hese screws are made from Grade 5 steel and are suitable for fastening most machinery and equipment. The dimensions meet ASME B18.2.1 specifications. The screws have a hex head and are marked with three radial lines. The length is measured from under the head. Black-oxide steel screws are mildly corrosion resistant in dry environments, zinc-plated steel screws resist corrosion in wet environments, and black corrosion-resistant-coated steel screws resist chemicals and withstand 1,000 hours of salt spray per ASTM B117. To maintain corrosion resistance, use these screws with black corrosion-resistant-coated nuts and washers.