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    2. ASME B18.3 Hexagon Socket Flat Countersunk Head Cap Screws

    ASME B18.3 Hexagon Socket Flat Countersunk Head Cap Screws

    Flush-mounting socket screws with a conical head, designed to sit level with the workpiece surface for aerodynamic and aesthetic finishes.

    ASME B18.3 Hexagon Socket Flat Countersunk Head Cap Screws are designed to be installed in countersunk holes so that the head sits flush with or below the surface of the material. Featuring a flat top and a conical bearing surface (typically with an 82-degree included angle), these screws combine the high-torque capability of an internal hex drive with a streamlined profile. This makes them ideal for applications where protruding heads would cause interference, create drag, or mar the aesthetic appearance of the product, such as in aerospace skins, firearm assemblies, and high-end consumer electronics. The ASME B18.3 standard specifies the precise geometry of the head and the socket to ensure proper seating and prevent cam-out during installation. They are available in various high-strength materials and finishes to suit demanding environments.

    ASME B18.3 Hexagon Socket Flat Countersunk Head Cap Screws

    ASME B18.3 Hexagon Socket Flat Countersunk Head Cap Screws Dimension (mm)

    Nominal Size Body Diameter, D Head Diameter, A Reference Head Height, H Protrusion Gage Diameter, G Protrusion, P Nominal Hexagon Socket Size, J Minimum Key Engagement, T Maximum Fillet Transition Diameter, F
    Max. Min. Max. Min. Max. Min. Max. Min. Min. Max.
    0 (0.0600) 1.524 1.442 3.505 2.971 1.117 1.981 1.955 1.117 0.660 0.902 0.889 0.635 1.828
    1 (0.0730) 1.854 1.765 4.267 3.632 1.371 2.565 2.540 1.219 0.711 1.295 1.270 0.787 2.260
    2 (0.0860) 2.184 2.087 5.003 4.267 1.625 3.149 3.124 1.295 0.787 1.295 1.270 0.965 2.692
    3 (0.0990) 2.514 2.410 5.740 4.902 1.854 3.759 3.733 1.371 0.838 1.613 1.588 1.117 3.022
    4 (0.1120) 2.844 2.730 6.477 5.537 2.108 4.368 4.343 1.447 0.914 1.613 1.588 1.397 3.454
    5 (0.1250) 3.175 3.053 7.137 6.096 2.286 4.978 4.953 1.498 0.939 2.009 1.984 1.549 3.886
    6 (0.1380) 3.505 3.375 7.797 6.680 2.463 5.588 5.562 1.524 0.939 2.009 1.984 1.676 4.267
    8 (0.1640) 4.165 4.025 9.118 7.899 2.844 6.781 6.756 1.600 0.990 2.418 2.38 1.930 4.927
    10 (0.1900) 4.826 4.673 10.439 9.118 3.225 7.950 7.924 1.676 1.041 3.226 3.175 2.209 5.588
    12 (0.2158) 5.481 5.339 11.430 10.414 3.429 9.144 9.118 1.752 1.066 3.226 3.175 2.387 6.248
    1/4 (0.2500) 6.350 6.184 13.487 12.192 4.089 10.769 10.744 1.828 1.092 4.031 3.967 2.819 7.112
    5/16 (0.3125) 7.937 7.747 16.662 15.240 5.029 13.690 13.665 1.981 1.193 4.826 4.763 3.429 8.712
    3/8 (0.3750) 9.525 9.342 19.837 18.288 5.943 16.586 16.560 2.235 1.270 5.631 5.555 4.038 10.287
    7/16 (0.4375) 11.112 10.906 21.437 19.837 5.943 17.526 17.475 2.641 1.600 6.426 6.35 4.038 11.887
    1/2 (0.5000) 12.700 12.494 23.825 22.148 6.375 18.770 18.745 3.327 2.209 8.026 7.938 4.368 13.462
    5/8 (0.6250) 15.875 15.654 30.175 28.244 8.229 24.434 24.409 3.708 2.438 9.627 9.525 5.588 16.637
    3/4 (0.7500) 19.050 18.811 36.525 34.417 10.058 30.124 30.099 4.318 2.667 12.827 12.7 5.588 19.812
    7/8 (0.8750) 22.225 21.963 42.875 40.741 11.887 35.839 35.814 4.191 2.997 14.427 14.288 6.299 22.987
    1 (1.0000) 25.400 25.110 49.225 46.761 13.716 41.529 41.503 4.597 3.302 16.027 15.875 7.543 26.162
    1 1/8 (1.1250) 28.575 28.158 55.575 52.806 15.519 47.218 47.193 4.978 3.556 19.228 19.05 8.255 30.149
    1 1/4 (1.2500) 31.750 31.333 61.925 58.826 17.348 52.908 52.882 5.384 3.810 22.479 22.225 9.093 33.324
    1 3/8 (1.3750) 34.925 34.462 68.275 64.846 19.177 58.572 58.547 5.791 4.114 22.479 22.225 10.210 36.499
    1 1/2 (1.5000) 38.100 37.637 74.625 70.891 21.005 64.262 64.236 6.172 4.394 25.908 25.4 11.049 39.674

    ASME B18.3 Hexagon Socket Flat Countersunk Head Cap Screws Length Tolerances(mm)

    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.
    Up to 25.4, incl. -0.762 -0.762 -1.270
    Over 25.4 to 63.5, incl. -1.016 -1.524 -2.540
    Over 63.5 to 152.4, incl. -1.524 -2.032 -3.556
    Over 152.4 -3.048 -3.048 -5.080

    Frequently Asked Questions

    What does ASME B18.3 hex socket flat countersunk head cap screws cover?
    ASME B18.3 is the inch dimensional standard for hex socket flat countersunk head cap screws. It defines head dimensions, body diameter, thread length, total length, drive recess, and tolerances so any compliant hex socket flat countersunk head cap screws interchanges with standard nuts, washers and wrenches across manufacturers.
    What size range is available for hex socket flat countersunk head cap screws under ASME B18.3?
    hex socket flat countersunk head cap screws under ASME B18.3 covers #0 through 1/2 in nominal diameter, 82° flush head. 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 hex socket flat countersunk head cap screws?
    The companion specification for hex socket flat countersunk head cap screws is ASME B18.3.5M metric flat countersunk. Both standards share equivalent functional dimensions but use the corresponding measurement system (inch vs metric ISO M-series).
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