As you can see in the picture, there are burn marks around SOME of my exhaust valve tops and not others (2 out of the 3 cylinders in the picture):
What could be the reason for them? Could it be pinging or premature combustion before the valves were sealed closed? Or perhaps that the valves weren't sealing tight. Since then, I have undergone a complete and detailed head and valve rebuild, cleaning all the valves, crud inside ports, lapped the valves and replaced the stem seals. I have also performed water tests to make sure the valves don't leak after the service I did.
It's a V6 Tacoma and the reason I took it apart is to replace the head gaskets (but then I decided to do the valves while I was at it) because the truck was overheating.