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.
| 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 |
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ASTM A1082/A1082M (2016) Table 1 lists 14 duplex (ferritic-austenitic) grades, S31100, S31260, S31803, S32101, S32202, S32205 (2205), S32304 (2304), S32506, S32550 (255), S32750 (2507), S32760, S32906, S32950 and S39277, and 5 precipitation-hardening grades, S15700 (Type 632), S17400 (Type 630), S17600 (Type 635), S17700 (Type 631) and S35500 (Type 634). Grade selection depends on design, service conditions and the mechanical properties required.
Dimensions follow ASME B18.3.1M, tabulated on this page. ASTM A1082/A1082M sets chemistry (Table 1), heat treatment and mechanical properties (Tables 2 to 4) and marking (Table 5). ASTM A962/A962M general requirements also apply, and A1082 prevails where the two conflict (clause 4.1). Inch-pound units apply unless the order specifies A1082M (clause 1.4).
ISO 15156-3 Table A.24 accepts wrought duplex stainless steel for any equipment when it is solution-annealed and liquid-quenched or rapidly cooled, has 35 % to 65 % ferrite and has had no ageing heat treatment. With 30 ≤ PREN ≤ 40 and Mo ≥ 1.5 % the limit is 232 °C (450 °F) and 10 kPa (1.5 psi) H2S partial pressure; with 40 < PREN ≤ 45 it is 232 °C (450 °F) and 20 kPa (3 psi). Table A.27 accepts wrought 17-4 PH (S17400) at 33 HRC max after a double age at 620 °C (1150 °F) or a modified double age, for named wellhead, valve and subsurface components, up to 3.4 kPa (0.5 psi) H2S and pH 4.5 or above.
The grade symbol comes from Table 5 (clause 11.1), for example 32205 for S32205 or 174G for S17400 in H1150. With Supplementary Requirement S2 the marking goes on the heads of bolts of all sizes; if the area is too small, the grade symbol may go on one end and the manufacturer's identification on the other. Bolts under 1/4 in. [6 mm] are marked by agreement between purchaser and manufacturer.
Table 4: solution annealed at 1880 to 2060 °F [1025 to 1125 °C], water quenched. Up to and including 2 in.: tensile 116 ksi [800 MPa] and yield 80 ksi [550 MPa] min. Over 2 in.: 110 ksi [760 MPa] and 75 ksi [515 MPa] min. Elongation 15 % min and hardness 33 HRC or 310 HB max for both. The marking grade symbol is 32750 (Table 5).
