Hexagon Head Bolt A2 (DIN 931). Hexagon Head. A2 Stainless Steel. M8 5/16" - 1.25 1 - 18 - 24 18 22. Fasteners in grades A2 and A4, tend to be amongst the most popular ones across global markets. Likewise, grade A4, or as known as 316 Stainless Steel is referred to as marine grade stainless steel.