ASME B18.3 Socket Head Cap Screw — Inch Series

The inch-series socket head cap screw covers tooling, dies, fixtures and machine components where the head must stay inside the envelope of the part. It is the US counterpart to DIN 912, and the practical reason for choosing it is the same: the hex key enters the head axially, so almost no radial clearance is needed around the fastener.

Sizes below #4 exist in the standard but are rarely stocked, which is why the range here starts at #4-40. Socket sizes are inch-series hex keys, so a 3/8-16 cap screw takes a 5/16 in key while the metric equivalent uses an 8 mm — a difference that matters when a tooling kit is assembled once and reused. Common stock grades are alloy steel at roughly Grade 8 equivalent strength, with stainless available in the A2 and A4 families at lower strength.

Nominal dimensions in inches as published in ASME B18.3. Tolerances, material requirements and marking are defined separately by the issuing body.

Head diameters and socket depths follow B18.3 rather than DIN 912, so a countersunk metric screw will not sit flush in an inch-dimensioned counterbore even where the thread matches. For tapped holes, use the inch tap drill charts — the metric rule of thumb (major diameter minus pitch) does not apply to TPI, where the drill is instead found from the inch thread tables.