A. The grade of a bolt indicates the tensile strength of the steel. The higher the number the harder it is. The most common grades are 2, 5 and 8. Grade 2 bolts are made out of mild steel and have no marks on the head, except maybe ‘A 307A’ . Grade 5 bolts are a medium hard bolt and have 3 lines on the head. Grade 8 bolts are made out of alloy steel and heat treated to a real high tensile. Grade 8 bolts have 6 lines on the head. The specification number SAE J429 is the standard that covers these grades, and many other grades that are not as commercially available as these three are.