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I have a stain on the top of the engine, coming from the spark plug hole. I believe it is anti-seeze that oozed or something like that.
I have tried soap & water, HD Boot Scuff Remover, S100, WD-40, scrubbing, and a soft bristle brush and I cannot remove it. I even let the scuff remover and WD-40 "soak" for 15 minutes trying to loosen it. No luck.
Harley makes this stuff called "engine brightner". Actually made to spray on the black sections of the motor and removes stains and makes the motor look like new. Really works. $14.99.