DIN 931 dimensions
Hexagon head bolts, partially threaded — a plain shank under the head.
The DIN number. Superseded by ISO 4014, and still what everyone orders by.
What DIN 931 is
DIN 931 is the partially threaded hex bolt. The unthreaded shank under the head is the point of it: it takes shear across the joint on solid metal rather than on the root of a thread, and it locates the parts more precisely than a thread can.
Why the shank is left plain
A DIN 931 bolt has a plain, unthreaded shank under the head, and the thread starts partway down. That is the whole distinction between this standard and DIN 933, which is threaded to the head.
The shank is not a saving of thread-cutting; it is the load-bearing part. Where a joint is loaded across the bolt rather than along it, the shear passes through solid metal instead of through a thread root. An M10 DIN 931 puts the full 10 mm diameter in the shear plane where a fully threaded bolt puts roughly 8.5 mm — the thread root sits near the diameter the hole was drilled to before tapping. That is on the order of 38% more area, and without the stress raiser that a thread root is.
The shank also locates. A plain cylinder in a reamed hole positions two parts to a fraction of a millimetre; a thread rattling in a clearance hole does not. That is why machine builders specify DIN 931 for anything that has to go back together in the same place.
The cost is that the grip length matters. The shank has to pass through the joint and the thread has to start beyond it, so a DIN 931 bolt of the wrong length bottoms the shank against the nut and never clamps — the failure that leaves a joint feeling tight while carrying nothing.
DIN 931 dimensions, M2 to M24
| Size | Pitch | Across flats | Across corners | Clearance hole |
|---|---|---|---|---|
| M2 | 0.4 mm | 4 mm | 4.62 mm | 2.4 mm |
| M2.5 | 0.45 mm | 5 mm | 5.77 mm | 2.9 mm |
| M3 | 0.5 mm | 5.5 mm | 6.35 mm | 3.4 mm |
| M4 | 0.7 mm | 7 mm | 8.08 mm | 4.5 mm |
| M5 | 0.8 mm | 8 mm | 9.24 mm | 5.5 mm |
| M6 | 1 mm | 10 mm | 11.55 mm | 6.6 mm |
| M8 | 1.25 mm | 13 mm | 15.01 mm | 9 mm |
| M10 | 1.5 mm | 17 mm (ISO 16) | 19.63 mm | 11 mm |
| M12 | 1.75 mm | 19 mm (ISO 18) | 21.94 mm | 13.5 mm |
| M14 | 2 mm | 22 mm (ISO 21) | 25.4 mm | 15.5 mm |
| M16 | 2 mm | 24 mm | 27.71 mm | 17.5 mm |
| M20 | 2.5 mm | 30 mm | 34.64 mm | 22 mm |
| M24 | 3 mm | 36 mm | 41.57 mm | 26 mm |
Across corners is the across-flats divided by cos 30°, which is what a socket has to clear. The standards quote a slightly smaller minimum because the corners may be rounded.
DIN 931 against ISO 4014
The threads are identical and the two are fully interchangeable in the hole. The heads are not identical. At M10, M12, M14 — DIN specifies 17 mm, 19 mm, 22 mm across the flats where ISO specifies 16, 18, 21 mm. Everywhere else in this range the two agree exactly.
That difference is small and extremely annoying. An M10 bolt made to ISO 4014 takes a 16 mm spanner and one made to DIN 931 takes 17 mm. Both are M10, both thread into the same hole, and a socket set built for one will skip a size on the other. It is why metric sets sold today usually carry both 16 and 17.
Frequently asked questions
What is DIN 931?
Hexagon head bolts, partially threaded — a plain shank under the head. Its ISO equivalent is ISO 4014.
Is DIN 931 the same as ISO 4014?
The threads are the same and the two are interchangeable in the hole, but the heads differ at M10, M12, M14. M10, M12, M14 — DIN specifies 17 mm, 19 mm, 22 mm across the flats where ISO specifies 16, 18, 21 mm. Everywhere else the two agree.
What spanner does an M10 DIN 931 take?
17 mm. DIN specifies that size across the flats; the other standard specifies 16 mm for the same M10 thread. This is why a socket set that covers 8, 10, 13, 17 and 19 occasionally meets a bolt it cannot grip.
Is DIN 931 still current?
Not formally — DIN 931 has been superseded by ISO 4014. It remains the number everyone uses: catalogues, drawings and purchase orders are still written in DIN numbers, and a supplier asked for DIN 931 will ship a part made to ISO 4014.
Other fastener standards
- ISO 4014 — Hexagon head bolts, partially threaded — the ISO standard that replaced DIN 931
- DIN 912 — Hexagon socket head cap screws — the cylindrical head driven by a hex key
- ISO 4762 — Hexagon socket head cap screws — the ISO standard matching DIN 912
- DIN 934 — Hexagon nuts — the plain hex nut
- ISO 4032 — Hexagon regular nuts, style 1 — the ISO standard that replaced DIN 934
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