Bolt vs Screw vs Stud: What's the Difference?

Ask five people what separates a bolt from a screw and you will get five answers, most of them about size or head shape. The catalogs are not much help either, since the same part gets sold under different names. The distinction that actually holds up is not about what the fastener looks like. It is about how it clamps the joint together.

Here is the clean version, plus where studs fit in and why the terms blur so often in practice.

The distinction that matters: how it clamps

Strip away the marketing and there is one honest difference between a bolt and a screw:

• A bolt passes through clearance holes in both parts and is tightened with a nut on the far side. The clamping force is created between the bolt head and the nut. The bolt itself does not thread into either part.
• A screw threads directly into a tapped hole in one of the parts. There is no nut. The clamping force comes from the screw pulling against the threads it engages.

That is the whole idea. A bolt needs a nut to do its job; a screw makes its own threads do the work. Everything else (size, head shape, drive type) is secondary.

Why the same part gets called both

Here is the catch that trips everyone up: a single fastener can be a bolt in one job and a screw in another. Take a standard hex head cap screw. Run it through two plates and tighten a nut on it, and it is functioning as a bolt. Thread that identical part into a tapped hole with no nut, and it is functioning as a screw. Nothing about the part changed; only how it clamps.

This is why the naming feels inconsistent. Manufacturers name products by convention and history as much as function, so you get "hex cap screws" that everyone on site calls bolts, and "machine screws" that behave exactly like small bolts when paired with a nut. In everyday shop talk, anything with a hex head gets called a bolt, and that is perfectly workable. The functional definition only really matters when you are specifying a joint or reading an engineering drawing.

Where the stud fits in

A stud is the odd one out: a rod threaded on both ends (or along its full length) with no head at all. One end screws permanently into a tapped hole; the parts slide over the projecting end, and a nut clamps them down. So a stud is a bit of both: screwed in at one end like a screw, clamped with a nut at the other like a bolt.

Studs earn their keep on joints that get opened repeatedly. Think cylinder heads, exhaust manifolds, and pump housings. Every time you undo a bolt threaded into an expensive aluminium casting, you wear those casting threads a little. Put a stud in permanently and the wear moves to the cheap, replaceable nut instead. Studs also make assembly easier: the parts locate over the projecting studs and hang there while you start the nuts, which is a real help on a heavy head you would otherwise be juggling.

Side by side

Does the difference change anything practical?

For picking a thread size or checking a torque figure, no. A 1/2-13 thread behaves the same whether the fastener clamps with a nut or in a tapped hole. Where it does matter is thread engagement. A screw or stud threading into a tapped hole needs enough engaged length to develop full strength, usually at least one diameter deep in steel and more in soft metals like aluminium. A bolt-and-nut joint sidesteps that worry because the nut is designed to carry the full load of the bolt.

It also affects what fails first. Over-torque a bolt and nut and the bolt usually gives way. Over-torque a screw in aluminium and the tapped threads often strip instead, which is exactly why coarse threads suit soft materials better.

The bottom line

Forget size and head shape. A bolt clamps with a nut, a screw threads into the part itself, and a stud does one at each end. The same hex fastener can play bolt or screw depending on the joint, so do not lose sleep over the label. What matters is how the clamp is made and, if it threads into a part, that there is enough thread engaged to hold. Once you know the thread size, the rest of the spec is on our thread database and the torque chart.