Socket Head/Allen Bolt is a bolt with a hexagonal socket in its head that is designed to be used with an Allen wrench. The Allen bolt is integral to the crank and remains in it when the crank is removed from the spindle. The primary advantage of Allen bolts is that they are perfect if you have a bolt thatâ??s down inside a narrow opening. You can stick the Allen key down into the hole and do up the bolt. A socket cap screws are machine screws with a cylindrical barrel-shaped head containing a hexagonal socket. The head has a smaller diameter than most other machine screw heads, typically just 1.5 times the nominal thread diameter. Socket cap screws may also be referred to as a socket head cap screw, a cap head socket screw, or a hex socket head screw.