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    2. ASME B18.2.1 Hex Cap Screws

    ASTM A1082 Hex Cap Screws: ASME B18.2.1 Dimensions

    Precision Inch-Series Cap Screws with Washer Face Bearing Surface for Controlled Clamping in Industrial Bolted Joints.

    ASME B18.2.1 Hex Cap Screws are inch-series threaded fasteners distinguished from standard hex bolts by their closer body diameter tolerances, mandatory washer face on the bearing surface, and the requirement for a fully threaded or specified partial thread length, features that combine to provide more precise clamping load control and superior bearing surface quality in bolted connections where tight dimensional tolerances and consistent clamp force are critical to joint performance. The ASME B18.2.1 standard defines precise dimensional requirements for hex cap screws including head height, width across flats, washer face thickness, body diameter, bearing surface runout (FIM), thread length, underhead fillet radii, and all manufacturing tolerances for nominal sizes from 1/4 inch to 3 inches. When manufactured to ASTM A1082, the specification for high-strength stainless steel bolting material, these hex cap screws deliver excellent corrosion resistance combined with the close dimensional tolerances and controlled bearing surface required in precision machine assemblies, pharmaceutical processing equipment, food-grade machinery, architectural metal connections, and marine structural assemblies. The washer face bearing surface ensures full 360° contact between the screw head and the mating surface, reducing the risk of head embedment and providing more consistent preload development compared to standard hex bolts during controlled torque installation. ASTM A1082 hex cap screws are available with full EN 10204 Type 3.1 certification for quality-critical industrial procurement.

    ASME B18.2.1 Hex Cap Screws

    ASME B18.2.1 Hex Cap Screws Dimensions (mm)

    Nominal Size or Basic Product Diameter Body Diameter, E Width Across Flats, F Width Across Corners, G Head Height, H Minimum Wrenching Height, J Nominal Thread Length for Screw Lengths, LT Transition Thread Length, Y
    Reference    		 Maximum Total Runout of Bearing Surface FIM
    Max. Min. Basic Max. Min. Max. Min. Basic Max. Min. 150mm (6") and Shorter Over 150mm (6")
    1/4 0.2500 6.350 6.223 11.113 11.125 10.871 12.827 12.395 3.969 4.140 3.810 2.692 19.050 25.400 6.350 0.254
    5/16 0.3125 7.938 7.785 12.700 12.700 12.421 14.656 14.148 5.159 5.359 4.953 3.556 22.225 28.575 7.061 0.279
    3/8 0.3750 9.525 9.373 14.288 14.275 13.995 16.510 15.951 5.953 6.172 5.740 4.064 25.400 31.750 7.925 0.305
    7/16 0.4375 11.113 10.935 15.875 15.875 15.545 18.339 17.729 7.144 7.391 6.909 4.953 28.575 34.925 9.068 0.330
    1/2 0.5000 12.700 12.522 19.050 19.050 18.694 21.996 21.336 7.938 8.204 7.671 5.461 31.750 38.100 9.779 0.356
    9/16 0.5625 14.288 14.084 20.638 20.625 20.269 23.825 23.114 9.128 9.423 8.839 6.350 34.925 41.275 10.592 0.381
    5/8 0.6250 15.875 15.672 23.813 23.825 23.419 27.508 26.695 9.922 10.236 9.601 6.833 38.100 44.450 11.557 0.432
    3/4 0.7500 19.050 18.821 28.575 28.575 27.940 32.995 31.852 11.906 12.268 11.557 8.230 44.450 50.800 12.700 0.508
    7/8 0.8750 22.225 21.996 33.338 33.325 32.639 38.506 37.211 13.891 14.300 13.487 9.601 50.800 57.150 14.122 0.584
    1 1.0000 25.400 25.146 38.100 38.100 37.313 43.993 42.545 15.478 15.926 15.011 10.566 57.150 63.500 15.875 0.660
    1 1/8 1.1250 28.575 28.296 42.863 42.875 41.427 49.505 47.219 17.463 18.237 16.713 11.709 63.500 69.850 18.136 0.737
    1 1/4 1.2500 31.750 31.471 47.625 47.625 46.025 54.991 52.476 19.844 20.650 19.025 13.462 69.850 76.200 18.136 0.838
    1 3/8 1.3750 34.925 34.620 52.388 52.375 50.648 60.503 57.734 21.431 22.301 20.574 14.453 76.200 82.550 21.158 0.914
    1 1/2 1.5000 38.100 37.795 57.150 57.150 55.245 65.989 62.992 23.813 24.740 22.911 16.256 82.550 88.900 21.158 0.991
    1 5/8 1.6250 41.275 40.970 61.913 61.925 59.842 71.501 68.224 25.400 26.365 24.435 17.628 88.900 95.250 23.089 1.092
    1 3/4 1.7500 44.450 44.145 66.675 66.675 64.465 76.987 73.482 27.781 28.804 26.772 18.999 95.250 101.600 25.400 1.168
    1 7/8 1.8750 47.625 47.320 71.438 71.425 69.063 82.499 78.715 29.369 30.429 28.296 20.371 101.600 107.950 25.400 1.245
    2 2.0000 50.800 50.495 76.200 76.200 73.660 87.986 83.972 30.956 32.080 29.845 20.955 107.950 114.300 28.219 1.321
    2 1/4 2.2500 57.150 56.845 85.725 85.725 82.855 98.984 94.463 34.925 36.144 33.706 23.698 ... 127.000 28.219 1.499
    2 1/2 2.5000 63.500 63.195 95.250 95.250 92.075 109.982 104.978 40.481 40.208 37.567 26.467 ... 139.700 31.750 1.651
    2 3/4 2.7500 69.850 69.545 104.775 104.775 101.295 120.980 115.468 42.863 44.298 41.453 29.235 ... 152.400 31.750 1.829
    3 3.0000 76.200 75.895 114.300 114.300 110.490 131.978 125.959 47.625 49.149 46.101 32.766 ... 165.100 31.750 2.007

    ASME B18.2.1 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 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 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 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 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) 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 hex cap screws tabulated on this page
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