Originally Posted by
hokiefyd
I can't use EITHER half of an ISO threadless headset in a JIS frame without modification, right? I'll have to have the head tube reamed out to use either of the cups from an ISO set, right?
All correct. Except for what I've posted previously, basically ignore all else.
You need to find a JIS threadless headset, or at a minimum the top parts. There is only a slim chance that your current upper cup will mate with the bearings etc from some store-bought unit.
Or get your frame reamed to take an ISO cupset, or just the top. I don't know of any shop that I would trust with this; an expensive disaster in the making.
Or what seems to be the only reasonable option remaining: mods to your existing headset.
BTW: attempting to install ISO cups into your JIS headtube will require immense compressive forces that will break either the installation tool, your frame, or both.