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Spdway,

Go down to your local Ace Hardware or similar and pick up a container of Wood Bleach. (Oxalic Acid)
It comes in powder form.

Mix it with water and let your part soak in that for a couple of days. Works very well. I've been using it for years on rusted parts.
Also if you have old spark plugs that are rusty and the porcelain is stained soak them in it too. Takes all the rust off and makes the porcelain look like new.