Originally Posted by
plonz
I've been looking at a headset in the FS section that has a 27.0mm crown race. The fork I would put it on is sized for a 26.4mm crown race. I would be using 25 5/32" loose ball bearings in each cup. For the sake of argument, let's assume the bearing cups of the headset fit the frame fine.
Could I theoretically use any 26.4mm crown race with the loose ball setup? If not, why?
Not all headsets use 5/32" balls. Your donor race would have to be from the subset that does use 5/32" balls. And not all crown races are the same thickness. If your donor race is too thick, it may change the overall stack height enough to prevent secure engagement of the upper locknut. If it is too thin, the edge of the lower pressed cup may drag on the fork crown.
Mix 'n' match is very much a trial and error affair.