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    2. ASME B18.2.1 Heavy Hex Screws

    ASTM A1082 Heavy Hex Screws: ASME B18.2.1 Dimensions

    Inch-Series Wide-Head Fasteners With Washer Face for Superior Bearing Area in Heavy Industrial Connections.

    ASME B18.2.1 Heavy Hex Screws are inch-series externally threaded fasteners featuring a wider hexagonal head (larger width across flats, same head height) than standard hex cap screws, specifically designed to provide a larger bearing footprint and greater resistance to loosening under heavy dynamic and static loads in structural steel connections, pressure equipment, and heavy-duty industrial bolted assemblies. In ASME B18.2.1 these are the same product as heavy hex cap screws, and they have a washer face on the bearing surface, 0.015 in. to 0.025 in. thick for sizes 3/4 in. and smaller and 0.015 in. to 0.035 in. thick for larger sizes, with bearing surface runout controlled by FIM limits. ASME B18.2.1 governs all dimensional requirements for heavy hex screws including head height, width across flats, body diameter, thread series, thread length, and associated tolerances for sizes ranging from 3/8 inch to 6 inches. When manufactured to ASTM A1082, the specification for high-strength stainless steel bolting material, these heavy hex screws provide outstanding corrosion resistance paired with high tensile strength, making them suitable for marine infrastructure, chemical processing equipment, coastal architectural structures, and industrial assemblies exposed to aggressive atmospheric and chemical conditions. Proper specification of both ASME B18.2.1 for dimensional compliance and ASTM A1082 for material and mechanical properties is essential when heavy hex screws are used in safety-critical or high-load service applications.

    ASME B18.2.1 Hex Screws

    ASME B18.2.1 Heavy Hex Screws Dimensions (mm)

    Nominal Size
    (Inch)    		 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, mm)    		 Maximum Total Runout of Bearing Surface FIM
    (mm)
    Frac. Dec. Max. Min. Basic Max. Min. Max. Min. Basic Max. Min. 6 in. and Shorter Over 6 in.
    3/8 0.3750 9.525 9.144 17.463 17.475 16.993 20.168 19.380 5.953 6.172 5.740 4.064 25.400 31.750 7.925 0.356
    1/2 0.5000 12.700 12.243 22.225 22.225 21.590 25.654 24.613 7.938 8.204 7.671 5.461 31.750 38.100 9.779 0.406
    5/8 0.6250 15.875 15.367 26.988 26.975 26.187 31.166 29.845 9.922 10.236 9.601 6.833 38.100 44.450 11.557 0.483
    3/4 0.7500 19.050 18.517 31.750 31.750 30.785 36.652 35.128 11.906 12.268 11.557 8.230 44.450 50.800 12.700 0.559
    7/8 0.8750 22.225 21.641 36.513 36.525 35.408 42.164 40.361 13.891 14.300 13.487 9.601 50.800 57.150 14.122 0.635
    1 1.0000 25.400 24.790 41.275 41.275 40.005 47.650 45.618 15.478 15.926 15.011 10.566 57.150 63.500 15.875 0.711
    1 1/8 1.1250 28.575 27.889 46.038 46.025 44.602 53.162 50.851 17.463 18.237 16.713 11.709 63.500 69.850 18.136 0.813
    1 1/4 1.2500 31.750 31.064 50.800 50.800 49.225 58.649 56.109 19.844 20.650 19.025 13.462 69.850 76.200 18.136 0.889
    1 3/8 1.3750 34.925 34.163 55.563 55.575 53.823 64.160 61.366 21.431 22.301 20.574 14.453 76.200 82.550 21.158 0.965
    1 1/2 1.5000 38.100 37.338 60.325 60.325 58.420 69.647 66.599 23.813 24.740 22.911 16.256 82.550 88.900 21.158 1.041
    1 5/8 1.6250 41.275 40.411 65.088 65.075 63.017 75.159 71.857 25.400 26.365 24.435 17.628 88.900 95.250 23.089 1.118
    1 3/4 1.7500 44.450 43.586 69.850 69.850 67.615 80.645 77.089 27.781 28.804 26.772 18.999 95.250 101.600 25.400 1.219
    1 7/8 1.8750 47.625 46.711 74.613 74.625 72.238 86.157 82.347 29.369 30.429 28.296 20.371 101.600 107.950 25.400 1.321
    2 2.0000 50.800 49.886 79.375 79.375 76.835 91.643 87.605 30.956 32.080 29.845 20.955 107.950 114.300 28.219 1.397
    2 1/4 2.2500 57.150 56.236 88.900 88.900 86.055 102.641 98.095 34.925 36.144 33.706 23.698 ... 127.000 28.219 1.549
    2 1/2 2.5000 63.500 62.509 98.425 98.425 95.250 113.640 108.585 38.894 40.208 37.567 26.467 ... 139.700 31.750 1.727
    2 3/4 2.7500 69.850 68.859 107.950 107.950 104.445 124.638 119.075 42.863 44.298 41.453 29.235 ... 152.400 31.750 1.880
    3 3.0000 76.200 75.209 117.475 117.475 113.665 135.636 129.591 47.625 49.149 46.101 32.766 ... 165.100 31.750 2.057
    3 1/4 3.2500 82.550 81.534 127.000 127.000 122.885 146.660 140.081 50.800 54.000 50.749 35.535 ... 177.800 31.750 2.311
    3 1/2 3.5000 88.900 87.909 136.525 136.525 132.080 157.658 150.571 57.150 57.302 53.848 37.694 ... 190.500 31.750 2.489
    3 3/4 3.7500 95.250 94.257 146.050 146.050 141.275 168.656 161.061 60.325 62.154 58.496 40.945 ... 203.200 31.750 2.667
    4 4.0000 101.600 100.607 155.575 155.575 150.495 179.654 171.577 63.500 65.430 61.570 43.104 ... 215.900 31.750 2.845
    4 1/4 4.2500 107.950 107.259 165.100 165.100 159.715 190.652 182.067 69.850 70.307 66.243 46.380 ... 228.600 31.750 3.023
    4 1/2 4.5000 114.300 113.607 174.625 174.625 168.910 201.651 192.557 73.025 73.558 69.291 48.514 ... 241.300 31.750 3.200
    4 3/4 4.7500 120.650 119.957 184.150 184.150 178.105 212.649 203.048 76.200 78.435 73.965 51.765 ... 254.000 31.750 3.378
    5 5.0000 127.000 126.304 193.675 193.675 187.325 223.647 213.563 79.375 81.712 77.038 53.924 ... 266.700 31.750 3.556
    5 1/4 5.2500 133.350 132.654 203.200 203.200 196.545 234.645 224.053 85.725 86.563 81.686 57.175 ... 279.400 31.750 3.734
    5 1/2 5.5000 139.700 139.004 212.725 212.725 205.740 245.643 234.544 88.900 89.865 84.785 59.360 ... 292.100 31.750 3.912
    5 3/4 5.7500 146.050 145.352 222.250 222.250 214.935 256.642 245.034 92.075 94.717 89.433 62.611 ... 304.800 31.750 4.089
    6 6.0000 152.400 151.702 231.775 231.775 224.155 267.640 255.524 95.250 97.993 92.507 64.745 ... 317.500 31.750 4.267

    ASME B18.2.1 Heavy Hex 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 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 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 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 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 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 screws tabulated on this page
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