What Kind Of Screw Is This? How To Identify One From A Photo

Before the head, before the drive, look at the thread itself. A machine screw has a uniform thread of constant diameter designed to enter a tapped hole or a nut, and usually a plain unthreaded shank near the head. A wood screw has a coarse, deep, sharply pointed thread on a tapering body. A sheet metal screw is threaded to the tip with a thread nearly as deep as the core is thick. A self-drilling screw has a small drill flute cut into the point.

This single distinction removes most of the catalogue. A 4 mm machine screw and a 4 mm wood screw look the same in a parts drawer and are not remotely interchangeable, because one cuts its own hole and the other needs a thread already there.

Ask whether the head sat flush or proud. A cone-shaped underside means the screw was pulled level into a countersunk hole; a flat underside means it bore on the surface. Put a flat-bottomed head into a countersunk hole and it never seats, and a countersunk head into a flat hole loads only the rim.

Width matters too. A very wide, very shallow head is there to spread load over thin or slotted material, and a tall narrow dome is there to be gripped or to look deliberate. If the original head was wide, a narrow replacement will pull through.

The recess is the best clue to where the assembly came from. Pozidriv is European, Robertson is Canadian, JIS is Japanese, Torx is modern and industrial, slotted is old or decorative. Getting this right also saves the head: a Phillips bit in a Pozidriv or JIS recess bears on the wrong flanks and rounds it out.

Look between the arms of a cross recess. Four fine lines at 45 degrees means Pozidriv. A single punched dot means JIS. Nothing at all means Phillips.

Everything above narrows the family. The answer itself is two numbers: the major diameter measured across the thread crests, and the pitch — millimetres between crests for metric, threads per inch for imperial.

Diameter alone is never enough, because every size has coarse and fine variants and because several metric sizes sit within a tenth of a millimetre of an inch size. M8 and 5/16 inch differ by 0.06 mm in diameter and by 0.16 mm in pitch, and it is the pitch that decides whether the screw threads home or strips the hole.

A photo settles the categorical questions well: thread style, head shape, drive type, point type, and whether the shank is partly plain. Those are shapes, and shape survives a photograph.

A photo cannot settle size on its own, because nothing in the image says how big anything is. Put a ruler, a caliper or a coin in the same plane as the fastener and the picture becomes measurable. Without a scale reference, two screws two sizes apart are the same photograph.