A socket cap screw, also known as a socket head capscrew, socket screw, or Allen bolt, is a type of cap screw with a cylindrical head and hexagonal drive hole. ... Socket head capscrews are commonly used in assemblies that do not provide sufficient clearance for a conventional wrench or socket. Socket head cap screws are a special type of fastener. Socket head cap screws are typically used in machine parts, die fixturing, and clamping. Socket head cap screws with a drilled head features a hole drilled in the head to accept a wire thread as a safety feature.