size or Basic Product Diameter | Body or Shoulder Diameter, E | Width Across Flats, F | Width Across Corners, G | Head Height, H | Minimum Shoulder Length, S | Radius of Fillet, R | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Max. | Min. | Basic | Max. | Min. | Max. | Min. | Basic | Max. | Min. | Max. | Min. | |||
| #10 | 0.1900 | 0.199 | 0.178 | 9/32 | 0.281 | 0.271 | 0.398 | 0.372 | 1/8 | 0.140 | 0.110 | 0.094 | 0.03 | 0.01 |
| 1/4 | 0.2500 | 0.260 | 0.237 | 3/8 | 0.375 | 0.362 | 0.530 | 0.498 | 11/64 | 0.188 | 0.156 | 0.094 | 0.03 | 0.01 |
| 5/16 | 0.3125 | 0.324 | 0.298 | 1/2 | 0.500 | 0.484 | 0.707 | 0.665 | 13/64 | 0.220 | 0.186 | 0.125 | 0.03 | 0.01 |
| 3/8 | 0.3750 | 0.388 | 0.360 | 9/16 | 0.562 | 0.544 | 0.795 | 0.747 | 1/4 | 0.268 | 0.232 | 0.125 | 0.03 | 0.01 |
| 7/16 | 0.4375 | 0.452 | 0.421 | 5/8 | 0.625 | 0.603 | 0.884 | 0.828 | 19/64 | 0.316 | 0.278 | 0.156 | 0.03 | 0.01 |
| 1/2 | 0.5000 | 0.515 | 0.482 | 3/4 | 0.750 | 0.725 | 1.061 | 0.995 | 21/64 | 0.348 | 0.308 | 0.156 | 0.03 | 0.01 |
| 5/8 | 0.6250 | 0.642 | 0.605 | 15/16 | 0.938 | 0.906 | 1.326 | 1.244 | 27/64 | 0.444 | 0.400 | 0.312 | 0.06 | 0.02 |
| 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 | 0.375 | 0.06 | 0.02 |
| 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 | 0.375 | 0.06 | 0.02 |
| 1 | 1.0000 | 1.022 | 0.976 | 1 1/2 | 1.500 | 1.450 | 2.121 | 1.991 | 21/32 | 0.684 | 0.628 | 0.625 | 0.09 | 0.03 |
| 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 | 0.625 | 0.09 | 0.03 |
| 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 | 0.625 | 0.09 | 0.03 |
ASME B18.2.1 Square Lag Screws Length 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. | +0.00 0.03 |
+0.00 0.03 |
+0.00 0.03 |
... | ... | ... |
| Over 1 in. to 2 1/2 in., incl. | +0.00 0.04 |
+0.00 0.06 |
+0.00 0.08 |
+0.00 0.10 |
+0.00 0.12 |
+0.00 0.18 |
| Over 2 1/2 in. to 4 in., incl. | +0.00 0.06 |
+0.00 0.08 |
+0.00 0.10 |
+0.00 0.14 |
+0.00 0.16 |
+0.00 0.20 |
| Over 4 in. to 6 in., incl. | +0.00 0.10 |
+0.00 0.10 |
+0.00 0.10 |
+0.00 0.16 |
+0.00 0.18 |
+0.00 0.22 |
| Longer than 6 in. | +0.00 0.18 |
+0.00 0.18 |
+0.00 0.18 |
+0.00 0.20 |
+0.00 0.22 |
+0.00 0.24 |
Frequently Asked Questions
- What does ASME B18.2.1 square lag screws cover?
- ASME B18.2.1 is the inch dimensional standard for square lag screws. It defines head dimensions, body diameter, thread length, total length, drive recess, and tolerances so any compliant square lag screws interchanges with standard nuts, washers and wrenches across manufacturers.
- What size range is available for square lag screws under ASME B18.2.1?
- square lag screws under ASME B18.2.1 covers #10 through 1 1/4 in nominal diameter, square head for wood/structural lag. 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 lag screws?
- The companion specification for square lag screws is DIN 571 metric coach screws. Both standards share equivalent functional dimensions but use the corresponding measurement system (inch vs metric ISO M-series).
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