In the middle of tearing down my 22r for a rebuild and noticed that several of the head bolts have a significant buildup of carbon on them over the entire length, including the threads. This made these certain bolts alot harder to remove also. Where and how do you think this buildup is occuring?

Pretty much everything (pistons/head/valves) is covered in carbon/burnt oil buildup, but I found it odd to also be on a couple of the head bolts and not the rest. I can get some pics if that would help.