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DIN 933 vs DIN 931: Full vs Partial Thread

DIN 933 is fully threaded; DIN 931 has an unthreaded shank. Same head, same material, same length measurement — the only difference is where the thread stops. If the joint clamps through a clearance hole and only the far end takes a nut, DIN 931 puts smooth shank in the shear plane and is the better part. If the bolt threads into a tapped hole, or the clamped thickness varies, DIN 933 is the safer choice because thread reaches the head.

The one real difference: where the thread stops

Both standards describe a hexagon head bolt with a thread tolerance of 6g and the same head geometry. Length is measured from under the head to the tip in both cases, so a “DIN 931 M10 × 100” and a “DIN 933 M10 × 100” are the same overall length and take the same 16 mm socket.

What changes is the thread run:

  • DIN 933 — thread from just under the head all the way to the tip. There is no unthreaded shank.
  • DIN 931 — thread only over the last part of the length. The rest is a smooth shank at roughly the nominal diameter.

The thread length on DIN 931 is not a free choice: the standard fixes it from the diameter and the overall length.

Bolt length (l) Thread length (b) Example at M10
l ≤ 125 mm b = 2d + 6 M10 × 100 → 26 mm thread, 74 mm shank
125 < l ≤ 200 mm b = 2d + 12 M10 × 150 → 32 mm thread, 118 mm shank
l > 200 mm b = 2d + 25 M10 × 220 → 45 mm thread, 175 mm shank

That formula is the single most useful thing on this page. It tells you, before you order, how much of the bolt is smooth — and therefore whether the nut lands on thread or on shank.

Head dimensions are identical — but check the ISO equivalent

Because the heads are shared between the two standards, one table covers both. Across-flats (s) is the wrench size, across-corners (e) is the clearance the head needs, and k is the head height.

d Pitch s — across flats e — across corners k — head height
M6 1.00 10 11.05 4.0
M8 1.25 13 14.38 5.3
M10 1.50 17 18.90 6.4
M12 1.75 19 21.10 7.5
M16 2.00 24 26.75 10.0
M20 2.50 30 33.53 12.5
M24 3.00 36 39.98 15.0

Watch M10, M12 and M14. DIN 931 and DIN 933 specify 17, 19 and 22 mm across flats. The ISO replacements — ISO 4014 for partial thread, ISO 4017 for full thread — use 16, 18 and 21 mm. The difference is one millimetre, which is exactly enough to make a 17 mm socket spin on an ISO head, or refuse to seat on a DIN one. If a drawing says “ISO 4017 M10” and the box says “DIN 933 M10”, check the head before you build the joint. Everything from M16 up is identical between the two systems.

The full cross-reference between the Chinese, German and ISO systems is in our GB, DIN and ISO fastener standards guide, including the GB/T 5782 and GB/T 5783 rows that map to these two bolts.

What the shank actually buys you

The unthreaded shank is not cosmetic. Three things follow from it.

1. Shear load lands on smooth material

In a joint where the bolt passes through two plates and the load tries to slide them past each other, the bolt is in shear. Thread roots are stress concentrators; a smooth shank is not. On a bearing-type joint, DIN 931 keeps the shear plane on plain shank as long as the plates sit within the grip length. DIN 933 puts thread — the weaker section — in the same plane.

How much weaker depends on the thread depth and the property class, and it is why structural bolting standards specify where the thread may sit relative to the shear plane. For general machine building the practical rule is: if the joint is loaded in shear and the bolt is a close fit in the hole, prefer DIN 931.

2. The shank protects the clamped parts

A threaded shank passing through a clearance hole bears on a narrow strip of thread crests. A smooth shank bears on its full cylindrical face. In an aluminium or plastic housing, or where the bolt is removed and refitted often, the smooth shank does less damage to the bore.

3. Long bolts become cheaper and stiffer

Thread rolling the full length of an M20 × 300 costs more than rolling 46 mm of it, and the full-thread version loses a little section over the whole length. For long tie rods and through-bolts, DIN 931 is both the cheaper and the stiffer part. It is also the reason long fully-threaded bolts are usually supplied as threaded rod rather than as DIN 933.

When the choice is not optional

Two cases decide it for you.

Situation Use Why
Bolt threads into a tapped hole in one part DIN 933 Thread has to reach the head; a shank would leave the first turns of the hole empty.
Bolted through a clearance hole with a nut DIN 931 Nut only needs thread at the far end, and the shear plane stays on shank.
Clamped thickness varies between assemblies DIN 933 Full thread tolerates a nut anywhere along the length.
Long through-bolt, 200 mm and up DIN 931 Cheaper, stiffer, and the thread length is fixed by the standard anyway.
Thread must not appear inside a bore or seal DIN 931 The shank is the sealing and bearing surface.

One trap sits in the first row. If you substitute DIN 931 for DIN 933 in a tapped hole, the bolt will start, feel tight, and then stop — because the unthreaded shank is trying to enter a threaded hole. It looks like a damaged thread. It is the wrong standard.

Common mistakes

Thinking partial thread means lower strength

Both bolts in the same property class have the same tensile capacity in the threaded section. A DIN 931 8.8 and a DIN 933 8.8 fail at the same load if the failure is in the thread. The difference is where the load passes through the shank, not how strong the bolt is.

Ordering DIN 931 for a short length

The formula does not care how short the bolt is, but the joint does. At M10 × 30, DIN 931 gives 26 mm of thread and a 4 mm shank — the shank is doing nothing, and you have paid for a feature you cannot use. Below roughly 3d, partial thread stops making sense; order DIN 933.

Forgetting that both are measured under the head

DIN hex bolts are measured from under the head to the tip, head excluded — the same convention as socket head cap screws, and the opposite of countersunk screws, which are measured overall. A DIN 931 M10 × 100 gives 100 mm from the bearing face, so the head adds 6.4 mm above the surface. If you are working to a total stack height, add the head.

Mixing DIN and ISO heads on the same assembly

M10 across flats is 17 mm on DIN 931/933 and 16 mm on ISO 4014/4017. Two bolts that look identical on the drawing will need two different sockets on the line. Pick one system per assembly and stay in it — or verify the head dimension on every delivery.

What we keep in stock

Our hex bolt range is stainless-first, sold by the piece with no minimum order:

  • DIN 933 — M1.6 through M24, in 304 (A2-70), 316 (A4-70) and PA66 nylon. Lengths from 3 mm at the small end to 300 mm at M10 and M12.
  • DIN 931 — M6, M8, M10 and M12 only, from 30 mm to 300 mm, in 304 and 316 stainless, plain finish.

The asymmetry is deliberate and worth stating plainly: we stock the full-thread bolt across sixteen diameters and the partial-thread bolt across four. DIN 931 demand is concentrated in through-bolted assemblies at M6–M12, and that is what we hold. If you need DIN 931 outside that range, ask — it is a production item, not a stock item, and the lead time is quoted per order rather than promised.

Grades, thread tolerances and what A2-70 actually means in a bolted joint are covered in A2-70 vs grade 8.8. If you are ordering small quantities from China for the first time, the no minimum order guide covers how per-piece ordering works.

FAQ

Is DIN 931 stronger than DIN 933?

No. In the same property class both bolts have the same tensile strength in the threaded section, and that is where a tensile failure occurs. DIN 931 keeps an unthreaded shank, which helps in shear and in fatigue, but it does not raise the tensile rating. A DIN 931 8.8 and a DIN 933 8.8 carry the same load.

Can I use DIN 933 instead of DIN 931?

Usually yes, with two caveats. Full thread in a shear plane is weaker than smooth shank, so on a joint designed around bearing-type shear the substitution reduces capacity. And a fully threaded bolt in a close-fit clearance hole will wear the bore faster if the joint is repeatedly dismantled. In a tapped hole or a nut-and-washer joint at varying thickness, DIN 933 is the safer of the two.

Can I use DIN 931 instead of DIN 933?

Only if the bolt passes through a clearance hole and the nut sits at the far end. In a tapped hole the unthreaded shank cannot enter the thread, so the bolt will stop short. This is the most common wrong-part failure we see in enquiries.

How is the thread length of DIN 931 determined?

By formula, from the diameter and the overall length: b = 2d + 6 for lengths up to 125 mm, b = 2d + 12 for 125 to 200 mm, and b = 2d + 25 above 200 mm. At M10 × 100 that gives 26 mm of thread and 74 mm of shank. The formula is part of the standard, not a supplier preference.

What is the wrench size for M10 hex bolts?

17 mm for DIN 931 and DIN 933, but 16 mm for the ISO 4014 and ISO 4017 equivalents. M12 is 19 mm against 18 mm, and M14 is 22 mm against 21 mm. From M16 upward the two systems agree. Always confirm which standard you have been supplied before torquing.

Are DIN 931 and DIN 933 still current standards?

Both have been withdrawn and replaced by ISO 4014 (partial thread) and ISO 4017 (full thread) respectively, but they remain the reference most drawings and catalogues still use, and the head dimensions of the DIN originals are what many buyers actually expect. Where the two differ — M10, M12 and M14 across flats — state which one you want.

Do you supply DIN 931 in steel grades?

Our stock DIN 931 is 304 and 316 stainless, plain finish, M6 to M12. Carbon and alloy steel versions in 8.8, 10.9 and 12.9 are available to order rather than from stock. See the DIN 931 size and price page for the live range, and the DIN 933 page for the full-thread equivalent.

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