GRUB SCREW #4 UNC X 1/4

A grub screw (also known as a set screw) is a fully threaded fastener without a protruding head. It is tightened using an internal drive, typically a hex/Allen socket. Grub screws are designed to secure one component against another — most commonly fixing a shaft, collar, gear, or pulley in place by applying pressure directly onto the mating surface.