M4-0.. Socket screws are commonly used in assemblies that do not provide sufficient clearance for a conventional wrench or socket.70 Socket Head Cap Screw, Steel, 8 mm Length, 10 PK A hex key is required to tighten or loosen a socket screw.
9 Steel Coarse Thread Cup Point Hex Socket Headless Set Screws 10PK The most common examples are securing a pulley or gear to a shaft.7 x 6mm Plain Class 14..4mm-0.