DIN 933 or DIN 931? Full thread vs half thread hex bolts

DIN 933 hex bolts are threaded all the way to the head; DIN 931 bolts keep a plain, unthreaded shank below the head. That one difference decides which bolt belongs in a joint: full thread where the nut must run close to the head or the bolt goes into a tapped hole, half thread where the smooth shank should sit in the hole and carry the load. Both come in the same hex head, the same metric and UNC sizes, and the same grades, so the choice is about the joint, not the bolt.

The two standards also have ISO twins: DIN 933 matches ISO 4017 and DIN 931 matches ISO 4014. A box marked either way is the same part for ordering purposes.

A full-thread hex bolt (often called a set screw or tap bolt) lets the nut travel the whole length, so it clamps thin parts where a plain shank would bottom out before the nut tightens. It is also the right bolt for a tapped hole in a steel plate or a machine casting, where there is no nut at all and the thread must engage as deep as the hole allows. Clamps, brackets, flanges, rails and anything bolted to a threaded base use DIN 933.

The trade-off is strength under shear: in a bolted joint that tries to slide, the thread sits in the shear plane and the bolt carries the load on its smaller thread core.

A half-thread bolt keeps an unthreaded shank sized to the nominal diameter. Set the length so that shank spans the parts being joined and only the thread sticks out for the nut and washer. The joint then bears on the full shank diameter, which is why structural steelwork, trailer couplings, conveyor frames, pumps and gearbox mounts specify DIN 931 or its ISO 4014 twin.

The thread length on a DIN 931 bolt follows a fixed rule from the standard, so you can check your grip length before you order: twice the diameter plus 6 mm for bolts up to 125 mm long, plus 12 mm from 125 to 200 mm, and plus 25 mm above 200 mm. An M16 x 100 therefore carries 38 mm of thread and 62 mm of plain shank.

• Add up the grip: every plate, washer and bracket the bolt passes through.
• DIN 931: pick a length whose plain shank covers the grip, leaving thread for the nut plus two to three threads showing past it.
• DIN 933 into a nut: any length that clears the nut by two threads works.
• DIN 933 into a tapped hole: aim for a thread engagement of at least one diameter in steel and one and a half to two diameters in aluminium or cast iron.

Common lengths are stepped in 5 mm up to 50 mm and 10 mm beyond, so round up and add a washer rather than down.

Either standard comes in property class 8.8 for general engineering, 10.9 where a higher preload is specified, and stainless A2 for damp or visible work; our guide to bolt grades 4.8 to 12.9 explains the numbers. The finish is a third choice: black (plain) for indoor machinery, electro-zinc plated (EPG) for covered steelwork, hot-dip galvanised for the weather, and stainless by the coast, as covered in stainless, zinc-plated or galvanised.

Match the nut and washer to the bolt: a DIN 934 nut of the same class and finish, and a flat washer under the turning part so the clamping face is not scored.