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    2. ASME B18.3 Hexagon Socket Head Cap Screws

    ASME B18.3 Hexagon Socket Head Cap Screws

    High-strength cylindrical head fasteners driven by an internal hex key, perfect for applications with limited clearance and high precision requirements.

    ASME B18.3 Hexagon Socket Head Cap Screws, universally known as Allen bolts, feature a cylindrical head with an internal hexagonal socket drive. This design allows for a compact head profile that fits into counterbores, making them ideal for applications where external head clearance is restricted or where a smooth, flush surface is desired. Despite their smaller head size, these screws are capable of withstanding very high tensile loads, often manufactured from alloy steel heat-treated to high hardness levels (e.g., Grade 8 or 12.9). They are a staple in the aerospace, automotive, mold-making, and robotics industries, where precision alignment and high clamping force are critical. The ASME B18.3 standard strictly defines the socket dimensions, head height, and thread engagement to prevent stripping and ensure reliable torque transmission during assembly.

    HEXAGON SOCKET HEAD CAP SCREW INCH THREAD

    ASME B18.3 Hexagon Socket Head Cap Screw Dimensions (mm)

    Nominal Size Basic Screw Diameter Body Diameter, D (mm) Head Diameter, A (mm) Head Height, H (mm) Maximum Chamfer or Radius, C (mm) Nominal Hexagon Socket Size, J Socket Width Across Flats, J (mm) Minimum Key Engagement, T (mm) Minimum Wall Thickness, G (mm) Maximum Chamfer or Radius, K (mm) Fillet Juncture Diameter, F (mm)
    Max. Min. Max. Min. Max. Min. Nom. Dec. (mm) Max. Min. Max. Min.
    #0 0.0600 1.524 1.443 2.438 2.311 1.524 1.448 0.102 ... 1.27 1.295 1.27 0.635 0.508 0.178 1.88 1.575
    #1 0.0730 1.854 1.765 2.997 2.845 1.854 1.778 0.127 1/16 1.575 1.613 1.588 0.787 0.635 0.178 2.21 1.905
    #2 0.0860 2.184 2.088 3.556 3.404 2.184 2.108 0.203 5/64 1.981 2.009 1.984 0.965 0.737 0.178 2.591 2.286
    #3 0.0990 2.515 2.41 4.089 3.912 2.515 2.413 0.203 5/64 1.981 2.009 1.984 1.118 0.864 0.178 2.921 2.591
    #4 0.1120 2.845 2.731 4.648 4.47 2.845 2.743 0.229 3/32 2.388 2.418 2.38 1.295 0.965 0.203 3.302 2.972
    #5 0.1250 3.175 3.053 5.207 5.029 3.175 3.073 0.305 3/32 2.388 2.418 2.38 1.448 1.092 0.203 3.683 3.353
    #6 0.1380 3.505 3.376 5.74 5.486 3.505 3.404 0.33 7/64 2.769 2.822 2.779 1.626 1.194 0.203 4.013 3.658
    #8 0.1640 4.166 4.026 6.858 6.528 4.166 4.039 0.356 9/64 3.581 3.622 3.571 1.956 1.422 0.203 4.775 4.369
    #10 0.1900 4.826 4.674 7.925 7.569 4.826 4.699 0.457 5/32 3.962 4.031 3.967 2.286 1.651 0.203 5.537 5.131
    #12 0.2160 5.486 5.321 8.23 7.976 5.486 5.334 0.559 5/32 3.962 4.031 3.967 2.616 2.083 0.254 - -
    1/4 0.2500 6.35 6.185 9.525 8.992 6.35 6.198 0.635 3/16 4.775 4.826 4.763 3.048 2.413 0.254 7.061 6.63
    5/16 0.3125 7.938 7.755 11.913 11.328 7.925 7.772 0.838 1/4 6.35 6.426 6.35 3.835 3.023 0.254 8.814 8.357
    3/8 0.3750 9.525 9.342 14.275 13.716 9.525 9.347 1.016 5/16 7.925 8.026 7.938 4.623 3.632 0.254 10.541 10.084
    7/16 0.4375 11.113 10.907 16.662 16.027 11.125 10.922 1.194 3/8 9.525 9.627 9.525 5.41 4.216 0.381 12.294 11.811
    1/2 0.5000 12.7 12.494 19.05 18.415 12.7 12.497 1.397 3/8 9.525 9.627 9.525 6.223 4.826 0.381 14.021 13.487
    9/16 0.5625 14.288 14.067 21.412 21.006 14.275 14.072 1.575 7/16 11.1 11.227 11.113 7.01 5.436 0.381 - -
    5/8 0.6250 15.875 15.654 23.825 23.216 15.875 15.647 1.778 1/2 12.7 12.827 12.7 7.798 6.045 0.381 17.501 16.866
    3/4 0.7500 19.05 18.811 28.575 27.788 19.05 18.796 2.159 5/8 15.875 16.027 15.875 9.398 7.239 0.381 21.031 20.32
    7/8 0.8750 22.225 21.963 33.325 32.791 22.225 21.946 2.54 3/4 19.05 19.228 19.05 10.973 8.458 0.508 24.46 23.673
    1 1.0000 25.4 25.11 38.1 37.49 25.4 25.095 2.896 3/4 19.05 19.228 19.05 12.573 9.652 0.508 27.94 27.127
    1 1/8 1.1250 28.575 28.158 42.875 42.291 28.575 28.219 3.277 7/8 22.225 22.479 22.225 14.148 10.871 0.508 31.369 30.429
    1 1/4 1.2500 31.75 31.333 47.625 47.041 31.75 31.394 3.658 7/8 22.225 22.479 22.225 15.748 12.065 0.508 34.798 33.858
    1 3/8 1.3750 34.925 34.463 52.375 51.765 34.925 34.544 4.064 1 25.4 25.908 25.4 17.323 13.284 0.508 38.227 37.236
    1 1/2 1.5000 38.1 37.638 57.15 56.49 38.1 37.719 4.47 1 25.4 25.908 25.4 18.923 14.478 0.508 41.656 40.665
    1 3/4 1.7500 44.45 43.929 66.675 65.964 44.45 44.044 5.258 1 1/4 31.75 32.385 31.75 22.098 16.891 0.508 48.514 47.473
    2 2.0000 50.8 50.241 76.2 75.438 50.8 50.368 6.045 1 1/2 38.1 38.862 38.1 25.273 19.304 0.508 55.372 54.051
    2 1/4 2.2500 57.15 56.591 85.725 84.938 57.15 56.693 6.833 1 3/4 44.45 45.339 44.45 28.448 21.717 0.914 62.23 60.909
    2 1/2 2.5000 63.5 62.896 95.25 94.412 63.5 63.017 7.62 1 3/4 44.45 45.339 44.45 31.623 24.13 0.914 69.088 67.615
    2 3/4 2.7500 69.85 69.246 104.775 103.886 69.85 69.342 8.433 2 50.8 51.816 50.8 34.798 26.543 0.914 75.946 74.574
    3 3.0000 76.2 75.596 114.3 113.386 76.2 75.667 9.195 2 1/4 57.15 58.293 57.15 37.973 28.956 0.914 82.804 81.432
    3 1/4 3.2500 82.55 81.946 123.825 122.86 82.55 81.991 10.008 2 1/4 57.15 58.293 57.15 41.148 31.369 0.914 89.662 88.29
    3 1/2 3.5000 88.9 88.296 133.35 132.359 88.9 88.341 10.82 2 3/4 69.85 71.247 69.85 44.323 33.782 0.914 96.52 95.148
    3 3/4 3.7500 95.25 94.646 142.875 141.834 95.25 94.666 11.633 2 3/4 69.85 71.247 69.85 47.498 36.195 0.914 103.378 102.006
    4 4.0000 101.6 100.996 152.4 151.333 101.6 100.99 12.421 3 76.2 77.724 76.2 50.673 38.608 0.914 110.236 108.864

    ASME B18.3 Hexagon Socket Head Cap Screw Length Tolerances(mm)

    Nominal Length Nominal Size, Tolerances on Lengths (mm)
    0 Through
    9.525, Incl.    		 11.112 Through
    19.05, Incl.    		 22.225 Through
    38.1, Incl.    		 Over 38.1
    Up to 25.4, incl. -0.762 -0.762 -1.27 ...
    Over 25.4 to 63.5, incl. -1.016 -1.524 -2.54 -4.572
    Over 63.5 to 152.4, incl. -1.524 -2.032 -3.556 -5.08
    Over 152.4 -3.048 -3.048 -5.08 -6.096

    ASME B18.3 Hexagon Socket Head Cap Screw Dimensions (Inch)

    Nominal Size Basic Screw Diameter Body Diameter, D Head Diameter, A Head Height, H Maximum Chamfer or Radius, C Nominal Hexagon Socket Size, J Socket Width Across Flats, J Minimum Key Engagement, T Minimum Wall Thickness, G Maximum Chamfer or Radius, K Fillet Juncture Diameter, F
    Max. Min. Max. Min. Max. Min. Nom. Dec. Max. Min. Max. Min.
    #0 0.0600 0.0600 0.0568 0.096 0.091 0.060 0.057 0.004 ... 0.050 0.0510 0.0500 0.025 0.020 0.007 0.074 0.062
    #1 0.0730 0.0730 0.0695 0.118 0.112 0.073 0.070 0.005 1/16 0.062 0.0635 0.0625 0.031 0.025 0.007 0.087 0.075
    #2 0.0860 0.0860 0.0822 0.140 0.134 0.086 0.083 0.008 5/64 0.078 0.0791 0.0781 0.038 0.029 0.007 0.102 0.090
    #3 0.0990 0.0990 0.0949 0.161 0.154 0.099 0.095 0.008 5/64 0.078 0.0791 0.0781 0.044 0.034 0.007 0.115 0.102
    #4 0.1120 0.1120 0.1075 0.183 0.176 0.112 0.108 0.009 3/32 0.094 0.0952 0.0937 0.051 0.038 0.008 0.130 0.117
    #5 0.1250 0.1250 0.1202 0.205 0.198 0.125 0.121 0.012 3/32 0.094 0.0952 0.0937 0.057 0.043 0.008 0.145 0.132
    #6 0.1380 0.1380 0.1329 0.226 0.216 0.138 0.134 0.013 7/64 0.109 0.1111 0.1094 0.064 0.047 0.008 0.158 0.144
    #8 0.1640 0.1640 0.1585 0.270 0.257 0.164 0.159 0.014 9/64 0.141 0.1426 0.1406 0.077 0.056 0.008 0.188 0.172
    #10 0.1900 0.1900 0.1840 0.312 0.298 0.190 0.185 0.018 5/32 0.156 0.1587 0.1562 0.090 0.065 0.008 0.218 0.202
    #12 0.2160 0.2160 0.2095 0.324 0.314 0.216 0.210 0.022 5/32 0.156 0.1587 0.1562 0.103 0.082 0.010 - -
    1/4 0.2500 0.2500 0.2435 0.375 0.354 0.250 0.244 0.025 3/16 0.188 0.1900 0.1875 0.120 0.095 0.010 0.278 0.261
    5/16 0.3125 0.3125 0.3053 0.469 0.446 0.312 0.306 0.033 1/4 0.250 0.2530 0.2500 0.151 0.119 0.010 0.347 0.329
    3/8 0.3750 0.3750 0.3678 0.562 0.540 0.375 0.368 0.040 5/16 0.312 0.3160 0.3125 0.182 0.143 0.010 0.415 0.397
    7/16 0.4375 0.4375 0.4294 0.656 0.631 0.438 0.430 0.047 3/8 0.375 0.3790 0.3750 0.213 0.166 0.015 0.484 0.465
    1/2 0.5000 0.5000 0.4919 0.750 0.725 0.500 0.492 0.055 3/8 0.375 0.3790 0.3750 0.245 0.190 0.015 0.552 0.531
    9/16 0.5625 0.5625 0.5538 0.843 0.827 0.562 0.554 0.062 7/16 0.437 0.4420 0.4375 0.276 0.214 0.015 - -
    5/8 0.6250 0.6250 0.6163 0.938 0.914 0.625 0.616 0.070 1/2 0.500 0.5050 0.5000 0.307 0.238 0.015 0.689 0.664
    3/4 0.7500 0.7500 0.7406 1.125 1.094 0.750 0.740 0.085 5/8 0.625 0.6310 0.6250 0.370 0.285 0.015 0.828 0.800
    7/8 0.8750 0.8750 0.8647 1.312 1.291 0.875 0.864 0.100 3/4 0.750 0.7570 0.7500 0.432 0.333 0.020 0.963 0.932
    1 1.0000 1.0000 0.9886 1.500 1.476 1.000 0.988 0.114 3/4 0.750 0.7570 0.7500 0.495 0.380 0.020 1.100 1.068
    1 1/8 1.1250 1.1250 1.1086 1.688 1.665 1.125 1.111 0.129 7/8 0.875 0.8850 0.8750 0.557 0.428 0.020 1.235 1.198
    1 1/4 1.2500 1.2500 1.2336 1.875 1.852 1.250 1.236 0.144 7/8 0.875 0.8850 0.8750 0.620 0.475 0.020 1.370 1.333
    1 3/8 1.3750 1.3750 1.3568 2.062 2.038 1.375 1.360 0.160 1 1.000 1.0200 1.0000 0.682 0.523 0.020 1.505 1.466
    1 1/2 1.5000 1.5000 1.4818 2.250 2.224 1.500 1.485 0.176 1 1.000 1.0200 1.0000 0.745 0.570 0.020 1.640 1.601
    1 3/4 1.7500 1.7500 1.7295 2.625 2.597 1.750 1.734 0.207 1 1/4 1.250 1.2750 1.2500 0.870 0.665 0.020 1.910 1.869
    2 2.0000 2.0000 1.9780 3.000 2.970 2.000 1.983 0.238 1 1/2 1.500 1.5300 1.5000 0.995 0.760 0.020 2.180 2.128
    2 1/4 2.2500 2.2500 2.2280 3.375 3.344 2.250 2.232 0.269 1 3/4 1.750 1.7850 1.7500 1.120 0.855 0.036 2.450 2.398
    2 1/2 2.5000 2.5000 2.4762 3.750 3.717 2.500 2.481 0.300 1 3/4 1.750 1.7850 1.7500 1.245 0.950 0.036 2.720 2.662
    2 3/4 2.7500 2.7500 2.7262 4.125 4.090 2.750 2.730 0.332 2 2.000 2.0400 2.0000 1.370 1.045 0.036 2.990 2.936
    3 3.0000 3.0000 2.9762 4.500 4.464 3.000 2.979 0.362 2 1/4 2.250 2.2950 2.2500 1.495 1.140 0.036 3.260 3.206
    3 1/4 3.2500 3.2500 3.2262 4.875 4.837 3.250 3.228 0.394 2 1/4 2.250 2.2950 2.2500 1.620 1.235 0.036 3.530 3.476
    3 1/2 3.5000 3.5000 3.4762 5.250 5.211 3.500 3.478 0.426 2 3/4 2.750 2.8050 2.7500 1.745 1.330 0.036 3.800 3.746
    3 3/4 3.7500 3.7500 3.7262 5.625 5.584 3.750 3.727 0.458 2 3/4 2.750 2.8050 2.7500 1.870 1.425 0.036 4.070 4.016
    4 4.0000 4.0000 3.9762 6.000 5.958 4.000 3.976 0.489 3 3.000 3.0600 3.0000 1.995 1.520 0.036 4.340 4.286

    ASME B18.3 Hexagon Socket Head Cap Screw Length Tolerances(Inch)

    Nominal Length Nominal Size, Tolerances on Lengths
    0 Through
    3/8, Incl.    		 7/16 Through
    3/4, Incl.    		 7/8 Through
    1 1/2, Incl.    		 Over 1 1/2
    Up to 1.00, incl. -0.03 -0.03 -0.05 ...
    Over 1.00 to 2.50, incl. -0.04 -0.06 -0.10 -0.18
    Over 2.5 to 6.00, incl. -0.06 -0.08 -0.14 -0.20
    Over 6.00 -0.12 -0.12 -0.20 -0.24

    Frequently Asked Questions

    What does ASME B18.3 hexagon socket head cap screws (allen bolts) cover?
    ASME B18.3 is the inch dimensional standard for hexagon socket head cap screws (allen bolts). It defines head dimensions, body diameter, thread length, total length, drive recess, and tolerances so any compliant hexagon socket head cap screws (allen bolts) interchanges with standard nuts, washers and wrenches across manufacturers.
    What size range is available for hexagon socket head cap screws (allen bolts) under ASME B18.3?
    hexagon socket head cap screws (allen bolts) under ASME B18.3 covers #0 through 4 in nominal diameter, internal hex drive. TorqBolt manufactures the full range in ASTM A193 Grades B7, B7M, B8, B8M and the full ASTM A320 cryogenic counterpart on customer specification.
    What is the metric/inch counterpart of ASME B18.3 hexagon socket head cap screws (allen bolts)?
    The companion specification for hexagon socket head cap screws (allen bolts) is ASME B18.3.1M (ISO 4762 / DIN 912 family). Both standards share equivalent functional dimensions but use the corresponding measurement system (inch vs metric ISO M-series).
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