Grade 8 Hex Bolt: The hex head bolt has a six sided hexagonal shaped head. This is a machine threaded bolt typically used with a nut or in a pre-threaded hole. The Grade 8 bolt can be distinguished by the 6 dashes (l) on top of the head of the bolt, these dashes on the head of the bolt are equally spaced over the circumference of the head, see photo below. Grade 8 hex bolts are made from medium carbon alloy steel and heat treated. A Grade 8 hex bolt has a greater tensile strength than a Grade 5 and Grade 2 hex bolt. They are designed for use in high stress applications. The length of a hex bolt is determined by measuring from under the head of the bolt to the base of the threads.