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    2. ASME B18.2.2 Heavy Hex Jam Nut Dimensional Standards

    Heavy Hex Jam Nuts | ASME B18.2.2 Dimensions

    ASME B18.2.2 Heavy Hex Jam Nut profiles provide critical engineering parameters outlining the dimensions for heavy-duty applications.
    While standard jam nuts are sufficient for general machinery, heavy industrial sectors—such as oil and gas refineries, marine construction, and power generation—rely extensively on heavy hex configurations.
    Consequently, when anti-vibration locking is required in these extreme environments, engineers specify the Heavy Hex Jam Nut under the ASME B18.2.2 dimensional framework.
    A heavy hex jam nut mirrors the exact Width Across Flats (WAF) footprint of a full heavy hex nut, ensuring that massive hydraulic torque wrenches and striking wrenches used by field crews fit perfectly over both the primary heavy nut and the secondary jam nut.
    However, like the standard jam nut, the heavy hex jam nut features a drastically reduced thickness profile.
    This is particularly critical in applications like high-pressure flange bolting (ASME B16.5) or turbine casing assembly, where the stud bolts are intentionally sized to leave minimal thread protrusion.
    A heavy hex jam nut can be utilized within this highly restricted spatial envelope to prevent the primary heavy hex nut from loosening due to the intense resonant vibrations produced by fluid flow or rotating machinery.
    Because heavy hex jam nuts are frequently deployed in the same highly corrosive environments as their full-sized counterparts, they are subject to identical material and coating stipulations.
    They are readily available in high-strength carbon steels (such as ASTM A563 Grade DH), alloys (Grade B8, B8M for extreme temperature environments), and Silicon Bronze.
    The standard accommodates the necessity for oversized tapping if the heavy jam nut is subjected to hot-dip galvanizing, ensuring that the zinc buildup does not compromise the locking installation sequence.

    ASME B18.2.2 Heavy Hex Jam Nuts Dimensions (mm)

    ASME B18.2.2 Heavy Hex Jam Nuts

    Nominal Size Basic Major Diameter of Thread Width Across Flats, F Width Across Corners, G Thickness Heavy Hex Jam Nuts, H1 Maximum Runout of Bearing Surface to Thread Axis, FIM
    Basic Min. Max. Min. Max. Basic Min. Max.
    All Strength Levels
    1/4 0.2500 1/2 0.488 0.500 0.556 0.577 11/64 0.156 0.188 0.017
    5/16 0.3125 9/16 0.546 0.562 0.622 0.650 13/64 0.186 0.220 0.020
    3/8 0.3750 11/16 0.669 0.688 0.763 0.794 15/64 0.216 0.252 0.021
    7/16 0.4375 3/4 0.728 0.750 0.830 0.866 17/64 0.247 0.285 0.022
    1/2 0.5000 7/8 0.850 0.875 0.969 1.010 19/64 0.277 0.317 0.023
    9/16 0.5625 15/16 0.909 0.938 1.037 1.083 21/64 0.307 0.349 0.024
    5/8 0.6250 1 1/16 1.031 1.062 1.175 1.227 23/64 0.337 0.381 0.025
    3/4 0.7500 1 1/4 1.212 1.250 1.382 1.443 27/64 0.398 0.446 0.027
    7/8 0.8750 1 7/16 1.394 1.438 1.589 1.660 31/64 0.458 0.510 0.029
    1 1.0000 1 5/8 1.575 1.625 1.796 1.876 35/64 0.519 0.575 0.031
    1 1/8 1.1250 1 13/16 1.756 1.812 2.002 2.093 39/64 0.579 0.639 0.033
    1 1/4 1.2500 2 1.938 2.000 2.209 2.309 23/32 0.687 0.751 0.035
    1 3/8 1.3750 2 3/16 2.119 2.188 2.416 2.526 25/32 0.747 0.815 0.038
    1 1/2 1.5000 2 3/8 2.300 2.375 2.622 2.742 27/32 0.808 0.880 0.041
    1 5/8 1.6250 2 9/16 2.481 2.562 2.828 2.959 29/32 0.868 0.944 0.044
    1 3/4 1.7500 2 3/4 2.662 2.750 3.035 3.175 31/32 0.929 1.009 0.048
    1 7/8 1.8750 2 15/16 2.844 2.938 3.242 3.392 1 1/32 0.989 1.073 0.051
    2 2.0000 3 1/8 3.025 3.125 3.449 3.608 1 3/32 1.050 1.138 0.055
    2 1/4 2.2500 3 1/2 3.388 3.500 3.862 4.041 1 13/64 1.155 1.251 0.061
    2 1/2 2.5000 3 7/8 3.750 3.875 4.275 4.474 1 29/64 1.401 1.505 0.068
    2 3/4 2.7500 4 1/4 4.112 4.250 4.688 4.907 1 37/64 1.522 1.634 0.074
    3 3.0000 4 5/8 4.475 4.625 5.102 5.340 1 45/64 1.643 1.763 0.081
    3 1/4 3.2500 5 4.838 5.000 5.515 5.774 1 13/16 1.748 1.876 0.087
    3 1/2 3.5000 5 3/8 5.200 5.375 5.928 6.207 1 15/16 1.870 2.006 0.094
    3 3/4 3.7500 5 3/4 5.562 5.750 6.341 6.640 2 1/16 1.990 2.134 0.100
    4 4.0000 6 1/8 5.925 6.125 6.755 7.073 2 3/16 2.112 2.264 0.107

    ASME B18.2.2 Heavy Hex Jam Nuts Dimensions (Inch)

    Nominal Size Basic Major Diameter of Thread Width Across Flats, F Width Across Corners, G Thickness Heavy Hex Jam Nuts, H1 Maximum Runout of Bearing Surface to Thread Axis, FIM
    Basic Min. Max. Min. Max. Basic Min. Max.
    All Strength Levels
    1/4 0.2500 1/2 0.488 0.500 0.556 0.577 11/64 0.156 0.188 0.017
    5/16 0.3125 9/16 0.546 0.562 0.622 0.650 13/64 0.186 0.220 0.020
    3/8 0.3750 11/16 0.669 0.688 0.763 0.794 15/64 0.216 0.252 0.021
    7/16 0.4375 3/4 0.728 0.750 0.830 0.866 17/64 0.247 0.285 0.022
    1/2 0.5000 7/8 0.850 0.875 0.969 1.010 19/64 0.277 0.317 0.023
    9/16 0.5625 15/16 0.909 0.938 1.037 1.083 21/64 0.307 0.349 0.024
    5/8 0.6250 1 1/16 1.031 1.062 1.175 1.227 23/64 0.337 0.381 0.025
    3/4 0.7500 1 1/4 1.212 1.250 1.382 1.443 27/64 0.398 0.446 0.027
    7/8 0.8750 1 7/16 1.394 1.438 1.589 1.660 31/64 0.458 0.510 0.029
    1 1.0000 1 5/8 1.575 1.625 1.796 1.876 35/64 0.519 0.575 0.031
    1 1/8 1.1250 1 13/16 1.756 1.812 2.002 2.093 39/64 0.579 0.639 0.033
    1 1/4 1.2500 2 1.938 2.000 2.209 2.309 23/32 0.687 0.751 0.035
    1 3/8 1.3750 2 3/16 2.119 2.188 2.416 2.526 25/32 0.747 0.815 0.038
    1 1/2 1.5000 2 3/8 2.300 2.375 2.622 2.742 27/32 0.808 0.880 0.041
    1 5/8 1.6250 2 9/16 2.481 2.562 2.828 2.959 29/32 0.868 0.944 0.044
    1 3/4 1.7500 2 3/4 2.662 2.750 3.035 3.175 31/32 0.929 1.009 0.048
    1 7/8 1.8750 2 15/16 2.844 2.938 3.242 3.392 1 1/32 0.989 1.073 0.051
    2 2.0000 3 1/8 3.025 3.125 3.449 3.608 1 3/32 1.050 1.138 0.055
    2 1/4 2.2500 3 1/2 3.388 3.500 3.862 4.041 1 13/64 1.155 1.251 0.061
    2 1/2 2.5000 3 7/8 3.750 3.875 4.275 4.474 1 29/64 1.401 1.505 0.068
    2 3/4 2.7500 4 1/4 4.112 4.250 4.688 4.907 1 37/64 1.522 1.634 0.074
    3 3.0000 4 5/8 4.475 4.625 5.102 5.340 1 45/64 1.643 1.763 0.081
    3 1/4 3.2500 5 4.838 5.000 5.515 5.774 1 13/16 1.748 1.876 0.087
    3 1/2 3.5000 5 3/8 5.200 5.375 5.928 6.207 1 15/16 1.870 2.006 0.094
    3 3/4 3.7500 5 3/4 5.562 5.750 6.341 6.640 2 1/16 1.990 2.134 0.100
    4 4.0000 6 1/8 5.925 6.125 6.755 7.073 2 3/16 2.112 2.264 0.107

    Related A194 Grades & References

    Heavy hex jam nuts per ASME B18.2.2 are commonly supplied in A194 Grade 2H, Grade 8 (304), and Grade 8M (316). For standard heavy hex form see heavy hex nuts; for non-heavy jam form see hex jam nuts. Full specification: ASTM A194 PDF. A194 hub.

    Frequently Asked Questions

    What is a heavy hex jam nut?

    A heavy hex jam nut is a thinner version of a heavy hex nut, typically about half the thickness of the standard heavy hex nut. Per ASME B18.2.2, sizes range from 1/4 to 4 inch, available in ASTM A194 grades for high-pressure/high-temperature service.

    Where are heavy hex jam nuts used?

    Heavy hex jam nuts are used as locknuts in pressure-vessel and flange assemblies, paired with a primary heavy hex nut. The greater width across flats compared to standard hex jam nuts allows higher torque without rounding.

    Which ASTM A194 grades are available as heavy hex jam nuts?

    Heavy hex jam nuts are typically supplied in ASTM A194 Grade 2H (carbon steel), Grade 7 (Cr-Mo alloy), Grade 8 (Type 304 stainless), and Grade 8M (Type 316 stainless) for matched bolting service.

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