Countersunk Flat Head Cap Screws, 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. The term socket head capscrew typically refers to a type of threaded fastener whose head diameter is nominally 1.5 times that of the screw shank (major) diameter, with a head height equal to the shank diameter (1960 series design). Forged heat-treated alloy examples are high strength fasteners intended for the most demanding mechanical applications, with special alloy formulations available that are capable of maintaining strength at temperatures in excess of 1000 degrees F.
other head designs include flat head, designed to be seated into countersunk holes. A hex key (sometimes referred to as an Allen wrench or Allen key) or hex driver is required to tighten or loosen a socket screw. Socket head capscrews are commonly used in assemblies that do not provide sufficient clearance for a conventional wrench or socket.