I used to fly model airplanes and the heads would get varnished due to the two-stroke oil.
I cleaned them by letting them soak for a day or two in a crock-pot full of antifreeze. I don't remember the temp setting, but they don't get that hot anyway. Did a great job of getting all the dirt, etc. out of the crevices.
"Human beings are the only creatures on Earth that claim a God, and the only living things that behave like they haven't got one." - The Rum Diary