Kyosho 1-S53005 M3x5mm Set Screw 10pcs

A pack of ten M3 x 5mm set screws, from Kyosho's universal hardware range.

A set screw, also called a grub screw, is headless. The entire body threads into the part so nothing protrudes above the surface, which is what makes it the right fastener for locking a component onto a rotating shaft: a pinion gear onto a motor shaft, a collar onto an axle, a clutch bell onto its pilot. A protruding head on a spinning part would catch on everything nearby.

They work by pressing their tip directly against the shaft. That means installation matters more than with a normal screw. Most shafts have a machined flat for the tip to seat against, and a set screw tightened onto the round part of a shaft will either work loose or dig in and damage it. Thread lock is standard practice, because vibration backs them out readily.

They are also easy to lose, being small and headless, and a pinion that has shed its grub screw usually strips the spur gear before you notice.

At 5mm this is the most commonly used length. This supersedes the older Kyosho number 1161.