[Argess]

I take it this was a polished manifold? If so, and it's not too bad, I'd use Mother's Aluminum Polish. If it's in bad shape, then a cotten wheel on a drill, or even a small one on a Dremel, WITH jeweler's rouge would be a good start, after that, the polish above.

If it's a natural or even gray surface, then just use Brake Klean and that's as good as it's going to get unless you want to polish it. That would be a lot of work,especially if it's still on the engine.