Flush-Mount Inch-Series Socket Screws for Smooth, Unobstructed Surfaces in Aerospace, Marine, and Precision Assemblies.
ASME B18.3 Hexagon Socket Flat Countersunk Head Cap Screws are inch-series precision fasteners featuring a flat conical head at an 82° included angle designed to sit flush with or below the mating surface when installed in a properly prepared countersunk hole , making them ideal for aerospace panels, precision tooling plates, machinery guards, optical instruments, and consumer product assemblies where protruding fastener heads are functionally or aesthetically unacceptable. The ASME B18.3 standard precisely defines dimensional requirements including head diameter, head height, socket size, hexagon key engagement depth, body diameter, thread length, and all associated tolerances, ensuring precision fit between screw and countersink across all compliant manufacturing sources. When manufactured to ASTM A1082, the specification for high-strength stainless steel bolting material, these socket flat countersunk cap screws offer high tensile strength, excellent corrosion resistance, and clean aesthetic appearance required in marine equipment, food processing machinery, medical device structures, and architectural metal assemblies. Proper countersink preparation at the correct 82° angle and depth in the mating component is critical to achieving full bearing contact between the conical head and countersunk surface, ensuring load is transferred efficiently and the head seats fully flush. All ASTM A1082 flat countersunk socket cap screws are supplied with full material test documentation for quality-critical applications.
| Nominal Size | Body Diameter, D | Head Diameter, A | Reference Head Height, H | Protrusion Gage Diameter, G | Protrusion, P | Socket Width Across Flats, J | Minimum Key Engagement, T | Maximum Fillet Transition Diameter, F | |||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Max. | Min. | Max. | Min. | Max. | Min. | Max. | Min. | Max. | Min. | ||||
| 0 (0.0600) | 1.524 | 1.442 | 3.505 | 2.971 | 1.117 | 1.981 | 1.955 | 1.117 | 0.660 | 0.902 | 0.889 | 0.635 | 1.828 |
| 1 (0.0730) | 1.854 | 1.765 | 4.267 | 3.632 | 1.371 | 2.565 | 2.540 | 1.219 | 0.711 | 1.295 | 1.270 | 0.787 | 2.260 |
| 2 (0.0860) | 2.184 | 2.087 | 5.003 | 4.267 | 1.625 | 3.149 | 3.124 | 1.295 | 0.787 | 1.295 | 1.270 | 0.965 | 2.692 |
| 3 (0.0990) | 2.514 | 2.410 | 5.740 | 4.902 | 1.854 | 3.759 | 3.733 | 1.371 | 0.838 | 1.613 | 1.588 | 1.117 | 3.022 |
| 4 (0.1120) | 2.844 | 2.730 | 6.477 | 5.537 | 2.108 | 4.368 | 4.343 | 1.447 | 0.914 | 1.613 | 1.588 | 1.397 | 3.454 |
| 5 (0.1250) | 3.175 | 3.053 | 7.137 | 6.096 | 2.286 | 4.978 | 4.953 | 1.498 | 0.939 | 2.009 | 1.984 | 1.549 | 3.886 |
| 6 (0.1380) | 3.505 | 3.375 | 7.797 | 6.680 | 2.463 | 5.588 | 5.562 | 1.524 | 0.939 | 2.009 | 1.984 | 1.676 | 4.267 |
| 8 (0.1640) | 4.165 | 4.025 | 9.118 | 7.899 | 2.844 | 6.781 | 6.756 | 1.600 | 0.990 | 2.418 | 2.38 | 1.930 | 4.927 |
| 10 (0.1900) | 4.826 | 4.673 | 10.439 | 9.118 | 3.225 | 7.950 | 7.924 | 1.676 | 1.041 | 3.226 | 3.175 | 2.209 | 5.588 |
| 12 (0.2158) | 5.481 | 5.339 | 11.430 | 10.414 | 3.429 | 9.144 | 9.118 | 1.752 | 1.066 | 3.226 | 3.175 | 2.387 | 6.248 |
| 1/4 (0.2500) | 6.350 | 6.184 | 13.487 | 12.192 | 4.089 | 10.769 | 10.744 | 1.828 | 1.092 | 4.031 | 3.967 | 2.819 | 7.112 |
| 5/16 (0.3125) | 7.937 | 7.747 | 16.662 | 15.240 | 5.029 | 13.690 | 13.665 | 1.981 | 1.193 | 4.826 | 4.763 | 3.429 | 8.712 |
| 3/8 (0.3750) | 9.525 | 9.342 | 19.837 | 18.288 | 5.943 | 16.586 | 16.560 | 2.235 | 1.270 | 5.631 | 5.555 | 4.038 | 10.287 |
| 7/16 (0.4375) | 11.112 | 10.906 | 21.437 | 19.837 | 5.943 | 17.526 | 17.475 | 2.641 | 1.600 | 6.426 | 6.35 | 4.038 | 11.887 |
| 1/2 (0.5000) | 12.700 | 12.494 | 23.825 | 22.148 | 6.375 | 18.770 | 18.745 | 3.327 | 2.209 | 8.026 | 7.938 | 4.368 | 13.462 |
| 5/8 (0.6250) | 15.875 | 15.654 | 30.175 | 28.244 | 8.229 | 24.434 | 24.409 | 3.708 | 2.438 | 9.627 | 9.525 | 5.588 | 16.637 |
| 3/4 (0.7500) | 19.050 | 18.811 | 36.525 | 34.417 | 10.058 | 30.124 | 30.099 | 4.318 | 2.667 | 12.827 | 12.7 | 5.588 | 19.812 |
| 7/8 (0.8750) | 22.225 | 21.963 | 42.875 | 40.741 | 11.887 | 35.839 | 35.814 | 4.191 | 2.997 | 14.427 | 14.288 | 6.299 | 22.987 |
| 1 (1.0000) | 25.400 | 25.110 | 49.225 | 46.761 | 13.716 | 41.529 | 41.503 | 4.597 | 3.302 | 16.027 | 15.875 | 7.543 | 26.162 |
| 1 1/8 (1.1250) | 28.575 | 28.158 | 55.575 | 52.806 | 15.519 | 47.218 | 47.193 | 4.978 | 3.556 | 19.228 | 19.05 | 8.255 | 30.149 |
| 1 1/4 (1.2500) | 31.750 | 31.333 | 61.925 | 58.826 | 17.348 | 52.908 | 52.882 | 5.384 | 3.810 | 22.479 | 22.225 | 9.093 | 33.324 |
| 1 3/8 (1.3750) | 34.925 | 34.462 | 68.275 | 64.846 | 19.177 | 58.572 | 58.547 | 5.791 | 4.114 | 22.479 | 22.225 | 10.210 | 36.499 |
| 1 1/2 (1.5000) | 38.100 | 37.637 | 74.625 | 70.891 | 21.005 | 64.262 | 64.236 | 6.172 | 4.394 | 25.908 | 25.4 | 11.049 | 39.674 |
| Nominal Length | Nominal Size, Tolerances on Lengths | ||
|---|---|---|---|
| 0 Through 3/8, Incl. |
7/16 Through 3/4, Incl. |
7/8 Through 1 1/2, Incl. |
|
| Up to 25.4, incl. | -0.762 | -0.762 | -1.270 |
| Over 25.4 to 63.5, incl. | -1.016 | -1.524 | -2.540 |
| Over 63.5 to 152.4, incl. | -1.524 | -2.032 | -3.556 |
| Over 152.4 | -3.048 | -3.048 | -5.080 |
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, 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).
