Hey,

I've been digging and testing, and I've come to the conclusion that the J-Spec rear is not 3.91 as rumored, but rather 3.64 as indicated on RealOEM . . . any J-Spec owners have any info/proof of the diff being anything other than 3.64? I thought mine was 3.91 based on road testing in '3', but when I jacked it up and did the wheel/driveshaft test, it came out to 3.64.

The other really interesting thing was on the diff itself, stamped 1213174 with a 'J' under it. My guess is that this corresponds to a BMW part number of:
33 10 1 213 174 — but this does not seem to exist.
The RealOEM part numbers end in 235 and 832 for the Jap/Australian 3.64 rear ends.

Very interested if anyone has anything else to add/suggest.

Those numbers can be confusing for sure..... you say on the diff itself, is that in the case casting or the tab under one of the rear cover bolts? My page there from the ETK confirms about it being a 3.64 as you said. My guess is the other is simply a casting code.

Thanks for the reply - what I see about 3.64 on RealOEM jives with that image you posted. I guess it's a casting number, just threw me off as it fits the pattern of diff. part numbers.

cheers