The SAE has established a sequence of grades from 0 to 8 for steel bolts, on the basis of the metal from which the bolt is made and the manner of manufacture. Available grades run from 2 to 8, with 8 the strongest. Higher grade numbers almost always mean increased strength (an exception is that some grade 6 bolts are stronger than grade 7). The heads of steel bolts are marked to identify their grade.
In the 1980s, large numbers of counterfeit bolts appeared in the United States, almost all imports. For this reason, the SAE grade markings can no longer be trusted unless one knows exactly who made and graded the bolt. Aerospace-grade bolts are also being counterfeited (even NASA has been duped, to the tune of one million dollars to disassemble the Astro 1 space lab to remove counterfeit and defective fasteners).

As a rule, when a bolt is installed the nut (over a washer) should be turned and not the bolt's head. Unless a torque wrench is used the tendency is to undertighten large bolts and overtighten small ones. Suggested torques are given below for the three most common grades. These suggestions do not apply if the bolt or nut has been specially lubricated.