DIN 7991 Countersunk Socket Screws

Sizes M1.6 to M20, 90° conical head that sits flush with the material. These are the only socket screws whose length is measured over the head rather than under it, which is why a DIN 7991 M6 × 20 engages 3.72 mm less thread than a DIN 912 M6 × 20 — and why you order the head height longer when converting a joint to a flush finish.
The recess has to be cut at 90° to at least the head diameter (13.44 mm for M6), and at least one thread diameter of sound material should stay under the countersink, so thin sheet is usually the wrong place for this head. Stock covers 10.9 (10B21 carbon steel), 12.9 (SCM435), A2-70 / 304 and A4-70 / 316 up to M20, fully threaded, in plain, black oxide, nickel, black nickel and black electrophoretic finishes. The length rule, head heights M3–M16 and the swap arithmetic are in the DIN 912 vs ISO 7380 vs DIN 7991 comparison.