Do you mean the nylon rings that fix the diaphragm to the piston on flat top Bings ?
Those rings were not originally available a a separate item nor was the diaphragm. You were supposed to buy the whole piston and diaphragm # 3 and it is still listed as a whole assembly by Bobs BMW. They do say you can fit a separate diaphragm but add the following cautionary note :-
Dismantling and installing of the diaphragm must be done carefully to avoid damaging the retaining ring which is not available separately!
They are usually a reasonably tight fit and some gentle heat is advised to remove and refit. A few people here have replaced diaphragm on flat tops so maybe someone found a source for the ring.
