5/16-18 x 3 1/2 A307A Hex Tap Bolt Zinc Plated. A307A Hex Tap Bolts are manufactured from low carbon steel. Our hex tap bolts meet our exceed ASTM A307 Grade A. All material is manufactured with rolled threads. Zinc plated material meets or exceeds ASTM F1941. Typical hardness will range from HRB 69-100 and tensile strength will meet a minimum 60,000 psi.