I can tell you this much from experience: a buddy had an E-Type rear in his street rod many years ago, and being the idiot I am, I noticed the carriers were aluminum, and offered to polish his (for free) while he had the rear end apart for bearings and u-joints. I have no idea what kind of aluminum they used, but it is as hard as a rock! But I stuck to my word and did it, and they came out great, but I would never open my mouth again about doing something like that for free.