After our DIN 912 vs ISO 4762 comparison, the countersunk sibling looks like it should be the same easy story: German standard withdrawn, ISO successor, fully interchangeable, done. It isn't. DIN 7991 and ISO 10642 both cover hexagon socket countersunk head cap screws — but this is one DIN-to-ISO transition where the head actually changed, and where "interchangeable" needs an asterisk.
What Both Standards Cover
Both define the flush-fitting workhorse of machine building: a 90° countersunk head with an internal hexagon drive, metric coarse thread, typically supplied in property classes 8.8, 10.9, and 12.9 or stainless A2/A4. DIN 7991 has been withdrawn; ISO 10642 is the current standard and the correct callout on new drawings.
The Real Difference: The Head Changed
Unlike the 912/4762 transition, the ISO committee revised the head geometry. Two things happened:
1. The head got larger — and stronger. DIN 7991 heads were relatively shallow, which gave the screws reduced load capacity: a known weakness of the old design. ISO 10642 revised head diameter and height, and defines the diameter to the theoretical sharp corner. The practical comparison:
| Size | DIN 7991 head dia. (nom./max) | ISO 10642 head dia. (theoretical max) |
|---|---|---|
| M6 | 12.0 mm | 13.44 mm |
| M8 | 16.0 mm | 17.92 mm |
| M10 | 20.0 mm | 22.40 mm |
2. Some sizes disappeared. ISO 10642 dropped the DIN-only diameters M18, M22, and M24 — and the departed M22/M24 were odd ducks anyway, carrying a 60° head angle instead of 90°.
Even with the improved head, one caveat carries over: due to the countersunk head configuration, these screws may not achieve the full ISO 898-1 minimum ultimate tensile load for their marked property class under head-loaded testing. Countersunk socket screws are for flush fastening, not maximum-strength joints — if the joint needs full class 12.9 capacity, a cylindrical-head DIN 912/ISO 4762 screw is the right tool.
Interchangeability: Check the Countersink
Thread, drive, and general fit are compatible. The interference point is the countersunk recess in the mating part:
- A countersink machined for a DIN 7991 head may leave the larger ISO 10642 head sitting proud of the surface — unacceptable in flush applications.
- The reverse swap can leave the smaller DIN head seated deep or bearing on a reduced contact area.
The rule: countersink dimensions (per DIN 74 / ISO 15065 practice) must match the screw standard. When drawings transition from DIN 7991 to ISO 10642, the countersink callout transitions with them — updating one without the other is the classic error in this product family.
Summary
| Aspect | DIN 7991 | ISO 10642 |
|---|---|---|
| Status | Withdrawn | Current |
| Head geometry | Shallower; reduced load capacity | Revised, larger head |
| Size range | Included M18, M22, M24 (M22/M24 at 60°) | M18/M22/M24 removed |
| Interchangeable? | Thread yes — head seating must be verified | — |
The Bottom Line
DIN 912 → ISO 4762 was a paperwork change; DIN 7991 → ISO 10642 was a design change. Specify ISO 10642 on new work, treat legacy M18/M22/M24 callouts as special-order items, and whenever a drawing migrates between the two standards, verify the countersink — because a socket countersunk screw that doesn't sit flush has failed at its only job.