Im going to resurrect this one as I have found out some more information...
I started a thread on the UK forum (
http://www.z4-forum.com/forum/viewtopic.php?f=9&t=50881) but thought I would share here also:
I have been going about a differential upgrade and acquired a genuine BMW Motorsport 4.10 pinion and crown set. I thought the easiest way would be to acquire another diff and get it rebuilt with the new gears.
I was under the impression that the e46 M3 and Z4m diff were the same except for the rear cooling fins which you can swap over. On initial inspection this would seem correct as the front pumpkin part of the case is identical however upon swapping the rear part of the case, the internals are somewhat different. Here are some pics:
I have had a hunt around and both are genuine variable m differentials and apparently there are 2 different types. The question is what are the differences??
For reference The Z4 one is the blacker one and the clean one is the M3.