I'd use a 20ga. shotgun bore brush (phosphor-bronze) and a 18" rifle cleaning rod to get down in the cooling jackets. Use a little bit of CLR in a squirt bottle and let it soak for a few minutes. It works at the molecule level so it won't eat a hole in there unless you let it sit for a very long time.
Rinse liberally with water and repeat till happy. Once you're done make sure you rinse it very well. You don't want that stuff to sit in there for days on end.
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