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    2. B18.2.1 Heavy Hex Cap Screw

    ASME B18.2.1 Heavy Hex Cap Screw – Dimensions, Specifications & Standards

    Heavy Hex Cap Screws for High-Load & High-Temperature Service.

    ASME B18.2.1 Heavy Hex Cap Screws are robust inch-series fasteners designed with a larger head footprint than standard hex cap screws, offering enhanced bearing surface and greater load distribution — ideal for critical structural, pressure vessel, and flange bolting. The standard covers sizes from 3 inches up to 6 inches in diameter, including head height, width across flats, wrenching height, underhead fillets, and washer face thickness. In high-performance applications, these screws comply with ASTM A1014/A1014M for UNS N07718 bolting, ensuring superior tensile strength, fatigue resistance, and corrosion resistance at elevated temperatures. Also available in ASTM A193 and ASTM A320 grades for high-temperature and low-temperature service. The dimensional standard does not address chemistry or mechanical strength — these are defined by the referenced ASTM specification.

    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
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