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    2. ASME B18.2.1 Heavy Hex Cap Screws

    ASME B18.2.1 Heavy Hex Cap Screws

    Structural fasteners with enlarged heads and bearing surfaces, engineered for high-load connections in steel bridges, buildings, and heavy infrastructure.

    ASME B18.2.1 Heavy Hex Cap Screws feature a larger head width-across-flats and a wider bearing surface compared to standard hex cap screws, specifically designed to accommodate the demands of structural steel construction. The increased bearing area helps distribute clamping forces over a larger surface, reducing the risk of crushing the connected material and allowing for higher preload tensions. These fasteners are the primary choice for connecting steel beams, columns, and girders in skyscrapers, bridges, and industrial facilities, often adhering to additional specifications like ASTM A325 or A490 for material properties. The heavy hex configuration also provides greater wrenching surface, enabling the application of higher torque values necessary for slip-critical joints. Manufactured to ASME B18.2.1 dimensions, they ensure compatibility with heavy hex nuts and washers in rigorous structural assemblies.

    ASME B18.2.1 Heavy Hex Cap Screws

    ASME B18.2.1 Heavy Hex Cap Screws Dimensions (mm)

    Nominal Size Body Diameter, E Width Across Flats, F Width Across Corners, G Head Height, H Minimum Wrenching Height, J Thread Length, LT Transition Thread Length, Y Maximum Total Runout of Bearing Surface FIM
    Max. Min. Basic Max. Min. Max. Min. Basic Max. Min. >6" <6" Reference
    3⁄8 9.53 9.53 9.14 17.46 17.48 16.99 20.17 19.38 5.95 6.17 5.74 4.06 25.40 31.75 7.92 0.36
    1⁄2 12.70 12.70 12.24 22.23 22.23 21.59 25.65 24.61 7.94 8.20 7.67 5.46 31.75 38.10 9.78 0.41
    5⁄8 15.88 15.88 15.37 26.99 26.97 26.19 31.17 29.85 9.92 10.24 9.60 6.83 38.10 44.45 11.56 0.48
    3⁄4 19.05 19.05 18.52 31.75 31.75 30.78 36.65 35.13 11.91 12.27 11.56 8.23 44.45 50.80 12.70 0.56
    7⁄8 22.23 22.23 21.64 36.51 36.53 35.41 42.16 40.36 13.89 14.30 13.49 9.60 50.80 57.15 14.12 0.64
    1 25.40 25.40 24.79 41.28 41.28 40.01 47.65 45.62 15.48 15.93 15.01 10.57 57.15 63.50 15.88 0.71
    1 1⁄8 28.58 28.58 27.89 46.04 46.02 44.60 53.16 50.85 17.46 18.24 16.71 11.71 63.50 69.85 18.14 0.81
    1 1⁄4 31.75 31.75 31.06 50.80 50.80 49.23 58.65 56.11 19.84 20.65 19.02 13.46 69.85 76.20 18.14 0.89
    1 3⁄8 34.93 34.93 34.16 55.56 55.58 53.82 64.16 61.37 21.43 22.30 20.57 14.45 76.20 82.55 21.16 0.97
    1 1⁄2 38.10 38.10 37.34 60.33 60.33 58.42 69.65 66.60 23.81 24.74 22.91 16.26 82.55 88.90 21.16 1.04
    1 5⁄8 41.28 41.28 40.41 65.09 65.07 63.02 75.16 71.86 25.40 26.37 24.43 17.63 88.90 95.25 23.09 1.12
    1 3⁄4 44.45 44.45 43.59 69.85 69.85 67.61 80.65 77.09 27.78 28.80 26.77 19.00 95.25 101.60 25.40 1.22
    1 7⁄8 47.63 47.63 46.71 74.61 74.63 72.24 86.16 82.35 29.37 30.43 28.30 20.37 101.60 107.95 25.40 1.32
    2 50.80 50.80 49.89 79.38 79.38 76.84 91.64 87.60 30.96 32.08 29.85 20.96 107.95 114.30 28.22 1.40
    2 1⁄4 57.15 57.15 56.24 88.90 88.90 86.06 102.64 98.09 34.93 36.14 33.71 23.70 ... 127.00 28.22 1.55
    2 1⁄2 63.50 63.50 62.51 98.43 98.43 95.25 113.64 108.59 30.96 40.21 37.57 26.47 ... 139.70 31.75 1.73
    2 3⁄4 69.85 69.85 68.86 107.95 107.95 104.44 124.64 119.08 26.99 44.30 41.45 29.24 ... 152.40 31.75 1.88
    3 76.20 76.20 75.21 117.48 117.48 113.67 135.64 129.59 47.63 49.15 46.10 32.77 ... 165.10 31.75 2.06
    3 1⁄4 82.55 82.55 81.53 127.00 127.00 122.89 146.66 140.08 50.80 54.00 50.75 35.53 ... 177.80 31.75 2.31
    3 1⁄2 88.90 88.90 87.91 136.53 136.53 132.08 157.66 150.57 57.15 57.30 53.85 37.69 ... 190.50 31.75 2.49
    3 3⁄4 95.25 95.25 94.26 146.05 146.05 141.27 168.66 161.06 60.33 62.15 58.50 40.94 ... 203.20 31.75 2.67
    4 101.60 101.60 100.61 155.58 155.58 150.50 179.65 171.58 63.50 65.43 61.57 43.10 ... 215.90 31.75 2.84
    4 1⁄4 107.95 107.95 107.26 165.10 165.10 159.72 190.65 182.07 69.85 70.31 66.24 46.38 ... 228.60 31.75 3.02
    4 1⁄2 114.30 114.30 113.61 174.63 174.63 168.91 201.65 192.56 73.03 73.56 69.29 48.51 ... 241.30 31.75 3.20
    4 3⁄4 120.65 120.65 119.96 184.15 184.15 178.10 212.65 203.05 76.20 78.44 73.96 51.77 ... 254.00 31.75 3.38
    5 127.00 127.00 126.30 193.68 193.68 187.33 223.65 213.56 79.38 81.71 77.04 53.92 ... 266.70 31.75 3.56
    5 1⁄4 133.35 133.35 132.65 203.20 203.20 196.55 234.65 224.05 85.73 86.56 81.69 57.18 ... 279.40 31.75 3.73
    5 1⁄2 139.70 139.70 139.00 212.73 212.73 205.74 245.64 234.54 88.90 89.87 84.79 59.36 ... 292.10 31.75 3.91
    5 3⁄4 146.05 146.05 145.35 222.25 222.25 214.93 256.64 245.03 92.08 94.72 89.43 62.61 ... 304.80 31.75 4.09
    6 152.40 152.40 151.70 231.78 231.78 224.16 267.64 255.52 95.25 97.99 92.51 64.74 ... 317.50 31.75 4.27
    • GENERAL NOTES:
      • (a) Wrenching Height, J. Wrenching height is a distance measured from the bearing surface up the side of the head at the corners. The width across corners shall be within specified limits for the full wrenching height.
      • (b) Refer to section 4 for further information.

    ASME B18.2.1 Heavy Hex Cap 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 Heavy Hex Cap Screws Dimensions (Inch)

    Nominal Size Body Diameter, E Width Across Flats, F Width Across Corners, G Head Height, H Minimum Wrenching Height, J Thread Length, LT Transition Thread Length, Y Maximum Total Runout of Bearing Surface FIM
    Max. Min. Basic Max. Min. Max. Min. Basic Max. Min. ">6"" "<6""< /th> Reference
    3⁄8 0.375 0.375 0.36 11⁄16 0.688 0.669 0.794 0.763 15⁄64 0.243 0.226 0.16 1 1.25 0.312 0.014
    1⁄2 0.5 0.5 0.482 7⁄8 0.875 0.85 1.01 0.969 5⁄16 0.323 0.302 0.215 1.25 1.5 0.385 0.016
    5⁄8 0.625 0.625 0.605 1 1⁄16 1.062 1.031 1.227 1.175 25⁄64 0.403 0.378 0.269 1.5 1.75 0.455 0.019
    3⁄4 0.75 0.75 0.729 1 1⁄4 1.25 1.212 1.443 1.383 15⁄32 0.483 0.455 0.324 1.75 2 0.5 0.022
    7⁄8 0.875 0.875 0.852 1 7⁄16 1.438 1.394 1.66 1.589 35⁄64 0.563 0.531 0.378 2 2.25 0.556 0.025
    1 1 1 0.976 1 5⁄8 1.625 1.575 1.876 1.796 39⁄64 0.627 0.591 0.416 2.25 2.5 0.625 0.028
    1 1⁄8 1.125 1.125 1.098 1 13⁄16 1.812 1.756 2.093 2.002 11⁄16 0.718 0.658 0.461 2.5 2.75 0.714 0.032
    1 1⁄4 1.25 1.25 1.223 2 2 1.938 2.309 2.209 25⁄32 0.813 0.749 0.53 2.75 3 0.714 0.035
    1 3⁄8 1.375 1.375 1.345 2 3⁄16 2.188 2.119 2.526 2.416 27⁄32 0.878 0.81 0.569 3 3.25 0.833 0.038
    1 1⁄2 1.5 1.5 1.47 2 3⁄8 2.375 2.3 2.742 2.622 15⁄16 0.974 0.902 0.64 3.25 3.5 0.833 0.041
    1 5⁄8 1.625 1.625 1.591 2 9⁄16 2.562 2.481 2.959 2.829 1 1.038 0.962 0.694 3.5 3.75 0.909 0.044
    13⁄4 1.75 1.75 1.716 2 3⁄4 2.75 2.662 3.175 3.035 1 3⁄32 1.134 1.054 0.748 3.75 4 1 0.048
    1 7⁄8 1.875 1.875 1.839 2 15⁄16 2.938 2.844 3.392 3.242 1 5⁄32 1.198 1.114 0.802 4 4.25 1 0.052
    2 2 2 1.964 3 1⁄8 3.125 3.025 3.608 3.449 1 7⁄32 1.263 1.175 0.825 4.25 4.5 1.111 0.055
    2 1⁄4 2.25 2.25 2.214 3 1⁄2 3.5 3.388 4.041 3.862 1 3⁄8 1.423 1.327 0.933 ... 5 1.111 0.061
    2 1⁄2 2.5 2.5 2.461 3 7⁄8 3.875 3.75 4.474 4.275 1 7⁄32 1.583 1.479 1.042 ... 5.5 1.25 0.068
    2 3⁄4 2.75 2.75 2.711 4 1⁄4 4.25 4.112 4.907 4.688 1 1⁄16 1.744 1.632 1.151 ... 6 1.25 0.074
    3 3 3 2.961 4 5⁄8 4.625 4.475 5.34 5.102 1 7⁄8 1.935 1.815 1.29 ... 6.5 1.25 0.081
    3 1⁄4 3.25 3.25 3.21 5 5 4.838 5.774 5.515 2 2.126 1.998 1.399 ... 7 1.25 0.091
    31⁄2 3.5 3.5 3.461 5 3⁄8 5.375 5.2 6.207 5.928 2 1⁄4 2.256 2.12 1.484 ... 7.5 1.25 0.098
    3 3⁄4 3.75 3.75 3.7109 5 3⁄4 5.75 5.562 6.64 6.341 2 3⁄8 2.447 2.303 1.612 ... 8 1.25 0.105
    4 4 4 3.9609 6 1⁄8 6.125 5.925 7.073 6.755 2 1⁄2 2.576 2.424 1.697 ... 8.5 1.25 0.112
    4 1⁄4 4.25 4.25 4.2228 6 1⁄2 6.5 6.288 7.506 7.168 2 3⁄4 2.768 2.608 1.826 ... 9 1.25 0.119
    4 1⁄2 4.5 4.5 4.4727 6 7⁄8 6.875 6.65 7.939 7.581 2 7⁄8 2.896 2.728 1.91 ... 9.5 1.25 0.126
    4 3⁄4 4.75 4.75 4.7227 7 1⁄4 7.25 7.012 8.372 7.994 3 3.088 2.912 2.038 ... 10 1.25 0.133
    5 5 5 4.9726 7 5⁄8 7.625 7.375 8.805 8.408 3 1⁄8 3.217 3.033 2.123 ... 10.5 1.25 0.14
    51⁄4 5.25 5.25 5.2226 8 8 7.738 9.238 8.821 3 3⁄8 3.408 3.216 2.251 ... 11 1.25 0.147
    51⁄2 5.5 5.5 5.4726 8 3⁄8 8.375 8.1 9.671 9.234 3 1⁄2 3.538 3.338 2.337 ... 11.5 1.25 0.154
    53⁄4 5.75 5.75 5.7225 8 3⁄4 8.75 8.462 10.104 9.647 3 5⁄8 3.729 3.521 2.465 ... 12 1.25 0.161
    6 6 6 5.9725 9 1⁄8 9.125 8.825 10.537 10.06 3 3⁄4 3.858 3.642 2.549 ... 12.5 1.25 0.168
    • GENERAL NOTES:
      • (a) Wrenching Height, J. Wrenching height is a distance measured from the bearing surface up the side of the head at the corners. The width across corners shall be within specified limits for the full wrenching height.
      • (b) Refer to section 4 for further information.

    ASME B18.2.1 Heavy Hex Cap Screws 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 heavy hex cap screws cover?
    ASME B18.2.1 is the inch dimensional standard for heavy hex cap screws. It defines head dimensions, body diameter, thread length, total length, drive recess, and tolerances so any compliant heavy hex cap screws interchanges with standard nuts, washers and wrenches across manufacturers.
    What size range is available for heavy hex cap screws under ASME B18.2.1?
    heavy hex cap screws under ASME B18.2.1 covers 1/4 in to 4 in nominal diameter with controlled head dimensions. 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 heavy hex cap screws?
    The companion specification for heavy hex cap screws is ASME B18.2.3.5 metric series. Both standards share equivalent functional dimensions but use the corresponding measurement system (inch vs metric ISO M-series).
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