Originally Posted by
dezork
If I bought a 26.4mm ISO headset to fit my new steerer tube, would I have to replace the JIS pressed frame races?
You may be able to find an ISO diameter crown race separately for whatever make/model your current headset is.
I noticed that the ISO standard cup outer diameter is 30.2, while the JIS is 30.0. So ISO cups won't fit my frame?
JIS head tubes can be reamed to ISO spec.
Sheldon Brown's site claims that both JIS and ISO have the same threads per inch on the headtube- so why doesn't my threaded race thread on?
They should be the same thread. You may want to chase the threads with a die, but extending the threaded portion will do this, if you decide to go that route.