Many metric automotive bolts are stamped with numbers creating a lot of confusion as to the actual grade (hardness) of the bolt. We see bolts with a 9, or a 7, perhaps even a 10 stamped in the head. These numbers do not give us any insight as to the size or hardness of the bolt. The other common mistake people make is to size their bolt based upon the wrench size. This also leads to misunderstanding the size and hardness of the bolt in question. The three most common Metric Head Markings (hardness) are:
- 8 Equivalent to our American Grade 5 hardness. PSI tensile strength of 120,000.
- 9 Equivalent to our American Grade 8 hardness. PSI tensile strength of 150,000.
- 9 Equivalent to our American Allen Head bolts. PSI tensile strength of 180,000.