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    2. ASME B18.2.1 Heavy Hex Cap Screws

    ASTM A1082 Heavy Hex Cap Screws: ASME B18.2.1 Dimensions

    Robust Inch-Series Cap Screws with Washer Face and Wide Heavy Hex Head for High-Load Process and Industrial Service.

    ASME B18.2.1 Heavy Hex Cap Screws are inch-series threaded fasteners that combine the precise dimensional accuracy and washer-face bearing surface of a cap screw with the wider hexagonal head profile (larger width across flats, same head height) of a heavy hex configuration, delivering superior clamping area, enhanced torque resistance, and improved load distribution compared to standard hex cap screws in heavy-duty mechanical, structural, and process equipment assemblies. The standard defines precise dimensional requirements for head height, width across flats, body diameter, thread length, washer face thickness, bearing surface runout (FIM), and underhead fillet radii for heavy hex cap screws from 3/8 inch to 6 inches in diameter, ensuring tight interchangeability across manufacturers. In demanding service environments, these cap screws are manufactured to ASTM A1082, the specification for high-strength stainless steel bolting material, which provides excellent corrosion resistance, sustained mechanical properties at elevated temperatures, and reliable performance in aggressive chemical and marine environments. They are particularly well-suited for pharmaceutical processing equipment, food-grade machinery, chemical reactors, and offshore structural assemblies where both strength and corrosion resistance are simultaneously critical. All heavy hex cap screws must comply with ASME B18.2.1 dimensional requirements while being independently assessed for mechanical and chemical properties per the ASTM A1082 material 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

    Frequently Asked Questions

    Which stainless steel grades does ASTM A1082 cover for heavy hex cap screws?

    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.

    Which standard sets the dimensions of these heavy hex cap screws?

    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).

    Can ASTM A1082 heavy hex cap screws be used in sour service (NACE MR0175 / ISO 15156)?

    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.

    How are ASTM A1082 heavy hex cap screws marked?

    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.

    What are the ASTM A1082 requirements for super duplex 2507 (S32750) heavy hex cap screws?

    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).

    References & Standards

    • ASTM A1082/A1082M: Standard Specification for High Strength Precipitation Hardening and Duplex Stainless Steel Bolting for Special Purpose Applications (2016 edition)
    • ASTM A962/A962M: Specification for Common Requirements for Bolting Intended for Use at Any Temperature from Cryogenic to the Creep Range
    • ASTM A370: Test Methods and Definitions for Mechanical Testing of Steel Products
    • ISO 15156-3 (NACE MR0175/ISO 15156): Cracking-resistant CRAs (corrosion-resistant alloys) and other alloys for H2S-containing environments in oil and gas production
    • ASME B18.2.1: dimensional standard for the heavy hex cap screws tabulated on this page
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