Originally Posted by sokoloff

Looking closely at #1, I noticed that there were a lot more threads showing sticking out of the head than the others. It turns out that #1 plug was replaced by the dealer about 18 months ago. It was done without removing the head. It is my theory that it was a little hard to reach and see and it wasn't screwed in all the way and thus did not seal, allowing carbon to enter and seal that sucker up solidly in the prechamber.