For a bike I'm building, I had my headset crown race press fitted to my fork crown by my LBS, along with my top and bottom cups into the headtube. The crown race was loose such that I could rotate it with my fingers, but there was no discernable lateral play. I was actually able to remove it with my hands, but it's not loose enough to put back on just by hand.
The headset is very good condition English-threaded Shimano 600 HP-6400. The fork is Italian threaded. My understanding is that the fork would expect a 26.5 mm diameter crown race, while the English headset has a 26.4 mm diameter race.
I measured the OD of the fork crown and it is exactly 26.4 mm (digital caliper). The crown race has an ID of 26.42 mm. What are my options? I really would like to use this headset.
(I hope it's clear to all that this is not an ISO vs JIS issue, and neither part is 27.0 mm.)