Hi Gearbox, the technical know-how belongs to others on here, however my understanding - which I think is captured in the thread - is all Z4 have the same rear discs and callipers from 20i to 35i with the 35is being the only exception. I’m sure I’ll get corrected if I’m wrong.
The callipers on the rear of my car were reconditioned by Bigg Red in Worcestershire here: http://biggred.co.uk/
and I have been fortunate to secure BMW replacement discs and pads to compliment.
Bigg Red appear to stock and sell refurbished brakes so may be worth a phone call. Otherwise it’s a case of sourcing a pair and having them reconditioned.
I believe the US issue you refer to was our friend trying to source the larger callipers from wherever possible to accompany a max size rear disc. Again I apologise if I’ve got that wrong!
I have to say they look a great match with the #…734 OEM Performance yellow front callipers, and again were easily fitted without any complications.
BMW will happily supply the front kit but will tell you they cannot recommend as, technically, there is no BBK upgrade for the Z4. This probably then calls in to doubt any future warranty, if you needed to do so. However my local BMW Specialist https://bartlett-auto.com/
supplied and fitted mine with no problems and if I had any issues it would go back to them.
It’s an expensive option, one that is unlikely to pay itself back when selling, but it looks sooo much better and probably gives improved stopping… I’ll let you know when I’ve actually done enough miles to bed them in.
Hope that helps a little. A