5/16-18 x 5 1/2 A307 Grade A Carriage Bolt Zinc Plated . A307 Grade A Carriage Bolts are manufactured from low carbon steel to meet or exceed ASTM A307 Grade A. Material is manufactured with rolled threads. Lengths out to 6 inches will be fully threaded. Lengths over 6 to 12 inches will have 6 inches of thread. Zinc plated material will meet or exceed ASTM F1941. Hot-Dip Galvanized material will meet or exceed ASTM A153. Typical hardness will range from HRB 69-100 and a minimum tensile strength of 60,000 psi is required.