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    2. ASME B18.2.2 Regular Hex Nut Dimensional Standard

    Hex Nuts ASME B18.2.2 — Sizes, Dimensions, Standards

    ASME B18.2.2 Regular Hex Nut documentation offers authoritative engineering data, detailing essential dimensional parameters including Width Across Flats, Width Across Corners, and overall thickness.
    The regular hex nut is unequivocally the foundational fastening component of modern mechanical engineering, serving as the standard counterpart to Unified National Coarse (UNC) and Unified National Fine (UNF) machine screws and bolts.
    Under the extensive ASME B18.2.2 specification, the regular hex nut is meticulously engineered to provide an optimal mathematical balance between material volume, internal thread shear strength, and rotational wrench clearance.
    Unlike the heavy hex variant designed for extreme structural loads, the regular hex nut features a moderate Width Across Flats (WAF) and thickness profile, making it ideally suited for general fastening applications across commercial machinery, automotive assemblies, and everyday industrial equipment where spatial efficiency is prioritized over absolute extreme clamping force.
    The dimensional integrity of the ASME B18.2.2 hex nut is maintained through strict manufacturing tolerances governing the Width Across Flats (F), the Width Across Corners (G), and the overall Thickness (H).
    To ensure that a bolted joint tightens uniformly without inducing off-axis bending moments into the bolt shank, the standard dictates that the axis of the tapped hole must be located at true position with respect to the axis of the nut body, constrained within highly specific diametric tolerance zones.
    Furthermore, the standard permits morphological options such as double chamfering (where both top and bottom faces are angled) or a singular washer-faced bearing surface with a chamfered top.
    This washer face is critical in dynamic assemblies, as it prevents the sharp hexagonal corners of the rotating nut from scoring, gouging, or galling the mating substrate during high-speed pneumatic torque applications.
    From a metallurgical and structural performance standpoint, plain and zinc-plated carbon steel hex nuts are typically evaluated based on Proof Load Stress—the absolute maximum tensile force the internal threads of the nut can withstand before undergoing permanent plastic deformation.
    Nuts manufactured to the ubiquitous ASTM A563 Grade A specification possess a proof load stress of approximately 90 ksi in their plain, unplated condition.
    However, the introduction of a zinc plating process alters the material dynamics and hydrogen embrittlement risks, subsequently reducing the rated proof load stress to 68 ksi.
    For highly corrosive environments requiring austenitic alloys, stainless steel variants governed by ASTM F594 exhibit proof load stresses beginning at 70 ksi, fluctuating depending on the specific alloy group and cold-working condition.
    Maintaining exact dimensional compliance with ASME B18.2.2 ensures that the internal thread shear area is perfectly calibrated against the corresponding bolt's tensile yield point.
    In proper mechanical design, the geometry of the nut ensures that an over-torqued assembly will result in the yielding or fracture of the bolt shank rather than the sudden, catastrophic stripping of the nut threads.

    ASME B18.2.2 Hex Nuts

    ASME B18.2.2 Hex Nuts Dimensions (mm)

    Nominal Size Basic Major Diameter of Thread Width Across Flats, F Width Across Corners, G Thickness, H Maximum Runout of Bearing Surface
    to Thread Axis, FIM
    Min. Max Min. Max Min. Max Up to 150,000 psi 150,000 psi and Greater
    1/4 6.35 10.8712 11.1252 12.3952 12.827 5.3848 5.7404 0.381 0.254
    5/16 7.9375 12.4206 12.7 14.1478 14.6558 6.5532 6.9342 0.4064 0.2794
    3/8 9.525 13.9954 14.3002 15.9512 16.51 8.128 8.5598 0.4318 0.3048
    7/16 11.1125 17.145 17.4752 19.5072 20.1676 9.271 9.779 0.4572 0.3302
    1/2 12.7 18.6944 19.05 21.336 21.9964 10.8458 11.3792 0.4826 0.3556
    9/16 14.2875 21.8694 22.225 24.9428 25.654 12.0142 12.5984 0.508 0.381
    5/8 15.875 23.4188 23.8252 26.6954 27.5082 13.589 14.1986 0.5334 0.4064
    3/4 19.05 27.6352 28.575 31.496 32.9946 15.6718 16.891 0.5842 0.4572
    7/8 22.225 32.2326 33.3248 36.7538 38.5064 18.3896 19.7104 0.635 0.508
    1 25.4 36.83 38.1 41.9862 43.9928 21.1074 22.5298 0.6858 0.5588
    1 1/8 28.575 41.4274 42.8752 47.2186 49.5046 23.8506 25.3746 0.762 0.635
    1 1/4 31.75 46.0248 47.625 52.4764 54.991 26.162 27.7876 0.8382 0.7112
    1 3/8 34.925 50.6476 52.3748 57.7342 60.5028 28.9052 30.6324 0.9144 0.7874
    1 1/2 38.1 55.245 57.15 62.992 65.9892 31.623 33.4518 0.9906 0.8636
    1 5/8 41.275 59.69 61.722 68.0466 71.247 34.6456 35.9664 1.1176 0.9652
    1 3/4 44.45 64.4652 66.675 73.4822 76.9874 37.084 39.116 1.2192 1.0414
    1 7/8 47.625 69.1388 71.4502 78.8162 82.4738 39.8018 41.9354 1.2954 1.1176
    2 50.8 73.66 76.2 83.9724 87.9856 42.545 44.7802 1.397 1.1938
    2 1/4 57.15 82.8802 85.725 94.4626 98.9838 48.006 50.4444 1.5494 1.3208
    2 1/2 63.5 92.075 95.25 104.9782 109.982 53.467 56.1086 1.7272 1.4732
    2 3/4 69.85 101.2952 104.775 115.4684 120.9802 58.9026 61.7474 1.8796 1.6256
    3 76.2 110.49 114.3 125.9586 131.9784 64.3636 67.4116 2.0574 1.778
    3 1/4 82.55 119.7102 123.825 136.4742 142.9766 69.8246 73.0758 2.2098 1.905
    3 1/2 88.9 128.905 133.35 146.9644 153.9748 75.2856 78.74 2.3876 2.0574
    3 3/4 95.25 138.1252 142.875 157.4546 164.973 80.7212 84.3788 2.54 2.2098
    4 101.6 147.32 152.4 167.9448 175.9712 86.1822 90.043 2.7178 2.3622

    ASME B18.2.2 Hex Nuts Dimensions (Inch)

    Nominal Size Basic Major Diameter of Thread Width Across Flats, F Width Across Corners, G Thickness, H Maximum Runout of Bearing Surface
    to Thread Axis, FIM
    Min. Max Min. Max Min. Max Up to 150,000 psi 150,000 psi and Greater
    1/4 0.2500 0.428 0.438 0.488 0.505 0.212 0.226 0.015 0.010
    5/16 0.3125 0.489 0.500 0.557 0.577 0.258 0.273 0.016 0.011
    3/8 0.3750 0.551 0.563 0.628 0.650 0.320 0.337 0.017 0.012
    7/16 0.4375 0.675 0.688 0.768 0.794 0.365 0.385 0.018 0.013
    1/2 0.5000 0.736 0.750 0.840 0.866 0.427 0.448 0.019 0.014
    9/16 0.5625 0.861 0.875 0.982 1.010 0.473 0.496 0.020 0.015
    5/8 0.6250 0.922 0.938 1.051 1.083 0.535 0.559 0.021 0.016
    3/4 0.7500 1.088 1.125 1.240 1.299 0.617 0.665 0.023 0.018
    7/8 0.8750 1.269 1.312 1.447 1.516 0.724 0.776 0.025 0.020
    1 1.0000 1.450 1.500 1.653 1.732 0.831 0.887 0.027 0.022
    1 1/8 1.1250 1.631 1.688 1.859 1.949 0.939 0.999 0.030 0.025
    1 1/4 1.2500 1.812 1.875 2.066 2.165 1.030 1.094 0.033 0.028
    1 3/8 1.3750 1.994 2.062 2.273 2.382 1.138 1.206 0.036 0.031
    1 1/2 1.5000 2.175 2.250 2.480 2.598 1.245 1.317 0.039 0.034
    1 5/8 1.6250 2.35 2.43 2.679 2.805 1.364 1.416 0.044 0.038
    1 3/4 1.7500 2.538 2.625 2.893 3.031 1.460 1.540 0.048 0.041
    1 7/8 1.8750 2.722 2.813 3.103 3.247 1.567 1.651 0.051 0.044
    2 2.0000 2.900 3.000 3.306 3.464 1.675 1.763 0.055 0.047
    2 1/4 2.2500 3.263 3.375 3.719 3.897 1.890 1.986 0.061 0.052
    2 1/2 2.5000 3.625 3.750 4.133 4.330 2.105 2.209 0.068 0.058
    2 3/4 2.7500 3.988 4.125 4.546 4.763 2.319 2.431 0.074 0.064
    3 3.0000 4.350 4.500 4.959 5.196 2.534 2.654 0.081 0.070
    3 1/4 3.2500 4.713 4.875 5.373 5.629 2.749 2.877 0.087 0.075
    3 1/2 3.5000 5.075 5.250 5.786 6.062 2.964 3.100 0.094 0.081
    3 3/4 3.7500 5.438 5.625 6.199 6.495 3.178 3.322 0.100 0.087
    4 4.0000 5.800 6.000 6.612 6.928 3.393 3.545 0.107 0.093

    Related A194 Grades & References

    ASME B18.2.2 hex nuts are typically manufactured in A194 Grade 2H (most common, Q&T carbon steel), Grade 8 (304 stainless), and Grade 8M (316 stainless). Heavy-duty pressure-vessel service prefers heavy hex nuts. For metric equivalents, see ISO 4032 or ASME B18.2.4.1M. Full specification: ASTM A194 PDF. A194 hub.

    Frequently Asked Questions

    What is the width across flats for a 1-inch hex nut per ASME B18.2.2?

    The width across flats (F) for a 1-inch hex nut per ASME B18.2.2 is 1-1/2 inches (1.500 in basic). For finished hex nuts the thickness is 55/64 inch (0.859 in); for heavy hex nuts the thickness is 63/64 inch (0.984 in) and width across flats is 1-5/8 inch.

    What is the difference between hex nuts and heavy hex nuts?

    Heavy hex nuts have a larger width across flats and greater thickness than standard hex nuts of the same nominal size, providing more thread engagement and load-bearing surface. Heavy hex nuts are required for ASTM A194 Grade 2H, 4, 7, 8 in high-pressure or high-temperature bolting applications.

    Which thread series do ASME B18.2.2 hex nuts use?

    ASME B18.2.2 hex nuts use Unified Coarse (UNC), Unified Fine (UNF), or 8-thread Series (8UN) thread series per ASME B1.1. Heavy hex nuts 1 inch and larger typically use 8UN series matching ASTM A193 stud bolt threading.

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