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    2. ASME B18.2.3.1M Metric Hex Cap Screws

    ASME B18.2.3.1M Metric Hex Cap Screws Dimensions

    Precision Metric Cap Screws with Controlled Bearing Surface for Global Industrial and Mechanical Bolted Assemblies.

    ASME B18.2.3.1M Metric Hex Cap Screws are metric-series fasteners that establish comprehensive dimensional requirements for hexagonal head cap screws in SI units — covering head height, width across flats, width across corners, washer face, body diameter, thread pitch, thread length, bearing surface angularity, and all associated dimensional tolerances — making them the standard reference for metric hex cap screw procurement in North American-designed systems operating under metric engineering standards and global supply chains. These screws differ from metric hex bolts in their closer body diameter tolerances, mandatory washer face bearing surface, and more controlled head dimensions, giving them superior clamping performance in precision mechanical and process equipment applications. When manufactured to ASTM A1082, the specification for high-strength stainless steel bolting material, these metric hex cap screws provide the combination of metric dimensional precision and outstanding corrosion resistance required for pharmaceutical machinery, food processing equipment, marine instrument enclosures, architectural cladding systems, and chemical plant mechanical assemblies. ASTM A1082 stainless steel metric hex cap screws are dimensionally compatible with ISO 4032 and ISO 4033 metric hexagon nuts, enabling fully corrosion-resistant metric bolted assemblies using matched stainless steel fastener components. Full EN 10204 Type 3.1 material test reports are available for all lots to support quality-controlled project procurement and inspection requirements.

    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.
    M5 5 4.82 8 7.78 9.24 8.79 3.65 3.35 2.4 0.5 0.2 7
    M6 6 5.82 10 9.78 11.55 11.05 4.15 3.85 2.8 0.5 0.2 8.9
    M8 8 7.78 13 12.73 15.01 14.38 5.5 5.1 3.7 0.6 0.3 11.6
    aM10 10 9.78 15 14.73 17.32 16.64 6.63 6.17 4.5 0.6 0.3 13.6
    M10 10 9.78 16 15.73 18.48 17.77 6.63 6.17 4.5 0.6 0.3 14.6
    M12 12 11.73 18 17.73 20.78 20.03 7.76 7.24 5.2 0.6 0.3 16.6
    M14 14 13.73 21 20.67 24.25 23.35 9.09 8.51 6.2 0.6 0.3 19.6
    M16 16 15.73 24 23.67 27.71 26.75 10.32 9.68 7 0.8 0.4 22.5
    M20 20 19.67 30 29.16 34.64 32.95 12.88 12.12 8.8 0.8 0.4 27.7
    M24 24 23.67 36 35 41.57 39.55 15.44 14.56 10.5 0.8 0.4 33.2
    M30 30 29.67 46 45 53.12 50.85 19.48 17.92 13.1 0.8 0.4 42.7
    M36 36 35.61 55 53.8 63.51 60.79 23.38 21.62 15.8 0.8 0.4 51.1
    M42 42 41.38 65 62.9 75.06 71.71 26.97 25.03 18.2 1 0.5 59.8
    M48 48 47.38 75 72.6 86.6 82.76 31.07 28.93 21 1 0.5 69
    M56 56 55.26 85 82.2 98.15 93.71 36.2 33.8 24.5 1 0.5 78.1
    M64 64 63.26 95 91.8 109.7 104.65 41.32 38.68 28 1 0.5 87.2
    M72 72 71.26 105 101.4 121.24 115.6 46.45 43.55 31.5 1.2 0.6 96.3
    M80 80 79.26 115 111 132.72 126.54 51.58 48.42 35 1.2 0.6 105.4
    M90 90 89.13 130 125.5 150.11 143.07 57.74 54.26 39.2 1.2 0.6 119.2
    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 stainless steel grades does ASTM A1082 cover for hex cap screws?

    ASTM A1082 covers duplex grades (S31803/2205, S32205/2205, S32750/2507, S32760/Zeron 100, S39277/AL-6XN) and precipitation-hardening grades (S17400/17-4 PH, S17700/17-7 PH, S15700/15-7 PH, S35500/AM-355) - all available for this product family at TorqBolt.

    What is the standard dimensional reference for these hex cap screws?

    This product follows ASME B18.2 3 1M, the dimensional standard for hex cap screws in stainless steel. Material chemistry, mechanical properties and heat treatment follow ASTM A1082/A1082M. TorqBolt supplies both inch-series and metric sizes per the applicable ASME or ISO drawing.

    Are A1082 hex cap screws suitable for sour service (NACE MR0175)?

    Yes, with hardness control. NACE MR0175 / ISO 15156 permits duplex grades S31803, S32205, S32750, S32760 at 28 HRC max for sour service. 17-4 PH (S17400) at H1150 condition (28 HRC) is the most common PH grade qualified for sour service. TorqBolt provides NACE-MR0175 compliant heat-treat reports.

    What is the typical strength of A1082 super-duplex 2507 (S32750) hex cap screws?

    Super-duplex 2507 (S32750) per A1082: tensile 116 ksi (800 MPa) min, yield 80 ksi (550 MPa) min, elongation 15% min, 32 HRC max. PREN ≈ 42 - suitable for seawater, brine and chloride-rich pressure-vessel service. Higher strength than austenitic A193 B8M while offering pitting resistance well above 304/316.

    References & Standards

    • ASTM A1082 / A1082M - Standard Specification for High-Strength Precipitation-Hardening and Duplex Stainless Steel Bolting for Special-Purpose Applications (current edition 2016, active)
    • ASTM A962 / A962M - General Requirements for Special-Purpose Bolting Materials
    • ASTM A370 - Mechanical Testing of Steel Products
    • NACE MR0175 / ISO 15156 - Sour-service hardness limits for petroleum & natural gas industries
    • ASME B18 series - Dimensional standards for bolts, screws, nuts, studs
    • ISO 4032 / 4033 / 4035 - Metric hexagon nut dimensional standards
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