I know you said that you tried every available chemical, but did you try oven cleaner? Try heavy duty easy off. Spray it on and when it dries in a half hour or so spray it again. Wait five minutes and wipe with a paper towel or a cloth. Should come off. As long as you are not doing any aluminum or pot-metal parts you should be good. Be sure to rinse well with water when you are done, and wear gloves! If you decide to try this let us know how well it works for you. I have used heavy duty easy off to remove old grease and carbon from just about everything.