Infinite Monkeys, if you end up pulling the head, look closely at the threads at the end (chamber side) of the plug hole. These threads at the very end are often folded over or otherwise in poor shape and might be filled w/carbon, and will need to be cleaned and/or dressed. This is yet another reason to use a tapered chaser/tap.
You can do a reasonably good job of cleaning the threads even on an assembled engine by chucking a brass 12 gauge shotgun bristle brush into a drill. Spin the brush in the hole using carb spray to clean the threads.
Last edited by cobalt327; 12-05-2012 at 12:32 PM.