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    2. B18.2.3.1M Metric Hex Cap Screws

    ASME B18.2.3.1M Metric Hex Cap Screws – Dimensions, Specifications & Standards

    Metric Hex Cap Screws – Global Standard for Metric Hex Fasteners.

    ASME B18.2.3.1M Metric Hex Cap Screws are metric-series fasteners with dimensional requirements for head height, width across flats, width across corners, body diameter, thread length, washer face, and bearing surface angularity per SI dimensions. These screws are specified for international projects where metric standards must align with ASME dimensional and ASTM material requirements. In demanding service, they are manufactured per ASTM A1014/A1014M for UNS N07718 (Inconel 718) alloy bolting, delivering excellent performance in high-temperature, high-pressure, and corrosive applications such as offshore platforms, LNG facilities, and aerospace structures. These are the metric counterpart to ASME B18.2.1 Hex Cap Screws. Proper documentation of both the dimensional standard and material specification is essential for critical service procurement.

    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
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