Made from Grade 8 steel, these screws can be used in high-stress applications, such as valves, pumps, motors, and automotive suspension systems. They are at least 25% stronger than medium-strength steel screws. Grade 8 steel hex head screws are marked with six radial lines. Length is measured from under the head. Zinc-flake-coated steel screws are about five times as corrosion resistant as zinc yellow-chromate plated screws and have good resistance to salt water and acids. Also known as ultra coat and armor coat.