Hex bolts are used to fasten two or more parts together to form an assembly either because it cannot be manufactured as a single part or to allow for maintenance and repair disassembly. By definition “A Bolt is a headed and externally threaded mechanical device designed for insertion through holes in assembled parts to mate with a nut and is normally intended to be tightened or released by turning that nut.” When used with a preformed internally threaded (tapped) hole, the hex bolt’s head is turned to tighten, which technically makes it a screw (see our Tech Data section for a discussion regarding the differences between bolts and screws). Hex bolts are also known as: hex head bolts, cap screws, hex cap screws, hex head cap screws, machine bolts, hex machine bolts, hex head machine bolts and, if fully threaded, tap bolts, hex tap bolts and hex head tap bolts. A hex bolt is often differentiated from a hex cap screw by its underhead bearing surface: if it has a circular boss, called a washer face, it’s a hex cap screw¡ªif it doesn’t then it’s a hex bolt.