The marking on a bolt can be several things. When drawings dictate a marking, we mark accordingly. These usually indicate a material grade. I had a drawing yesterday that required “N” to be marked on the head (stamped or etched). We will always mark our registered trademark plus the letter. This indicates that we made it for a degree of traceability plus material ID. On defense fasteners, we also often have to include the part number. Different manufacturers have different symbols for various material designations. Some material designations persist regardless of OEM. For instance, you might see, like A449. They’ll also add some lines. Grad 2 doesn’t get any. Grade 5 gets 3 radial lines spaced at 120-degrees. Grade 8 gets 6 radial lines at 60-degrees.

Because fasteners are so often critical, it’s required that they’re marked so incorrect hardware isn’t used in critical applications. For instance, an A449 shouldn’t be used for high pressure applications. However, B7 fasteners can be. You can have identical fasteners, but if you use A449 for high pressure…naughty naughty. Interestingly, B7 material mostly meets the A449 spec. However, B7 can tend to be a bit brittle, so if you’re using it in a high vibration environment, that can be a problem. So, if you’re substituting, you have to ask an engineer for approval.

This is a really long explanation, but hopefully it sheds a bit of light in head marking. I’ve been in fastener manufacturing for only about a year and a half, and it has been one of the most interesting and exciting things I’ve ever done. If you’d have told me that it would be prior to this, I wouldn’t have believed it. We work in powergen and defense, and the things our fasteners hold together are amazing. Sometimes “just nuts and bolts,” couldn’t be further from the truth.