Identifying a screw when the manufacturer never published the size

Step 1: is it a machine screw or a self-tapping screw?

Machine screws have even, shallow threads and go into a metal thread or a nut. Self-tapping screws have deeper, sharper, more widely spaced threads and usually bite straight into plastic or thin metal.

This decides everything after it. A machine screw is identified by thread size; a self-tapping screw is identified by its outside diameter and the boss it goes into.

Step 2: measure across the threads

Caliper across the outside of the threads. Round to the nearest common size. A reading of 2.9 to 3.05 mm is an M3; 3.4 to 3.6 mm is more likely a #6 imperial or an ST3.5 self-tapper.

Readings that land between metric sizes are your warning that the screw may be imperial.

Step 3: confirm the thread with a nut or a tap

Spin a known nut on by hand. If it runs freely down the full length, you have your size. If it binds after a turn, stop and try the neighbouring size or the imperial equivalent.

Never force a test nut. Two turns of resistance is all the information you need, and forcing it damages the screw you are trying to identify.

Step 4: measure the hole, not just the screw

If the screw is lost, measure the hole. A metal thread reads roughly one pitch under the screw size — a 2.5 mm threaded hole takes an M3.

For a plastic boss, measure the inner hole and add: a boss around 2.0 to 2.2 mm normally takes a self-tapper of about 2.9 to 3.0 mm outside diameter.

Step 5: get the length right

Depth-gauge the hole with the tail of a caliper or a cocktail stick marked with your thumbnail. Subtract the thickness of whatever the screw passes through.

When in doubt go shorter. A short screw holds less; a long screw destroys something behind the hole.

Common questions

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