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    2. ASME B18.3 Spline Socket Head Cap Screws

    ASME B18.3 Spline Socket Head Cap Screws

    High-torque fasteners featuring a spline drive recess, engineered to resist cam-out and handle extreme installation forces in critical applications.

    ASME B18.3 Spline Socket Head Cap Screws utilize a star-shaped spline drive recess rather than a standard hexagon, providing significantly greater surface contact between the driver and the fastener. This design allows for the transmission of much higher torque levels without the risk of "cam-out" (slipping) or rounding off the drive recess, which is a common failure mode with hex sockets under extreme load. Consequently, these screws are preferred in aerospace, heavy equipment, and safety-critical applications where precise and high clamping forces are mandatory. The spline drive also tends to extend the life of the driving tool and reduces operator fatigue. Compliant with ASME B18.3, these fasteners maintain the same external head dimensions as standard socket cap screws, allowing them to fit into existing counterbores while offering superior drive performance.

    Spline Socket HEAD CAP SCREW INCH THREAD

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

    Nominal Size
    (Basic Screw Diameter)    		 Body Diameter, D Head Diameter, A Head Height, H Chamfer
    or Radius, C    		 Nominal
    Spline Socket
    Size, M    		 Key
    Engagement, T    		 Wall
    Thickness, G    		 Chamfer
    or Radius, K    		 Thread Length,
    L T    		 Total
    Thread Length,
    L TT
    Max. Min. Max. Min. Max. Min. Max. Min. Min. Max. Min. Max.
    0 (0.0600) 1.52 1.45 2.44 2.31 1.52 1.45 0.10 1.52 0.64 0.51 0.18 12.70 15.75
    1 (0.0730) 1.85 1.78 3.00 2.84 1.85 1.78 0.13 1.83 0.79 0.64 0.18 15.75 19.56
    2 (0.0860) 2.18 2.08 3.56 3.40 2.18 2.11 0.20 2.44 0.97 0.74 0.18 15.75 20.32
    3 (0.0990) 2.51 2.41 4.09 3.91 2.51 2.41 0.20 2.44 1.12 0.86 0.18 15.75 21.08
    4 (0.1120) 2.84 2.74 4.65 4.47 2.84 2.74 0.23 2.82 1.30 0.97 0.20 19.05 25.15
    5 (0.1250) 3.18 3.05 5.21 5.03 3.18 3.07 0.30 2.82 1.45 1.09 0.20 19.05 25.40
    6 (0.1380) 3.51 3.38 5.74 5.54 3.51 3.40 0.33 3.38 1.63 1.19 0.20 19.05 26.67
    8 (0.1640) 4.17 4.04 6.86 6.65 4.17 4.04 0.36 4.27 1.96 1.42 0.20 22.35 30.23
    10 (0.1900) 4.83 4.67 7.92 7.70 4.83 4.70 0.46 4.65 2.29 1.65 0.20 22.35 32.26
    1/4 (0.2500) 6.35 6.20 9.53 9.27 6.35 6.20 0.64 5.49 3.05 2.41 0.25 25.40 38.10
    5/16 (0.3125) 7.95 7.75 11.91 11.61 7.92 7.77 0.84 7.39 3.84 3.02 0.25 28.45 43.43
    3/8 (0.3750) 9.53 9.35 14.27 13.97 9.53 9.35 1.02 9.45 4.62 3.63 0.25 31.75 49.28
    7/16 (0.4375) 11.13 10.90 16.66 16.31 11.13 10.92 1.19 11.53 5.41 4.22 0.38 35.05 55.12
    1/2 (0.5000) 12.70 12.50 19.05 18.67 12.70 12.50 1.40 11.53 6.22 4.83 0.38 38.10 60.45
    5/8 (0.6250) 15.88 15.65 23.83 23.39 15.88 15.65 1.78 15.11 7.80 6.05 0.38 44.45 71.63
    3/4 (0.7500) 19.05 18.82 28.58 28.12 19.05 18.80 2.16 15.75 9.40 7.24 0.38 50.80 82.55
    7/8 (0.8750) 22.23 21.97 33.32 32.84 22.23 21.95 2.54 17.73 10.97 8.46 0.51 57.15 93.73
    1 (1.0000) 25.40 25.12 38.10 37.57 25.40 25.10 2.90 20.07 12.57 9.65 0.51 63.50 104.65
    1 1/8 (1.1250) 28.58 28.17 42.88 42.29 28.58 28.22 3.28 ... 14.15 10.87 0.51 71.37 118.11
    1 1/4 (1.2500) 31.75 31.34 47.63 47.04 31.75 31.39 3.66 ... 15.75 12.07 0.51 79.25 129.29
    1 3/8 (1.3750) 34.93 34.47 52.37 51.77 34.93 34.54 4.06 ... 17.32 13.28 0.51 87.38 143.51
    1 1/2 (1.5000) 38.10 37.64 57.15 56.49 38.10 37.72 4.47 ... 18.92 14.48 0.51 95.25 154.43
    1 3/4 (1.7500) 44.45 43.94 66.68 65.96 44.45 44.04 5.26 ... 22.10 16.89 0.51 111.25 181.10
    2 (2.0000) 50.80 50.24 76.20 75.44 50.80 50.37 6.05 ... 25.27 19.30 0.51 127.00 206.00
    2 1/4 (2.2500) 57.15 56.59 85.73 84.94 57.15 56.69 6.83 ... 28.45 21.72 0.91 142.75 228.35
    2 1/2 (2.5000) 63.50 62.89 95.25 94.41 63.50 63.02 7.62 ... 31.62 24.13 0.91 158.75 254.00
    2 3/4 (2.7500) 69.85 69.24 104.78 103.89 69.85 69.34 8.43 ... 34.80 26.54 0.91 174.75 276.10
    3 (3.0000) 76.20 75.59 114.30 113.39 76.20 75.67 9.22 ... 37.97 28.96 0.91 190.50 298.45
    3 1/4 (3.2500) 82.55 81.94 123.83 122.86 82.55 81.99 10.01 ... 41.15 31.37 0.91 206.25 320.80
    3 1/2 (3.5000) 88.90 88.29 133.35 132.36 88.90 88.34 10.82 ... 44.32 33.78 0.91 222.25 342.90
    3 3/4 (3.7500) 95.25 94.64 142.88 141.83 95.25 94.67 11.63 ... 47.50 36.20 0.91 238.25 365.00
    4 (4.0000) 101.60 100.99 152.40 151.33 101.60 100.99 12.42 ... 50.67 38.61 0.91 254.00 387.35

    ASME B18.3 Spline Socket Head Cap Screw 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.    		 Over 1 1/2
    Up to 25.4, incl. -0.762 -0.762 -1.270 ...
    Over 25.4 to 63.5, incl. -1.016 -1.524 -2.540 -4.572
    Over 63.5 to 152.4, incl. -1.524 -2.032 -3.556 -5.080
    Over 152.4 -3.048 -3.048 -5.080 -6.096

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

    Nominal Size
    (Basic Screw Diameter)    		 Body Diameter, D Head Diameter, A Head Height, H Chamfer
    or Radius, C    		 Nominal
    Spline Socket
    Size, M    		 Key
    Engagement, T    		 Wall
    Thickness, G    		 Chamfer
    or Radius, K    		 Thread Length,
    L T (in)    		 Total
    Thread Length,
    L TT (in)
    Max. Min. Max. Min. Max. Min. Max. Min. Min. Max. Min. Max.
    0 (0.0600) 0.060 0.057 0.096 0.091 0.060 0.057 0.004 0.06 0.025 0.020 0.007 0.50 0.62
    1 (0.0730) 0.073 0.070 0.118 0.112 0.073 0.070 0.005 0.072 0.031 0.025 0.007 0.62 0.77
    2 (0.0860) 0.086 0.082 0.140 0.134 0.086 0.083 0.008 0.096 0.038 0.029 0.007 0.62 0.80
    3 (0.0990) 0.099 0.095 0.161 0.154 0.099 0.095 0.008 0.096 0.044 0.034 0.007 0.62 0.83
    4 (0.1120) 0.112 0.108 0.183 0.176 0.112 0.108 0.009 0.111 0.051 0.038 0.008 0.75 0.99
    5 (0.1250) 0.125 0.120 0.205 0.198 0.125 0.121 0.012 0.111 0.057 0.043 0.008 0.75 1.00
    6 (0.1380) 0.138 0.133 0.226 0.218 0.138 0.134 0.013 0.133 0.064 0.047 0.008 0.75 1.05
    8 (0.1640) 0.164 0.159 0.270 0.262 0.164 0.159 0.014 0.168 0.077 0.056 0.008 0.88 1.19
    10 (0.1900) 0.190 0.184 0.312 0.303 0.190 0.185 0.018 0.183 0.090 0.065 0.008 0.88 1.27
    1/4 (0.2500) 0.250 0.244 0.375 0.365 0.250 0.244 0.025 0.216 0.120 0.095 0.010 1.00 1.50
    5/16 (0.3125) 0.313 0.305 0.469 0.457 0.312 0.306 0.033 0.291 0.151 0.119 0.010 1.12 1.71
    3/8 (0.3750) 0.375 0.368 0.562 0.550 0.375 0.368 0.040 0.372 0.182 0.143 0.010 1.25 1.94
    7/16 (0.4375) 0.438 0.429 0.656 0.642 0.438 0.430 0.047 0.454 0.213 0.166 0.015 1.38 2.17
    1/2 (0.5000) 0.500 0.492 0.750 0.735 0.500 0.492 0.055 0.454 0.245 0.190 0.015 1.50 2.38
    5/8 (0.6250) 0.625 0.616 0.938 0.921 0.625 0.616 0.070 0.595 0.307 0.238 0.015 1.75 2.82
    3/4 (0.7500) 0.750 0.741 1.125 1.107 0.750 0.740 0.085 0.62 0.370 0.285 0.015 2.00 3.25
    7/8 (0.8750) 0.875 0.865 1.312 1.293 0.875 0.864 0.100 0.698 0.432 0.333 0.020 2.25 3.69
    1 (1.0000) 1.000 0.989 1.500 1.479 1.000 0.988 0.114 0.79 0.495 0.380 0.020 2.50 4.12
    1 1/8 (1.1250) 1.125 1.109 1.688 1.665 1.125 1.111 0.129 ... 0.557 0.428 0.020 2.81 4.65
    1 1/4 (1.2500) 1.250 1.234 1.875 1.852 1.250 1.236 0.144 ... 0.620 0.475 0.020 3.12 5.09
    1 3/8 (1.3750) 1.375 1.357 2.062 2.038 1.375 1.360 0.160 ... 0.682 0.523 0.020 3.44 5.65
    1 1/2 (1.5000) 1.500 1.482 2.250 2.224 1.500 1.485 0.176 ... 0.745 0.570 0.020 3.75 6.08
    1 3/4 (1.7500) 1.750 1.730 2.625 2.597 1.750 1.734 0.207 ... 0.870 0.665 0.020 4.38 7.13
    2 (2.0000) 2.000 1.978 3.000 2.970 2.000 1.983 0.238 ... 0.995 0.760 0.020 5.00 8.11
    2 1/4 (2.2500) 2.250 2.228 3.375 3.344 2.250 2.232 0.269 ... 1.120 0.855 0.036 5.62 8.99
    2 1/2 (2.5000) 2.500 2.476 3.750 3.717 2.500 2.481 0.300 ... 1.245 0.950 0.036 6.25 10.00
    2 3/4 (2.7500) 2.750 2.726 4.125 4.090 2.750 2.730 0.332 ... 1.370 1.045 0.036 6.88 10.87
    3 (3.0000) 3.000 2.976 4.500 4.464 3.000 3.979 0.363 ... 1.495 1.140 0.036 7.50 11.75
    3 1/4 (3.2500) 3.250 3.226 4.875 4.837 3.250 3.228 0.394 ... 1.620 1.235 0.036 8.12 12.63
    3 1/2 (3.5000) 3.500 3.476 5.250 5.211 3.500 3.478 0.426 ... 1.745 1.330 0.036 8.75 13.50
    3 3/4 (3.7500) 3.750 3.726 5.625 5.584 3.750 3.727 0.458 ... 1.870 1.425 0.036 9.38 14.37
    4 (4.0000) 4.000 3.976 6.000 5.958 4.000 3.976 0.489 ... 1.995 1.520 0.036 10.00 15.25

    ASME B18.3 Spline 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 spline socket head cap screws cover?
    ASME B18.3 is the inch dimensional standard for spline socket head cap screws. It defines head dimensions, body diameter, thread length, total length, drive recess, and tolerances so any compliant spline socket head cap screws interchanges with standard nuts, washers and wrenches across manufacturers.
    What size range is available for spline socket head cap screws under ASME B18.3?
    spline socket head cap screws under ASME B18.3 covers #10 through 1 in nominal diameter, internal spline 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 spline socket head cap screws?
    The companion specification for spline socket head cap screws is ANSI B18.3.5M. Both standards share equivalent functional dimensions but use the corresponding measurement system (inch vs metric ISO M-series).
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