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    2. ASME B18.3.1M Metric Socket Head Cap Screws

    ASME B18.3.1M Metric Socket Head Cap Screws Dimensions

    High-Strength Metric Allen Drive Cap Screws for Confined-Space Mechanical, Hydraulic, and Aerospace Bolting.

    ASME B18.3.1M Metric Socket Head Cap Screws are metric-series precision fasteners featuring a cylindrical head with an internal hexagonal drive socket, manufactured to metric SI dimensional requirements and engineered for applications where high clamping force must be achieved in confined areas without access for external wrenching tools — making them the standard fastener specification for precision machine tools, hydraulic manifold blocks, medical devices, aerospace subassemblies, industrial robots, and automated production machinery. The ASME B18.3.1M standard specifies metric dimensions for head diameter, head height, socket size, key engagement depth, body diameter, thread pitch, thread length, and all dimensional tolerances, providing a globally recognized metric socket cap screw dimensional reference compatible with ISO 4762 for international project procurement. In corrosion-resistant service environments, these socket head cap screws are manufactured to ASTM A1082, the specification for high-strength stainless steel bolting material, delivering excellent corrosion resistance in saline, acidic, and high-humidity environments combined with high tensile strength required for precision mechanical applications in marine instruments, food processing machinery, chemical plant control equipment, and pharmaceutical production facilities. The compact cylindrical head also offers a smaller installation footprint than equivalent hex head fasteners, making them ideal for space-constrained assemblies where every millimeter of clearance is critical. All ASTM A1082 metric socket head cap screw lots are available with EN 10204 Type 3.1 material certification.

    ASME B18.3.1M Socket Head Cap Screws

    Nom. Sizeand Thread Pitch Body Diameter, D Head Diameter A Head Height H Chamferor Radius S Hexagon Socket Sizea J Spline Socket Sizea M Key Engagement T Transition Dia. B a
    Max Min. Max Min. Max Min. Max Nom. Nom. Min. Max
    M1.6 1.6 1.46 3 2.87 1.6 1.52 0.16 1.5 1.829 0.8 2
    M2 2 1.86 3.8 3.65 2 1.91 0.2 1.5 1.829 1 2.6
    M2.5 2.5 2.36 4.5 4.33 2.5 2.4 0.25 2 2.438 1.25 3.1
    M3 3 2.86 5.5 5.32 3 2.89 0.3 2.5 2.819 1.5 3.6
    M4 4 3.82 7 6.8 4 3.88 0.4 3 3.378 2 4.7
    M5 5 4.82 8.5 8.27 5 4.86 0.5 4 4.648 2.5 5.7
    M6 6 5.82 10 9.74 6 5.85 0.6 5 5.486 3 6.8
    M8 8 7.78 13 12.7 8 7.83 0.8 6 7.391 4 9.2
    M10 10 9.78 16 15.67 10 9.81 1 8 ... 5 11.2
    M12 12 11.73 18 17.63 12 11.79 1.2 10 ... 6 14.2
    M14 14 13.73 21 20.6 14 13.77 1.4 12 ... 7 16.2
    M16 16 15.73 24 23.58 16 15.76 1.6 14 ... 8 18.2
    M20 20 19.67 30 29.53 20 19.73 2 17 ... 10 22.4
    M24 24 23.67 36 35.48 24 23.7 2.4 19 ... 12 26.4
    M30 30 29.67 45 44.42 30 29.67 3 22 ... 15 33.4
    M36 36 35.61 54 53.37 36 35.64 3.6 27 ... 18 39.4
    M42 42 41.61 63 62.31 42 41.61 4.2 32 ... 21 45.6
    M48 48 47.61 72 71.27 48 47.58 4.8 36 ... 24 52.6
    • a See also Table 25.
    • b The M14 ×2 size is not recommended for use in new designs.
    • All dimensions are in millimeters
    • LG is grip length and LB is body length. For length of complete thread.
    • For additional manufacturing and acceptance specifications, see ASME B18.3.1M-1986.

    Frequently Asked Questions

    What stainless steel grades does ASTM A1082 cover for socket head 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 socket head cap screws?

    This product follows ASME B18.3 1M, the dimensional standard for socket head 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 socket head 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) socket head 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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