These screws are manufactured with alloy steel, which contains elements to help the screws become more responsive to heat and mechanical treatments.- Grade B7 hex head cap screws with 1/2-13 and larger thread sizes provide an increased bearing surface and thereby qualify as heavy hex bolts. Fastener Length / 2 1/2 in Finish / Plain Material / Carbon Steel Head Height / 5/16 in Head Width / 7/8 in Tensile Strength / 125,000 psi Thread Size / 13 Thread Type / UNC (Coarse) System of Measurement / Inch Thread Style / Partially Threaded Thread Direction / Right Hand Rockwell Hardness / C35 Type / Medium Strength