Grade 5 medium-strength square head bolts are used in applications where visibility is limited but durable fastening is required. These bolts have large, flat sides and are easy to tighten machinery and equipment with a wrench. Grade 5 steel hex head screws are marked with three radial lines. Square heads are best used in tight or dark spaces where a machinist or fabricator must tighten the bolt blind. It is simple to fit a tool over the square shape even when there are no visual markers available. These bolts come in a black oxide finish, which is corrosion resistant in dry environments, or are zinc-yellow coated for corrosion resistance in wet environments.

Low-strength square head bolts are made from standard hardware-grade steel. These low-strength square-head bolts can be used on metal or wood. They are a low-cost fastener for use in light-duty applications where the bolt is not subject to extreme temperatures or stress.

Grade 8 square head bolts are used in applications where visibility is limited but durable fastening is required. Square heads are best used in tight or dark spaces where a machinist or fabricator must tighten the bolt blind. It is simple to fit a tool over the square shape even when there are no visual markers available. These bolts come in a black oxide finish, which is corrosion resistant in dry environments.