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    2. ASME B18.2.1 Heavy Hex Bolts

    ASTM A1082 Heavy Hex Bolts: ASME B18.2.1 Dimensions

    Wide-Head Inch-Series Bolts for High-Load Structural, Flange, and Pipeline Bolted Connections.

    ASME B18.2.1 Heavy Hex Bolts are inch-series fasteners featuring a wider hexagonal head (larger width across flats, same head height) than standard hex bolts, providing an increased bearing surface that distributes clamping load more effectively across the joint and reduces the risk of surface embedment under high preload , making them the preferred choice for heavy structural steel connections, pressure vessel flange bolting, and pipeline assembly joints. The ASME B18.2.1 standard governs all critical dimensional parameters for heavy hex bolts including head height, width across flats, width across corners, body diameter, thread length, fillet radius, and all associated manufacturing tolerances for sizes ranging from 3/8 inch through 3 inches in diameter. When manufactured to ASTM A1082, the specification for high-strength stainless steel bolting material, these bolts deliver excellent tensile strength combined with superior corrosion resistance, making them ideal for coastal structures, chemical processing plants, offshore platforms, and environments where carbon steel bolting would degrade rapidly. The larger head footprint also provides better wrench engagement and torque resistance compared to standard hex bolts, reducing the risk of bolt rotation during installation in high-preload structural bolting applications. Specifying both ASME B18.2.1 for dimensional compliance and ASTM A1082 for material properties ensures a fully defined heavy hex bolt procurement that meets both dimensional and performance requirements.

    Heavy Hexagon Bolt Inch Thread

    ASME B18.2.1 Heavy Hex Bolts Dimensions (mm)

    Nominal Size or Basic Product Diameter Full-Size Body Diameter, E Width Across Flats, F Width Across Corners, G Head Height, H Radius of Fillet, R Nominal Thread Length, LT
    Max. Min. Basic Max. Min. Max. Min. Basic Max. Min. Max. Min. 152.4mm and Shorter Over 152.4mm
    3/8 9.525 9.855 9.144 17.463 17.475 16.993 20.168 19.380 6.350 6.807 5.740 0.762 0.254 25.400 31.750
    1/2 12.700 13.081 12.243 22.225 22.225 21.590 25.654 24.613 8.731 9.246 7.671 0.762 0.254 31.750 38.100
    5/8 15.875 16.307 15.367 26.988 26.975 26.187 31.166 29.845 10.716 11.278 9.601 1.524 0.508 38.100 44.450
    3/4 19.050 19.507 18.517 31.750 31.750 30.785 36.652 35.128 12.700 13.310 11.557 1.524 0.508 44.450 50.800
    7/8 22.225 22.733 21.641 36.513 36.525 35.408 42.164 40.361 14.684 15.342 13.487 1.524 0.508 50.800 57.150
    1 25.400 25.959 24.790 41.275 41.275 40.005 47.650 45.618 17.066 17.780 15.011 2.286 0.762 57.150 63.500
    1 1/8 28.575 29.185 27.889 46.038 46.025 44.602 53.162 50.851 19.050 19.812 16.713 2.286 0.762 63.500 69.850
    1 1/4 31.750 32.436 31.064 50.800 50.800 49.225 58.649 56.109 21.431 22.250 19.025 2.286 0.762 69.850 76.200
    1 3/8 34.925 35.662 34.163 55.563 55.575 53.823 64.160 61.366 23.019 23.876 20.574 2.286 0.762 76.200 82.550
    1 1/2 38.100 38.887 37.338 60.325 60.325 58.420 69.647 66.599 25.400 26.314 22.911 2.286 0.762 82.550 88.900
    1 5/8 41.275 42.113 40.411 65.088 65.075 63.017 75.159 71.857 27.781 28.346 24.841 2.286 0.762 88.900 95.250
    1 3/4 44.450 45.339 43.586 69.850 69.850 67.615 80.645 77.089 29.369 30.378 26.772 3.048 1.016 95.250 101.600
    1 7/8 47.625 48.565 46.711 74.613 74.625 72.238 86.157 82.347 31.750 32.410 28.702 3.048 1.016 101.600 107.950
    2 50.800 51.791 49.886 79.375 79.375 76.835 91.643 87.605 34.131 35.255 29.845 3.048 1.016 107.950 114.300
    2 1/4 57.150 58.547 56.236 88.900 88.900 86.055 102.641 98.095 38.100 39.319 33.706 4.826 1.524 120.650 127.000
    2 1/2 63.500 64.999 62.509 98.425 98.425 95.250 113.640 108.585 42.069 43.383 37.567 4.826 1.524 133.350 139.700
    2 3/4 69.850 71.806 68.859 107.950 107.950 104.445 124.638 119.075 46.038 47.473 41.453 4.826 1.524 146.050 152.400
    3 76.200 78.257 75.209 117.475 117.475 113.665 135.636 129.591 50.800 52.324 46.101 4.826 1.524 158.750 165.100

    ASME B18.2.1 Heavy Hex Bolts 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. +0.508
    0.762    		 +0.508
    0.762    		 +0.508
    0.762    		 ... ... ...
    Over 25.4 mm to 63.5 mm, incl. +0.508
    1.016    		 +1.016
    1.524    		 +1.524
    2.032    		 +2.032
    2.54    		 +3.048
    3.048    		 +4.572
    4.572
    Over 63.5 mm to 101.6 mm, incl. +1.016
    1.524    		 +1.524
    2.032    		 +2.032
    2.54    		 +2.54
    3.556    		 +4.064
    4.064    		 +5.08
    5.08
    Over 101.6 mm to 152.4 mm, incl. +1.524
    2.54    		 +2.032
    2.54    		 +2.54
    2.54    		 +3.048
    4.064    		 +4.572
    4.572    		 +5.588
    5.588
    Longer than 152.4 mm +2.54
    4.572    		 +3.048
    4.572    		 +3.556
    4.572    		 +4.064
    5.08    		 +5.588
    5.588    		 +6.096
    6.096

    Frequently Asked Questions

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

    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 bolts?

    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 bolts 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 bolts 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 bolts?

    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 bolts tabulated on this page
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