5/16-24 x 1/2 Hex Cap Screw SS 18-8 (A2) . 18-8 stainless steel hex cap screws will have a typical composition of 18% chromium and 8% nickel. These percentages exist in 303, 304 and 305 series stainless steels so 18-8 is used at times to designate any and all of those alloy groups. 18-8 stainless steel offers good corrosion resistance and machines easily but will over time stain in coastal environments. They are also acceptable for use in ACQ pressure-treated lumber.