[mossman]

I am rebuilding the bottom end of a dirt bike and soaked one of the cast aluminum crankcase halves in POR 15 cleaner/degreaser (http://www.por15.com/POR-15-Cleaner-Degreaser_p_14.html) at a ratio of 1:10 and let it soak for approximately three hours.  Upon removing the case, it is a dull dark gray color with black powdery residue.  What is the best way to remove this film and get the case back to its original bright silver color?  Keep in mind this is an engine case and media blasting isn't the ideal solution.  I need a chemical solution to gently etch away whatever is on the surface without affecting the underlying metal.  Some of the recommendations I have been given: vinegar, lemon juice, aluminum jelly, aluminum mag wheel cleaner.

[Intanjir]

Unfortunately the manufacturer's website does not have the MSDS available. It lists the product as being 'formerly MARINE CLEAN' and with a search for 'por 15 cleaner degreaser msds' I was able to locate an MSDS for Marine Clean.
http://www.frost.co.uk/fileuploader/download/download/?d=0&file=custom%2Fupload%2FFile-1358433843.pdf

The ingredients listed are Potassium Hydroxide and Ethylene Glycol Monobutyl Ether.
From this I would guess that the 'film' you are seeing is really just aluminium and that it is darker and dull because the surface has been chemically roughened.
Aluminium's oxide/hydroxide is soluble in a strong potassium hydroxide and you can get lots of black gunk which probably consists of partially reacted aluminium.
If this is right then you simply need to gently polish the surface.
If this is wrong and something organic is responsible then you might try to fully combust the organic matter somehow.
I don't think a chemical solution is the right coarse since the MSDS does not list any inorganics except potassium hydroxide and that would just wash out.

If you don't want to damage the piece with whatever you might try then I would suggest duplicating the problem with a scrap piece of aluminium and testing on that.

Here is a relevant recent thread:
http://www.chemicalforums.com/index.php?topic=80455

[mossman]

then you might try to fully combust the organic matter somehow.

Examples?  Hit it with a blow torch?  Bake it?

[Intanjir]

EDIT: Read Bottom

Most any clean burning flame would probably be enough for most organics. If it burns sooty then there might not be enough oxygen and you would be depositing soot besides, so cleaner is better.

Baking at a normal temperature is not enough to fully combust many organics. Hence why ovens accumulate char. However, self-cleaning ovens can reach considerably higher temperatures when the clean feature is utilized. This would also be sufficient for most organic substances. Just keep in mind that doing it in a dirty oven would be a bad idea as there would be no available oxygen and you would get pyrolysis without combustion.

However, I really doubt that the problem is organic.

EDIT: I just realized that aluminium's melting point is really low. I am not a metallurgist so I really can't say whether trying to burn the organic matter off in an oven is a good idea. Actually I would recommend against it without getting the opinion of someone qualified. Engines are serious business and potentially weakening the metal could prove deadly. Brief exposure to a flame would be less of a concern than a self-cleaning oven that might take it to 550 celsius.

[Arkcon]

I really don't know what this black film is, it may be organic residue, or it may be a small amount of powdery oxide formed from the action of this degreaser. 

This sort of thing comes up often on this board, someone wants an intensive chemical explanation for a phenomena that really needs a lengthy discourse by an expert.

Briefly, when I've degreased or derusted a large metal object, such as the metal floor of my Jeep or the cast iron railing at my mother's house, there's a residue like this one.  Not in color, because we're talking about steel and iron and not aluminum in my case, but in the physical attributes of the powdery coating.  In my case, I use solvent to remove the powdery residue, and let the solvent evaporate,before I paint the metal object.