Thread: Coarse / Full Thread / Metric Class 12.9 / Black Oxide Recommended Hex Socket Allen Key Wrench: 3 Black Oxide is not a protective finish and will rust if exposed to moisture. Stainless Steel material is recommended for environments with moisture An externally threaded fastener with unified threads, a cylindrical head with a chamfered top surface, knurled cylindrical sides and hexagonal recess, made from alloy Steel. Ideal for precision assembly work with close tolerances and applications needing a well tooled appearance. Supplies greater tensile strength than equivalent sizes of Grade 5 or Grade 8 hex head cap screws while requiring less surface area or counterbore since the fastener is internally wrenched.