Countersunk Bolt Spec Table

In-Depth Guide

90° (ISO 10642 / DIN 7991 / JIS) vs 82° (ANSI inch) head angle — a purchasing and substitution trap

A metric flat-head screw has a 90° head angle (ISO 10642 / DIN 7991 / JIS B 1194), while an inch socket flat head (ANSI B18.3) is 82°; aerospace also uses 100° and 120°. If a drawing calls for an inch 1/4"-20 flat head and you swap in a nearby M6, the head angle, head diameter and head height all differ and it will not seat in the countersink. A flat head has a head diameter of roughly 2d and a head height of about 0.5d, so the head is shallow — if the countersink angle does not match the head angle, the head contacts only at the rim, wobbles, and will not sit flush. Always confirm the head-angle family before buying or substituting.

Quantifying the weaker tension — 0.5d head height, no high preload like a cap screw (1d)

A flat socket head is about 0.5d tall, roughly half the head and under-head material of a socket-head cap screw (about 1.0d). So under tension the head can pull off, or the wedge effect can split it. Maker catalogs do not rate a flat head tensile capacity as equal to a same-size cap screw. So use flat heads for shear or light clamping, and substitute a socket-head cap screw on tension joints that need high preload. If you need both a flush surface and strength, consider a counterbore plus a cap screw instead.

No positive stop — over-tightening sinks and cracks the material

A socket-head cap screw stops at a positive hard stop — the counterbore bottom or a flange — but a flat head keeps wedging deeper along the countersink, so there is no positive stop. The more you tighten, the deeper the head drives, and in brittle materials (plastics) or thin plate the base material sinks, cracks, or cold-flows (deforms over time). Always manage torque, and in plastics use a metal insert or boss to spread the load rather than driving a flat head straight into the plastic. The sharper the cone angle (82°), the higher the wedging force and the greater the risk of sinking.

The drive — Phillips cam-out vs hex socket/TORX, and stainless galling

When seating a flat head flush and tightening hard, a Phillips drive cams out (the bit lifts), wearing the recess and failing to transmit full torque. For high torque or power-driver tightening, a hex socket or TORX drive is the right choice. Stainless flat heads (A2/A4) also rub over a large conical contact area, generating friction heat, and stainless-on-stainless readily galls (cold-welds), so the screw can seize permanently while tightening — apply anti-seize to the threads and cone, or tighten slowly at low speed.

Under-head fillet and hole-edge burr interference → flush failure

Even when the head angle and countersink angle match, the head can fail to seat flush. If the fillet radius under the head interferes with the edge where the countersink meets the through-hole, the head cannot go all the way down; and if a machining burr is left at the hole edge, the head sits on the burr and stands proud. So make the countersink diameter slightly larger than the head so the fillet does not contact, or lightly chamfer the intersection, and always deburr after machining the countersink and through-hole. Do not blame a flush failure on the angle alone — check for fillet and burr interference too.