Screw thread

A screw thread is the helical ridge wrapped around a cylindrical shaft that converts rotational motion into linear force. It is the fundamental feature that makes screws, bolts, and threaded rods work.

How it works

A thread is essentially an inclined plane wound around a cylinder. When you turn a screw, the thread rides against the mating thread of a nut or tapped hole, pushing the screw forward (or backward) with each revolution. The mechanical advantage depends on the thread’s pitch: a finer pitch (more threads per unit length) moves the screw less per turn but generates more clamping force for the same torque.

Key measurements

• Pitch: The distance between adjacent thread crests. In metric threads, pitch is measured in millimetres (e.g., M8 × 1.25 means 1.25 mm between threads). In imperial threads, it is expressed as threads per inch (e.g., 1/4“-20 means 20 threads per inch).
• Major diameter: The outermost diameter of the thread.
• Minor diameter: The innermost diameter, at the root of the thread.
• Thread angle: The angle between the flanks of the thread. Most metric and imperial threads use 60°; Whitworth threads use 55°; pipe threads use various angles depending on standard.

Metric vs. imperial

Metric threads (M-series) are specified by diameter and pitch: M6 × 1.0 is a 6 mm diameter bolt with 1.0 mm pitch. Imperial threads are specified by diameter and threads per inch: 1/4“-20 is a quarter-inch bolt with 20 threads per inch. The two systems are not interchangeable — mixing metric and imperial threads strips the threads and destroys the fastener.

Left-hand vs. right-hand threads

Most threads tighten clockwise (right-hand). Left-hand threads exist for special applications: the left pedal of a bicycle, gas cylinder valves, and some rotating machinery where normal rotation would loosen a right-hand thread.

How to identify a thread

Identifying an unknown thread comes down to three measurements. First, the major diameter: measure across the outside of the threads with a caliper. Second, the pitch: hold a ruler against the threads and count the crests over a known length (or use a pitch gauge, a small blade set that fans out like a pocket knife). Third, the handedness: a right-hand thread tightens clockwise when viewed from the head; if it tightens anticlockwise, you are holding a left-hand thread, which is rare outside the applications listed above.

A quick sanity check separates metric from imperial: metric pitches are round numbers or simple decimals (1.0, 1.25, 1.5 mm), while imperial threads-per-inch values are whole counts (20, 24, 32 tpi). An M8 measures just under 8 mm across the threads; a 5/16“ imperial bolt measures 7.94 mm — nearly identical, which is exactly why the two strip each other when mixed.

Common confusion: fine pitch vs. standard pitch

The most expensive thread mistake is swapping fine and standard pitch of the same diameter. An M10 × 1.5 nut starts smoothly onto an M10 × 1.25 fine bolt for two or three turns — the threads look compatible — then binds hard. Backing it off and forcing it again gouges both threads. If a nut turns freely then suddenly stops partway, suspect a pitch mismatch before reaching for a bigger wrench.