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Headsets
This seems to be a common problem. At the moment my partner's son is searching for a set that would fit his old British frame. It seems to me there are more than two possible sizes - at a guess I suspect faulty conversion between imperial and metric.
What to do? If it's the crown race loose on the steerer (common) one crude solution is to take a chisel to the steerer and hope that the race will be held in place by the burrs you have created.
A better method is to put some brass or silver solder onto the steerer and then turn (or file) it down until you get a tight fit. This will probably burn some of the paint on the fork crown and care must be taken not to damage the brazing which holds the steerer into the crown.
Third method - Use metal loaded epoxy glue to take up the slack. This will work in the short term, but doesn't last for ever.
Of course the best way is to find the correct headset, but that's easier said than done.
clubman-
I’ve mixed and matched bearings, cups and crown races amongst jis and iso headsets to get something that fits.
If the crown race seat really is undersized then I’d go with @midlife suggestion of centre punching around it.
Adding some silver sounds like a good idea but I reckon you’ll struggle to get a very even application and cutting a decent seat into an uneven surface is not easy.
I’d strongly recommend against adding any brass to a fork that has any kind of crown lug as you’ll be taking it up to a temperature that will melt out the silver from the original construction.
M_V
Weird. So the headset I have is a Campagnolo Chorus 1" threaded and according to that table it should be 30.2mm but it is in fact 29.8mm. Does. Not. Compute.