5/16-18 x 1 3/4 Hex Tap Bolt 18-8 (A2) Stainless Steel . 18-8 stainless steel hex tap bolts 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.