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ID:	9740A cheap way to clean bare aluminum is to use an S.O.S pad. Takes a little effort but is cheap and no nasty chemicals. I think the soap in the S.O.S pad is even biodegradable.

I even did this to 4 cyl, aluminum head and and had to replace a the valve cover gasket 6 years later and it still looks looked shiny and prevented varnish from sticking.