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    2. ASME B18.2.2 Square Nut Dimensional Data and Standard

    Square Nuts | ASME B18.2.2 Dimensions

    ASME B18.2.2 Square Nut provides comprehensive engineering specifications, tolerance limits, and dimensional charts for American National Standard square nuts under the ASME B18.2.2 framework.
    The square nut, while largely superseded by the hexagon nut in modern high-density mechanical assemblies, remains a highly relevant component in specific industrial sectors, including agricultural machinery, utility pole construction, timber framing, and legacy rail systems.
    According to the ASME B18.2.2 dimensional standard, Square nuts are characterized by their four-sided geometry, which provides an exceptionally large wrenching surface area.
    This extended bearing surface significantly mitigates the risk of wrench slippage and prevents the rounding of the fastener corners during high-torque manual installations.
    However, the geometric constraint of a square nut necessitates a minimum 90-degree rotational clearance for tool engagement, rendering it physically unsuited for confined mechanical envelopes compared to the 60-degree rotational requirement of modern hexagon variants.
    The dimensional limits for ASME B18.2.2 regular square nuts are defined across nominal sizes ranging primarily from 1/4 inch up to 1-1/2 inches.
    The specification strictly dictates the Width Across Flats (F), the Width Across Corners (G), and the overall Thickness (H) to ensure uniform load distribution across the engaged substrate.
    The standard deliberately accommodates both flat-bottom and chamfered-bottom configurations, allowing engineers to specify variable bearing surface topographies based on the hardness and material properties of the mating surface.
    When applied in standard industrial environments, these fasteners are typically manufactured from carbon steel to meet the rigorous mechanical property requirements of ASTM A563 Grade A or SAE J995 Grade 2.
    Nuts of other materials, including corrosion-resistant stainless steel, brass, bronze, and aluminum alloys, are also governed by this standard, requiring cross-referencing with ASTM F594 for stainless properties and ASTM F467 for nonferrous requirements.
    Because square nuts are frequently utilized in highly exposed external environments—such as high-voltage outdoor power transmission structures—the dimensional tolerances must inherently account for heavy protective coatings.
    When electroplated finishes are specified by the purchaser, the nut dimensions must adhere to the allowances of ASTM F1941.
    More critically, when hot-dip galvanizing is employed to provide decades of cathodic protection, the process deposits between 2.2 to 5 mils of zinc onto the internal threads.
    To accommodate this anti-corrosion layer without inducing thread binding, the internal threads of the square nuts must be tapped oversize in accordance with standard thread gaging allowances.

    ASME B18.2.2 Square Nuts

    ASME B18.2.2 Square Nuts Dimensions (mm)

    Nominal Size (in) Basic Major Diameter Width Across Flats, F Width Across Corners, G Thickness Runout to Thread Axis, FIM
    Basic Min. Max. Min. Max. Basic Min. Max.
    1/4 6.350 11.11 10.80 11.13 14.07 15.72 5.56 5.16 5.97 0.28
    5/16 7.938 14.29 13.89 14.27 18.31 20.19 6.75 6.32 7.19 0.38
    3/8 9.525 15.88 15.39 15.88 20.37 22.45 8.33 7.87 8.79 0.41
    7/16 11.113 19.05 18.49 19.05 24.64 26.95 9.53 9.04 10.01 0.48
    1/2 12.700 20.64 20.02 20.62 26.72 29.18 11.11 10.62 11.63 0.56
    5/8 15.875 25.40 24.61 25.40 33.02 35.92 13.89 13.34 14.45 0.66
    3/4 19.050 28.58 27.64 28.58 37.19 40.41 16.67 16.05 17.27 0.74
    7/8 22.225 33.34 32.23 33.32 43.48 47.14 19.45 18.80 20.12 0.86
    1 25.400 38.10 36.83 38.10 49.81 53.87 22.23 21.51 22.94 0.99
    1 1/8 28.575 42.86 41.43 42.88 56.11 60.60 25.40 24.64 26.16 0.74
    1 1/4 31.750 47.63 46.02 47.63 62.43 67.36 27.78 26.97 28.60 0.81
    1 3/8 34.925 52.39 50.65 52.37 68.78 74.09 30.56 29.69 31.42 0.89
    1 1/2 38.100 57.15 55.25 57.15 75.08 80.82 33.34 32.41 34.24 0.99

    ASME B18.2.2 Square Nuts Dimensions (Inch)

    Nominal Size Basic Major Diameter of Thread Width Across Flats, F Width Across Corners, G Thickness Bearing Surface Runout to Thread Axis, FIM
    Basic Min. Max. Min. Max. Basic Min. Max.
    1/4 0.2500 7/16 0.425 0.438 0.554 0.619 7/32 0.203 0.235 0.011
    5/16 0.3125 9/16 0.547 0.562 0.721 0.795 17/64 0.249 0.283 0.015
    3/8 0.3750 5/8 0.606 0.625 0.802 0.884 21/64 0.310 0.346 0.016
    7/16 0.4375 3/4 0.728 0.750 0.970 1.061 3/8 0.356 0.394 0.019
    1/2 0.5000 13/16 0.788 0.812 1.052 1.149 7/16 0.418 0.458 0.022
    5/8 0.6250 1 0.969 1.000 1.300 1.414 35/64 0.525 0.569 0.026
    3/4 0.7500 1 1/8 1.088 1.125 1.464 1.591 21/32 0.632 0.680 0.029
    7/8 0.8750 1 5/16 1.269 1.312 1.712 1.856 49/64 0.740 0.792 0.034
    1 1.0000 1 1/2 1.450 1.500 1.961 2.121 7/8 0.847 0.903 0.039
    1 1/8 1.1250 1 11/16 1.631 1.688 2.209 2.386 1 0.970 1.030 0.029
    1 1/4 1.2500 1 7/8 1.812 1.875 2.458 2.652 1 3/32 1.062 1.126 0.032
    1 3/8 1.3750 2 1/16 1.994 2.062 2.708 2.917 1 13/64 1.169 1.237 0.035
    1 1/2 1.5000 2 1/4 2.175 2.250 2.956 3.182 1 5/16 1.276 1.348 0.039

    Related A194 Grades & References

    Square nuts per ASME B18.2.2 can be manufactured in A194 Grade 8 (304 stainless) or Grade 8M (316), though heavy hex is the standard form for A194 high-pressure/high-temperature bolting. For hex form see hex nuts or heavy hex nuts. Full specification: ASTM A194 PDF. A194 hub.

    Frequently Asked Questions

    Where are square nuts used?

    Square nuts per ASME B18.2.2 are commonly used in structural steel construction, machine assemblies, and applications where the nut must be held in a slot or channel. The four flat sides resist rotation when wedged against a flat surface.

    What thread series are used on ASME B18.2.2 square nuts?

    Square nuts per ASME B18.2.2 are available in Unified Coarse (UNC) and Unified Fine (UNF) thread series per ASME B1.1, typically in sizes from 1/4 to 1-1/2 inch.

    Are square nuts available in stainless steel?

    Yes. Square nuts can be manufactured in ASTM A194 Grade 8 (Type 304), Grade 8M (Type 316), or other stainless grades, although the standard nut form for stainless A194 grades is heavy hex.

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