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    2. ASME B18.2.3.1M Metric Hex Cap Screws

    ASME B18.2.3.1M Metric Hex Cap Screws

    Precision metric hex bolts with washer faces and chamfered points, designed for high-performance machinery and automotive assemblies.

    ASME B18.2.3.1M Metric Hex Cap Screws are the metric equivalent of the inch-series finished hex cap screw, featuring a hexagonal head with an integral washer face and a chamfered point. These features are critical for precision applications as the washer face protects the mating surface and ensures consistent torque-tension relationships, while the chamfer aids in starting the thread. Widely used in automotive engines, industrial machinery, and equipment manufactured to metric standards, these screws offer superior dimensional accuracy compared to standard metric hex bolts. The ASME B18.2.3.1M standard aligns closely with international standards such as DIN 931 (partial thread) and DIN 933 (full thread), facilitating their use in global supply chains. They are available in various property classes (e.g., 8.8, 10.9) to meet specific strength requirements for dynamic and static loading conditions.

    ASME B18.2.3.1m Metric Hex Cap Screws

    ASME B18.2.3.1m Metric Hex Cap Screws Dimensions

    Nominal Dia.,D,and Thread Pitch Body Dia., D s Width Across Flats, S Width Across Corners, E Head Height, K Wrenching Height, K1 1 Washer Face Thick., C Washer Face Dia., DW
    Max Min. Max Min. Max Min. Max Min. Min. Max Min. Min.
    ANSI B18.2.3.1M - M5 5 4.82 8 7.78 9.24 8.79 3.65 3.35 2.4 0.5 0.2 7
    ANSI B18.2.3.1M - M6 6 5.82 10 9.78 11.55 11.05 4.15 3.85 2.8 0.5 0.2 8.9
    ANSI B18.2.3.1M - M8 8 7.78 13 12.73 15.01 14.38 5.5 5.1 3.7 0.6 0.3 11.6
    ANSI B18.2.3.1M - aM10 10 9.78 15 14.73 17.32 16.64 6.63 6.17 4.5 0.6 0.3 13.6
    ANSI B18.2.3.1M - M10 10 9.78 16 15.73 18.48 17.77 6.63 6.17 4.5 0.6 0.3 14.6
    ANSI B18.2.3.1M - M12 12 11.73 18 17.73 20.78 20.03 7.76 7.24 5.2 0.6 0.3 16.6
    ANSI B18.2.3.1M - M14 14 13.73 21 20.67 24.25 23.35 9.09 8.51 6.2 0.6 0.3 19.6
    ANSI B18.2.3.1M - M16 16 15.73 24 23.67 27.71 26.75 10.32 9.68 7 0.8 0.4 22.5
    ANSI B18.2.3.1M - M20 20 19.67 30 29.16 34.64 32.95 12.88 12.12 8.8 0.8 0.4 27.7
    ANSI B18.2.3.1M - M24 24 23.67 36 35 41.57 39.55 15.44 14.56 10.5 0.8 0.4 33.2
    ANSI B18.2.3.1M - M30 30 29.67 46 45 53.12 50.85 19.48 17.92 13.1 0.8 0.4 42.7
    ANSI B18.2.3.1M - M36 36 35.61 55 53.8 63.51 60.79 23.38 21.62 15.8 0.8 0.4 51.1
    ANSI B18.2.3.1M - M42 42 41.38 65 62.9 75.06 71.71 26.97 25.03 18.2 1 0.5 59.8
    ANSI B18.2.3.1M - M48 48 47.38 75 72.6 86.6 82.76 31.07 28.93 21 1 0.5 69
    ANSI B18.2.3.1M - M56 56 55.26 85 82.2 98.15 93.71 36.2 33.8 24.5 1 0.5 78.1
    ANSI B18.2.3.1M - M64 64 63.26 95 91.8 109.7 104.65 41.32 38.68 28 1 0.5 87.2
    ANSI B18.2.3.1M - M72 72 71.26 105 101.4 121.24 115.6 46.45 43.55 31.5 1.2 0.6 96.3
    ANSI B18.2.3.1M - M80 80 79.26 115 111 132.72 126.54 51.58 48.42 35 1.2 0.6 105.4
    ANSI B18.2.3.1M - M90 90 89.13 130 125.5 150.11 143.07 57.74 54.26 39.2 1.2 0.6 119.2
    ANSI B18.2.3.1M - M100 100 99.13 145 140 167.43 159.6 63.9 60.1 43.4 1.2 0.6 133
    • (a) This size with width across flats of 15 mm is not standard. Unless specifically ordered, M10 hex
    • cap screws with 16 mm width across flats will be furnished.
    • All dimensions are in millimeters.
    • Basic thread lengths, B, are the same as given in .
    • Transition thread length, X, includes the length of incomplete threads and tolerances on grip gaging
    • length and body length. It is intended for calculation purposes.
    • For additional manufacturing and acceptance specifications, reference should be made to the
    • ASME B18.2.3.1M-1979 (R1995).
    ASME B18.2.3.1m Metric Hex Cap Screws

    Nominal
    Screw Diameter
    and
    Thread Pitch    		 Fillet Transition Diameter,
    Da    		 Fillet Length, Lf Fillet
    Radius for
    Short and
    Long
    Screws,
    R, Min.    		 Body
    Diameter
    for Short
    Screws,
    Dsi, Min.
    For Short
    Screws,
    Min.    		 For Short
    and Long
    Screws,
    Max.    		 For Long
    Screws,
    Max.    		 For Short
    Screws, Max.
    M5 5.1 5.7 1.2 0.7 0.2 4.36
    M6 6.2 6.8 1.4 0.9 0.25 5.21
    M8 8.3 9.2 2.0 1.1 0.4 7.04
    M10 10.2 11.2 2.0 1.2 0.4 8.86
    M12 12.2 13.7 3.0 1.3 0.6 10.68
    M14 14.1 15.7 3.0 1.4 0.6 12.50
    M16 16.5 17.7 3.0 1.6 0.6 14.50
    M20 20.7 22.4 4.0 2.1 0.8 18.16
    M24 24.5 26.4 4.0 2.3 0.8 21.80
    M30 30.8 33.4 6.0 3.0 1.0 27.46
    M36 36.6 39.4 6.0 3.1 1.0 33.12
    M42 . . . 45.6 8.0 3.4 1.2 38.78
    M48 . . . 52.6 10.0 4.0 1.6 44.43
    M56 . . . 63.0 12.0 4.8 2.0 52.09
    M64 . . . 71.0 13.0 5.0 2.0 59.74
    M72 . . . 79.0 13.0 5.0 2.0 67.74
    M80 . . . 87.0 13.0 5.0 2.0 75.74
    M90 . . . 97.0 13.0 5.0 2.5 85.74
    M100 . . . 107.0 13.0 5.5 2.5 95.72

    ASME B18.2.3.1m Metric Hex Cap Screws Thread Length

    Nominal Screw Diameter and Thread Pitch Thread Length, B, Ref Transition Thread Length, X, Ref Screws Threaded Full Length
    Screw Lengths, L ≤ 125 Screw Lengths, L > 125 & ≤ 200 Screw Lengths, L > 200 Screw Lengths, L, Under Unthreaded Length Under Head, A, Max. [Note (1)] Screw Lengths, L Unthreaded Length Under Head, A,
    Max.
    At Least Under
    M5 16 22 35 4.0 10 1.2 10 25 2.4
    M6 18 24 37 5.0 12 1.5 12 30 3.0
    M8 22 28 41 6.25 16 1.9 16 40 4.0
    M10 26 32 45 7.5 20 2.2 20 45 4.5
    M12 30 36 49 8.75 24 2.6 24 50 5.3
    M14 34 40 53 10.0 28 3.0 28 60 6.0
    M16 38 44 57 10.0 32 3.0 32 65 6.0
    M20 46 52 65 12.5 40 3.8 40 80 7.5
    M24 54 60 73 15.0 ... ... ... 90 9.0
    M30 66 72 85 17.5 ... ... ... 110 10.5
    M36 ... 84 97 20.0 ... ... ... 140 12.0
    M42 ... 96 109 22.5 ... ... ... 160 13.5
    M48 ... 108 121 25.0 ... ... ... 180 15.0
    M56 ... ... 137 27.5 ... ... ... 220 16.5
    M64 ... ... 153 30.0 ... ... ... 260 18.0
    M72 ... ... 169 30.0 ... ... ... 280 18.0
    M80 ... ... 185 30.0 ... ... ... 300 18.0
    M90 ... ... 205 30.0 ... ... ... 340 18.0
    M100 ... ... 225 30.0 ... ... ... 360 18.0

    ASME B18.2.3.1m Metric Hex Cap Screws Length Tolerance

    Nominal Length Nominal Screw Diameter
    Over Thru M5 M6 M8 M10 M12 M14 M16-M24 >M24
    6 10 ±0.29 ±0.29 ±0.29 ±0.29 ... ... ... ...
    10 18 ±0.35 ±0.35 ±0.35 ±0.35 ±0.35 ±0.35 ±0.35 ...
    18 30 ±0.42 ±0.42 ±0.42 ±0.42 ±0.42 ±0.42 ±0.42 ±1.05
    30 50 ±0.50 ±0.50 ±0.50 ±0.50 ±0.50 ±0.50 ±0.50 ±1.25
    50 60 ±1.50 ±0.60 ±0.60 ±0.60 ±0.60 ±0.60 ±0.60 ±1.50
    60 80 ±1.50 ±1.50 ±0.60 ±0.60 ±0.60 ±0.60 ±0.60 ±1.50
    80 100 ±1.75 ±1.75 ±1.75 ±0.70 ±0.70 ±0.70 ±0.70 ±1.75
    100 120 ±1.75 ±1.75 ±1.75 ±1.75 ±0.70 ±0.70 ±0.70 ±1.75
    120 140 ±2.00 ±2.00 ±2.00 ±2.00 ±2.00 ±0.80 ±0.80 ±2.00
    140 150 ±2.00 ±2.00 ±2.00 ±2.00 ±2.00 ±2.00 ±0.80 ±2.00
    150 180 ±2.00 ±2.00 ±2.00 ±2.00 ±2.00 ±2.00 ±2.00 ±2.00
    180 250 ±2.30 ±2.30 ±2.30 ±2.30 ±2.30 ±2.30 ±2.30 ±2.30
    250 315 ±2.60 ±2.60 ±2.60 ±2.60 ±2.60 ±2.60 ±2.60 ±2.60
    315 400 ±2.85 ±2.85 ±2.85 ±2.85 ±2.85 ±2.85 ±2.85 ±2.85
    400 500 ±3.15 ±3.15 ±3.15 ±3.15 ±3.15 ±3.15 ±3.15 ±3.15

    Frequently Asked Questions

    What does ASME B18.2.3.1M metric hex cap screws cover?
    ASME B18.2.3.1M is the metric dimensional standard for metric hex cap screws. It defines head dimensions, body diameter, thread length, total length, drive recess, and tolerances so any compliant metric hex cap screws interchanges with standard nuts, washers and wrenches across manufacturers.
    What size range is available for metric hex cap screws under ASME B18.2.3.1M?
    metric hex cap screws under ASME B18.2.3.1M covers M1.6 through M48 nominal diameter (ISO 4014/4017 family). 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 B18.2.3.1M metric hex cap screws?
    The companion specification for metric hex cap screws is ASME B18.2.1 inch hex cap screws. Both standards share equivalent functional dimensions but use the corresponding measurement system (inch vs metric ISO M-series).
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