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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):

enter image description here

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.

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    The only real place I'm seeing discoloration in the picture is around the spark plug hole, not around the exhaust ports. Are you seeing at something I'm not? – Pᴀᴜʟsᴛᴇʀ2 Aug 3 '15 at 15:22
  • that's what i am referring to, it is "near" the exhaust valves – amphibient Aug 3 '15 at 15:23
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    Maybe someone else has some ideas, but for me personally, this is nothing I'd be worried about. Just looks like some discoloration and not burnt marks. It looks really clean and that's the main thing I'd be worried about. – Pᴀᴜʟsᴛᴇʀ2 Aug 3 '15 at 15:25
  • Did you examine the head gaskets after they were removed to see where they failed? If so does the point of failure correspond with the clean exhaust ports? – mikes Aug 3 '15 at 20:15
  • the head gaskets, which are layered stainless steel painted black, were all in one piece although the layers were flaking apart, which is normal. there was no visible sign of breach, which i understand is very common and basically that it is virtually impossible to tell, just judging by the looks of an old gasket, whether they were bad or not – amphibient Aug 3 '15 at 20:23
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This is nothing I'd be worried about. It Just looks like some discoloration and not burnt marks. It looks really clean and that's the main thing I'd be worried about.

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