I thought that also but it turns out the Z3 fits into the small 168 differential found on the 2002's, 320i's, e30's etc. You are correct if it was for the Z3 6 cylinder cars which use the medium 188 diffs. The 4 cylinder Z3's however used the smaller ones that should fit. Dave from Aardvarc has come available with new guts and for grins I think I might try one to see how it performs on a street driver.