When I had a BMW motorcycle I used oven cleaner (eazy off) to take a layer off of oxidized aluminum.  It EATS aluminum so be careful where you use it.  I used it to my advantage on the motorcycle.  I don't know what it does to other metals but I would say use with caution.

For pistons in Continentals (with major lead and carbon deposits) I have used a product called BH38 from Spartan chemicals.  We just soaked them in it undiluted.  An hour later we removed them and rinsed them with water and they were like new.  BH38 is also great for removing oil and grease on the engines and hose it off with water.  It doesn't hurt aluminum nor rubber.  We have used it here for years with no problems.

Jeff