Bolt

A bolt is a threaded fastener designed to pass through a hole in two or more parts and be secured with a nut on the opposite side. Unlike a screw, which threads directly into a tapped hole, a bolt relies on a separate nut to clamp the assembly together.

How it works

The bolt’s shank is smooth for most of its length, with threads cut near the tip. It passes through clearance holes in the parts being joined, and a nut is threaded onto the protruding end. As the nut is tightened, it pulls the bolt head and nut together, compressing the parts between them. This clamping force — called preload — is what holds the joint tight.

Head shapes

Bolt heads come in several standardised shapes: hexagonal (the most common), square, hex socket (Allen), and Torx. The hex head is dominant because it can be gripped by a wrench or socket from six different angles, making it practical in tight spaces. The 12-point box-end wrench was designed specifically to maximise engagement with hex bolt heads.

Grades and strength

Bolts are manufactured in different grades that specify their tensile strength. In the metric system, grade 8.8 bolts have a tensile strength of 800 megapascals; grade 10.9 bolts reach 1,040 MPa. Higher-grade bolts are used in structural steel, automotive engines, and other applications where failure would be dangerous. The grade is typically stamped on the bolt head.

Bolt vs. screw

The distinction is functional, not visual: a bolt is used with a nut; a screw is threaded into a tapped hole or drives into wood. In practice the terms overlap — many fasteners called “screws” could technically be called “bolts” and vice versa.

How to identify a bolt

Three measurements pin down almost any bolt: diameter (the shank’s outer width, e.g. M8 or 5/16“), length (measured from under the head to the tip, not including the head itself), and thread pitch (coarse or fine; see the screw thread entry). The head marking adds the strength grade — three short lines on an imperial bolt or “8.8” stamped on a metric one.

The fastest field test for bolt-vs-screw: look at the threads. A bolt has machine threads — uniform, shallow, parallel-sided — and no taper at the tip. A wood screw’s threads cut into material and its tip is pointed; a bolt’s is blunt, because it is meant to be caught by a nut, not to cut its own way in. If the fastener needs a nut to do its job, it is behaving as a bolt regardless of what the package calls it.

Common confusion: grade swapping

Mixing bolt grades within a joint is a silent failure mode. A grade 4.6 bolt in a joint designed around grade 8.8 looks identical once installed — same head size, same thread — but carries roughly half the tensile strength. Always match nut grade to bolt grade too: a hard bolt with a soft nut strips the nut’s threads; a soft bolt with a hard nut pulls the bolt’s threads out.