DIN 931 vs DIN 933 vs DIN 934: what actually changes between them

DIN 931 vs DIN 933 vs DIN 934: what actually changes between them

DIN 931 is a hex bolt with a partial thread. DIN 933 is the same bolt threaded along its whole length. DIN 934 is not a bolt at all, it is the hex nut that pairs with both. If the joint carries shear, the plain shank of DIN 931 is usually what you want. If you are threading into a tapped hole or the grip length varies, use DIN 933.

Ask for a hex bolt at any fastener counter in the UAE and you get a question straight back: 931 or 933? In a catalogue photo they look identical. The difference is in the threading, and it matters more than most buyers expect. Get it wrong and you either weaken a shear joint or end up with a bolt that will not clamp down on a short stack of plates.

What a DIN number actually tells you

DIN standards define dimensions and tolerances. Head size, thread pitch, lengths. They say nothing about strength. Strength comes from the property class stamped on the head, such as 8.8 or 10.9.

So “DIN 933 M20 x 60, class 8.8, HDG” is a complete order: geometry, grade and coating in one line. “M20 bolt” is not an order, it is the start of a phone call.

DIN 931, the partially threaded one

On a DIN 931 bolt the thread covers only part of the length. The rest is plain shank. That plain section is the whole point of the standard.

When two plates are bolted together and the load tries to slide them past each other, the bolt takes that load in shear. Size a DIN 931 correctly and the shear plane passes through solid steel rather than through a thread root. On an M20 the stressed area drops from 314 mm2 in the shank to 245 mm2 in the thread. About a fifth of your steel disappears exactly where the load is.

That is why structural drawings, flange connections and machinery joints call for 931.

DIN 933, threaded all the way

DIN 933 threads run right up to the head. That makes it the bolt for tapped holes, short clamp lengths, and anywhere you cannot predict the grip in advance. Maintenance teams like it because one box covers a lot of jobs.

The trade-off is the one above. Put a 933 in shear across its threads and capacity drops. In a pure tension joint the difference barely registers.

DIN 934, the nut that goes with both

DIN 934 is the standard metric hex nut, same thread as both bolts, so it runs on either.

Two things to get right. Match the property class, because a soft nut on a hard bolt strips before the bolt reaches its rated preload: class 8 nuts with 8.8 bolts, class 10 with 10.9. And if the assembly is hot dip galvanized, the nuts need tapping oversize after coating or the zinc layer jams the thread. Any decent supplier handles that automatically, but say “HDG assembly” rather than ordering bolts and nuts as two separate line items.

The three standards side by side

DIN or ISO, and the socket size trap

For most joints the two series are interchangeable in function. ISO 4014, 4017 and 4032 replaced the old DIN numbers.

The catch is width across flats. A handful of sizes changed in the switch, including M10, M12, M14 and M22. An M12 DIN bolt takes a 19 mm socket while the ISO version takes 18 mm. If a site crew has torque tooling set up for one series and a mixed delivery turns up, that is an afternoon gone. Specify one series per project and hold the supplier to it in writing.

Choosing between them in practice

• Joint in shear, or plates that could slide relative to each other: DIN 931, with enough plain shank that the shear plane misses the thread
• Tapped hole, or clamp length that varies job to job: DIN 933
• Nuts: DIN 934, class matched to the bolt, oversize tapped if galvanized
• Outdoor or coastal installation: specify the coating with the same care as the size, because plain zinc plating will not last a Gulf summer outdoors

Frequently asked questions

What is the difference between DIN 931 and DIN 933?

DIN 931 has a plain shank under the head, DIN 933 is threaded the whole way. The plain shank belongs in the shear plane.

Can I use DIN 933 where the drawing specifies DIN 931?

Not without engineer approval. Threads in the shear plane reduce joint capacity.

Do DIN 934 nuts fit both bolt types?

Yes, same thread. Match the property class and order oversize tapping for galvanized assemblies.

Is DIN 931 the same as ISO 4014?

Functionally yes. Watch the width across flats on M10, M12, M14 and M22 if your tooling is already set.

Which DIN bolt sizes are stocked in the UAE?

M6 to M36 in class 8.8, zinc plated or hot dip galvanized, covers nearly everything. Larger diameters and class 10.9 come against order with mill certificates.