Same hex head, same spanner, same family — so what's actually different? A set screw is threaded all the way up to the underside of the head, while a hex bolt has a plain unthreaded shank behind the head. That one difference decides which one the job needs, and picking wrong either leaves you short of thread or puts threads where they shouldn't be.
Here's the short version, then the detail:
Quick reference — set screw vs hex bolt
| Set screw | Hex bolt | |
| Thread | Fully threaded up to the underside of the head (DIN 933) | Partially threaded — plain shank behind the head (DIN 931) |
| How it fixes | Screws into a tapped hole, or through with a nut | Passes through a clearance hole, held by a nut and washer |
| The shank does | All thread — grip can go as deep as you need | The plain shank takes shear and lines the joint up |
| Best for | Tapped holes, thin material, short or adjustable grips | Structural through-joints, shear loads, timber and plate with room for a nut |
| Standards | DIN 933 | DIN 931 |
| We stock | M4–M20, lengths up to 200mm, grade 8.8 zinc plated and A2 stainless | M8–M20, zinc plated and A2 stainless |
Set screws — the fully threaded hex head
A hexagon set screw is a fully threaded fastener: the thread runs right up to the underside of the hexagon head, with no plain shank. That's the whole point of it. Screw it into a tapped hole and you get the maximum possible thread engagement for the length you're using, or pair it with a nut and washer and you can clamp material of any thickness — there's no unthreaded section to run out of grip.
Where it earns its keep: anything that threads into a tapped hole — steel plate, threaded inserts, machined parts — plus thin sections where a partially threaded bolt would leave the joint hanging on the last two threads, and adjustable joints where something needs to be screwed in and out.
One trap worth knowing: in the UK a "set screw" is this fully threaded hex head fastener. In the US, "set screw" means a headless grub screw — a completely different part. If a drawing or spec from a US source says "set screw", check which one it means before ordering.
What we stock: hexagon set screws M4–M20 in lengths up to 200mm, in grade 8.8 high tensile zinc plated steel and grade A2 stainless steel, conforming to DIN 933.
Shop the range: Hexagon Set Screws
Hex bolts — the partially threaded bolt
A hex bolt (hexagon bolt) carries thread for only part of its length, with a plain shank directly behind the head. You drill a clearance hole, push the bolt through, and tighten a nut and washer on the other side. Because the shank is unthreaded, there are no threads sitting in the hole — the joint can pull up tighter, and the smooth shank acts like a dowel: it lines the parts up accurately and it's the part that carries shear loads.
That makes the hex bolt the choice for structural through-joints — brackets, plates, timber-to-timber connections — anywhere the bolt passes fully through the work and shear strength or alignment matters more than thread depth.
What we stock: hex bolts M8–M20 in a wide range of lengths, in zinc plated and A2 stainless steel, conforming to DIN 931.
Shop the range: Hex Bolts (Hexagon Bolts)
Choosing by job
- Bolting into a tapped hole or threaded insert: set screw, every time — it's the only one that can bite to full depth.
- Through-bolt of two plates or timber with a nut on the back: hex bolt where shear loads or alignment matter; a set screw will do the job on a short grip where a partially threaded shank would waste length.
- Brackets under shear load — shelves, handrails, machinery mounts: hex bolt. The unthreaded shank is the right engineering for loads acting across the bolt.
- Thin material or a short, tight grip: set screw — full thread means no wasted length and no joint riding on the last threads.
- Adjustable joints, or anything that screws in and out: set screw — thread right up to the head gives you full engagement at any position.
- Gates and timber frames: for chunky timber fasteners with "coach" in the name, that's a different decision — see our Coach Bolts vs Coach Screws guide.
Which size and thread?
- Diameters: set screws run M4 to M20; hex bolts M8 to M20. Length is always measured from under the head, not overall.
- Rule of thumb for a tapped hole: aim for at least one diameter of thread engagement in steel — an M10 set screw wants 10mm of thread in the hole. For a through-bolt, the nut should be fully engaged with a couple of threads standing proud.
- Thread type: both are metric coarse as standard — M8 = 1.25mm pitch, M10 = 1.5mm, M12 = 1.75mm. That's the default for construction and general engineering, and it's what our range is built on. Fine pitch (like M10 × 1.0) turns up in automotive and precision work — check the spec before ordering.
- The nut is the same either way: an M10 nut fits an M10 set screw or an M10 hex bolt. The thread is identical — only how much of the fastener carries it changes.
Which finish?
- Zinc plated grade 8.8 — the everyday high-tensile choice for internal and sheltered use.
- A2 stainless steel — wet, coastal and long-life jobs where rust streaks would spoil the finish, and for food, marine and outdoor equipment.
Not sure which fits the job? Call the team on 01270 252 326 — lines open Mon–Fri, 8am–4:30pm. With over 25 years supplying the trade, we can size a set screw or hex bolt for whatever you're building.
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