Hex head bolts (cap bolts) are hexagonal holes in a cylindrical head, and the best feature is to use a hex wrench for tightening. It is also called cap screws and socket screws, but as the name is a "hexagonal bolt" or "cap bolts" and the surface treatment is generally black dyed. The dimensions of steel hexagonal bolts (cap screws). The size of the screw diameter x the neck length. (Not total length) )