GRADE 8 CARRIAGE BOLTS: Carriage bolts have a round smooth head with a square neck under the head. The square neck keeps the bolt from turning as a nut is tightened on to the bolt. Grade 8 Carriage bolts are made from medium carbon alloy steel and heat treated. A Grade 8 Carriage bolt has a greater tensile strength than Grade 5 and Grade 2 Carriage bolts. They are designed for use in high stress applications. The length of a Carriage bolt is determined by measuring from under the head of the bolt to the base of the threads.