Screw Drive Types and How to Tell Them Apart

Torque goes into a screw through the contact between the driver flanks and the recess walls. A correct bit spreads that contact over the full depth of the recess. A bit that is one size small contacts near the mouth of the recess only, on a smaller area, and the pressure at that contact goes up sharply. Steel gives way, the corners round off, and the recess stops being a socket.

This is why the failure is sudden. Nothing looks wrong for the first half turn, because the undersized bit does grip until the moment the corners yield. After that no bit will grip, including the correct one you should have used, because the socket you needed is gone.

The practical rule is that the largest bit that fully enters the recess is the right one. If a bit drops in and rocks, it is too small. If it sits proud and will not seat, it is too big, and forcing it flares the recess mouth.

Three cross recesses look alike across a workbench and behave differently under load. Phillips has tapered flanks and a rounded root. That taper is deliberate: it makes the driver climb out of the recess above a certain torque, which was a way of limiting torque before torque-limiting drivers were cheap. The climb-out is called cam-out, and it is the mechanism that rounds Phillips screws.

Pozidriv looks like Phillips with four short tick marks stamped between the arms. Its flanks are close to parallel, so it resists cam-out and takes far more torque. A Phillips bit will enter a Pozidriv recess, contact high on the flanks, hold perhaps a third of the intended torque, and then slip. This is the standard failure on European flat-pack hardware.

JIS is the Japanese cross, usually marked with a single small dot punched next to the recess on the head. Its root is flatter and the driver seats deeper. A Phillips driver in a JIS recess bottoms early, carries load on a narrow band, and cams out. On Japanese vehicles, cameras and motorcycles this is the single most common cause of a rounded screw.

Torx replaced the cross on anything that needed real torque in a small head. Six lobes with straight walls means the driver does not climb out, so the fastener can be set tighter and the bit lasts longer. ISO calls the shape a hexalobular internal driving feature; everybody else calls it Torx.

Security Torx adds a pin standing in the center of the recess, so a solid bit cannot seat at all. The bit you need is hollow and is marked with an H or the word security. Consoles, appliance housings and public fixtures use it.

Pentalobe has five rounded lobes and no pin. It is shallow, it is small, and it is used on phone and slim laptop cases. A Torx bit of roughly the right size will enter a pentalobe recess and turn about a quarter turn before the rounded lobes give up. Tri-wing has three wings set off center like a pinwheel; tri-point has three legs at even 120 degree spacing. They are not interchangeable with each other, and neither is a Torx.

Count the contact features first, before worrying about size. One straight groove is slotted. A cross is one of the three crosses; look for tick marks between the arms for Pozidriv, and a dot beside the recess for JIS. Three features is tri-wing or tri-point, and the difference is whether the legs are offset like a pinwheel or even like a Y. Five is pentalobe. Six with a pin is security Torx. Six with straight walls and no pin is Torx. Six flat sides rather than lobes is a hex socket.

A phone camera in macro mode, held still with light coming across the head rather than straight down, resolves this in seconds. Raking light throws shadows into the recess and makes the feature count obvious in a way that overhead light does not.

If the head is already damaged, count what is left of the outer edge rather than the middle. The mouth of the recess wears last.

Callouts run in two directions. Phillips and Pozidriv count up from tiny: PH000, PH00, PH0, then PH1, PH2, PH3. Torx counts up as a plain number that tracks the point-to-point dimension across the lobes. Hex sockets are named by their across-flats dimension outright, in millimeters or in inches.

The metric and inch hex sets are the trap. A 5/32 inch key measures 3.97 mm and will enter a 4 mm socket, contacting on six narrow lines instead of six faces. It turns until it does not. The same goes for 3/16 inch at 4.76 mm in a 5 mm socket, and 1/4 inch at 6.35 mm against a 6 mm socket.

Bit dimensions on this site are the nominal callout figures, which is what packaging prints. A bit that measures under its own callout, which cheap bits often do, behaves like the next size down.