Originally Posted by GCRyder
If mixing JIS crown races and ISO cups is an issue, I guess I'll find out someday, because I have two bikes that have that combination and have been that way for a long time. That happened by way of experiences pretty much the opposite of bigbossman's - I ordered new ISO headsets only find that their crown races wouldn't fit on the forks I wanted use. So, I just polished up the old JIS crown races and installed the rest of the ISO headsets. No apparent issues so far.
Of course, this occurred before I discovered that JensonUSA has an entire page of just
crown races, and that I had overlooked the option box for selecting either 26.4 or 27.0 when ordering the headsets.
You may have had a headset that used some of both standards, I believe it's fairly common for a fork to have the base of the steerer tube where the crown race seats sized for a JIS 27.0mm crown race and for the frame's head tube to be sized for ISO cups. Even if the head tube is sized for JIS, it can usually be sanded or reamed out enough to make ISO cups fit (it's only a few tenths of a millimeter difference in diameter).............But if you have a headset that's JIS-sized in the crown race
and bearing cups, putting those cups in a headtube that's meant for an ISO headset probably won't work because the cups will be too small, you won't have the proper interference fit. Hopefully, the headset that bigbossman wants to use only has the JIS crown race and the bearing cups are ISO-