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    2. ASME B18.2.1 Hex Bolts

    ASME B18.2.1 Hex Bolts Dimensions

    The Industry-Standard Inch-Series Hex Fastener for Structural, Mechanical, and Process Bolting Worldwide.

    ASME B18.2.1 Hex Bolts are among the most widely used inch-series fasteners across global engineering, construction, and manufacturing industries, governing dimensional requirements for hexagonal head bolts in a broad range of nominal sizes from 1/4 inch to 4 inches diameter, including specifications for head height, width across flats, body diameter, thread length in both UNC and 8UN thread series, and all related dimensional tolerances recognized as American National Standards. ASME B18.2.1 serves as the foundational dimensional reference that most ASTM bolt material specifications defer to, since material specifications focus on chemistry and mechanical properties while B18.2.1 governs the physical form and dimensional compliance of the fastener. When manufactured to ASTM A1082, the specification for high-strength stainless steel bolting material, these hex bolts deliver dimensional precision, excellent tensile strength, and superior resistance to corrosion, chloride attack, and oxidation — making them the preferred grade for marine construction, chemical plant structures, food processing buildings, coastal infrastructure, and industrial facilities where carbon steel bolting cannot provide adequate service life. Both full-size body and reduced-body configurations are available in ASTM A1082 stainless steel, giving design engineers flexibility in choosing between maximum shank strength and improved installation clearance. Specifying ASME B18.2.1 dimensions alongside ASTM A1082 material requirements provides a fully traceable, performance-defined hex bolt for any critical corrosion-resistant bolting application.

    ASME B18.2.1 Hex Bolts

    ASME B18.2.1 Hex Bolts Dimensions (mm)

    Nominal Size 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 >152.4 mm <152.4 mm
    1/4 6.604 6.020 11.113 11.125 10.795 12.827 12.294 4.366 4.775 3.810 0.762 0.254 19.050 25.400
    5/16 8.230 7.569 12.700 12.700 12.294 14.656 14.021 5.556 5.969 4.953 0.762 0.254 22.225 28.575
    3/8 9.855 9.144 14.288 14.275 13.818 16.510 15.748 6.350 6.807 5.740 0.762 0.254 25.400 31.750
    7/16 11.481 10.693 15.875 15.875 15.316 18.339 17.450 7.541 8.026 6.909 0.762 0.254 28.575 34.925
    1/2 13.081 12.243 19.050 19.050 18.415 21.996 20.980 8.731 9.246 7.671 0.762 0.254 31.750 38.100
    5/8 16.307 15.367 23.813 23.825 23.012 27.508 26.238 10.716 11.278 9.601 1.524 0.508 38.100 44.450
    3/4 19.507 18.517 28.575 28.575 27.635 32.995 31.496 12.700 13.310 11.557 1.524 0.508 44.450 50.800
    7/8 22.733 21.641 33.338 33.325 32.233 38.506 36.754 14.684 15.342 13.487 1.524 0.508 50.800 57.150
    1 25.959 24.790 38.100 38.100 36.830 43.993 41.986 17.066 17.780 15.011 2.286 0.762 57.150 63.500
    1 1/8 29.185 27.889 42.863 42.875 41.427 49.505 47.219 19.050 19.812 16.713 2.286 0.762 63.500 69.850
    1 1/4 32.436 31.064 47.625 47.625 46.025 54.991 52.476 21.431 22.250 19.025 2.286 0.762 69.850 76.200
    1 3/8 35.662 34.163 52.388 52.375 50.648 60.503 57.734 23.019 23.876 20.574 2.286 0.762 76.200 82.550
    1 1/2 38.887 37.338 57.150 57.150 55.245 65.989 62.992 25.400 26.314 22.911 2.286 0.762 82.550 88.900
    1 5/8 42.113 40.411 61.913 61.925 59.842 71.501 66.446 26.194 28.346 24.841 2.286 0.762 88.900 95.250
    1 3/4 45.339 43.586 66.675 66.675 64.465 76.987 73.482 27.781 30.378 26.772 3.048 1.016 95.250 101.600
    1 7/8 48.565 46.711 71.438 71.425 69.063 82.499 78.715 31.750 32.410 28.702 3.048 1.016 101.600 107.950
    2 51.791 49.886 76.200 76.200 73.660 87.986 83.972 34.131 35.255 29.845 3.048 1.016 107.950 114.300
    2 1/4 58.547 56.236 85.725 85.725 82.855 98.984 94.463 38.100 39.319 33.706 4.826 1.524 120.650 127.000
    2 1/2 64.999 62.509 95.250 95.250 92.075 109.982 104.978 42.069 43.383 37.567 4.826 1.524 133.350 139.700
    2 3/4 71.806 68.859 104.775 104.775 101.295 120.980 115.468 46.038 47.473 41.453 4.826 1.524 146.050 152.400
    3 78.257 75.209 114.300 114.300 110.490 131.978 125.959 50.800 52.324 46.101 4.826 1.524 158.750 165.100
    3 1/4 84.709 81.534 123.825 123.825 119.685 142.977 136.449 55.563 57.175 49.174 4.826 1.524 171.450 177.800
    3 1/2 91.161 87.909 133.350 133.350 128.905 153.975 146.964 58.738 60.452 52.248 4.826 1.524 184.150 190.500
    3 3/4 97.993 94.640 142.875 142.875 138.100 164.973 157.429 63.500 65.329 56.921 4.826 1.524 196.850 203.200
    4 104.419 100.965 152.400 152.400 147.320 175.971 167.945 68.263 70.206 61.570 4.826 1.524 209.550 215.900

    ASME B18.2.1 Hex Bolts Length Tolerance (mm)

    Nominal Length Size
    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

    What stainless steel grades does ASTM A1082 cover for hex bolts?

    ASTM A1082 covers duplex grades (S31803/2205, S32205/2205, S32750/2507, S32760/Zeron 100, S39277/AL-6XN) and precipitation-hardening grades (S17400/17-4 PH, S17700/17-7 PH, S15700/15-7 PH, S35500/AM-355) - all available for this product family at TorqBolt.

    What is the standard dimensional reference for these hex bolts?

    This product follows ASME B18.2 1, the dimensional standard for hex bolts in stainless steel. Material chemistry, mechanical properties and heat treatment follow ASTM A1082/A1082M. TorqBolt supplies both inch-series and metric sizes per the applicable ASME or ISO drawing.

    Are A1082 hex bolts suitable for sour service (NACE MR0175)?

    Yes, with hardness control. NACE MR0175 / ISO 15156 permits duplex grades S31803, S32205, S32750, S32760 at 28 HRC max for sour service. 17-4 PH (S17400) at H1150 condition (28 HRC) is the most common PH grade qualified for sour service. TorqBolt provides NACE-MR0175 compliant heat-treat reports.

    What is the typical strength of A1082 super-duplex 2507 (S32750) hex bolts?

    Super-duplex 2507 (S32750) per A1082: tensile 116 ksi (800 MPa) min, yield 80 ksi (550 MPa) min, elongation 15% min, 32 HRC max. PREN ≈ 42 - suitable for seawater, brine and chloride-rich pressure-vessel service. Higher strength than austenitic A193 B8M while offering pitting resistance well above 304/316.

    References & Standards

    • ASTM A1082 / A1082M - Standard Specification for High-Strength Precipitation-Hardening and Duplex Stainless Steel Bolting for Special-Purpose Applications (current edition 2016, active)
    • ASTM A962 / A962M - General Requirements for Special-Purpose Bolting Materials
    • ASTM A370 - Mechanical Testing of Steel Products
    • NACE MR0175 / ISO 15156 - Sour-service hardness limits for petroleum & natural gas industries
    • ASME B18 series - Dimensional standards for bolts, screws, nuts, studs
    • ISO 4032 / 4033 / 4035 - Metric hexagon nut dimensional standards
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