Hex Head Screws are cap screws with a hexagonal head, designed to be driven by a wrench (spanner). An ASME B18.2.1 compliant screw has somewhat tighter tolerances than a hex bolt for the head height and the shaft length. The nature of the tolerance difference allows an ASME B18.2.1 hex head screw to always fit where a hex bolt is installed but a hex bolt could be slightly too large to be used where a hex head screw is designed in.