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Is it possible to get rid of these dark blotches on the cast part the case without media blasting the surface? They don't appear to be oil/grease stains. Perhaps some sort of corrosion?

A wire brush (not drill powered) can improve it.
For difficult to get at corners, I pull some wires out of a normal wire brush, stick them in a copper plumbing tube and hammer them flat to retain, to get a sort of paintbrush style wire brush.

Jordan wrote:A wire brush (not drill powered) can improve it.
For difficult to get at corners, I pull some wires out of a normal wire brush, stick them in a copper plumbing tube and hammer them flat to retain, to get a sort of paintbrush style wire brush.

Do you use the wire brush dry?

Yes, by hand you can't generate enough heat to be a problem.
You can do lots of damage with a powered rotary wire brush on aluminium.
This is for the unpolished parts only, of course.

http://www.machinerycleanery.com/

I did buy a set of these (the stainless steel brushes, not the nylon ones) but have never really pursued using them. Too busy I guess.

A friend of mine did buy a set for his BSA engine and had good results.

Jim

I now favor gluing strips of Scotch-Brite abrasive nylon pads, or equivalent, to hand implements like rods, spatulas or hacksaw blade metal to get into difficult places. For example, I have used a hot glue gun and wrapped a pad strip around an old hacksaw blade to fit perfectly in between the cylinder fins. Use a little rubbing compound and the areas polish nicely without diminishing the "cast" appearance of the aluminum. Reheat the implement to remove and replace the abrasive pad. With a little imagination, home made abrasive implements can be fitted onto drills or reciprocating power tools to make the job easier.

This is driving insane. The more I rub using a fine abrasive scotch pad, the darker the stains become.

Sounds like a you are leaving some scotch pad material on there. I've seen that myself. Id try wetting a rag with carb cleaner or acetone and rubbing it now and see if it lightens Be careful around any paint.

Kev

Maybe try a little metal polish after the Scotchbrite treatment? I've had really good results with Mother's Billet polish: http://www.europaparts.com/mothers-bill ... fgodQPAJkQ

I'm not so sure you'd want to polish the unpolished castings.