Originally Posted by operator
Ok. So you're saying you can use a upper headset race with a fork which doesn't have threads extending low enough for the race to actually thread onto anything? And use spacers with a locknut to keep it in place?
Moxfyre has interpreted my answer correctly. No, you can't fit a fork with the threads too short to allow the top race to thread on properly but you can use spacers above it to take up the slack if the total fork length is too long to fit the lock nut correctly. Sorry for the poor initial explanation.
In fact, what you describe was done as a way to allow Shimano headsets (all for threaded forks) to be used with a threadless steerer. Team mechanic machined out the threads from the top race and slipped it over a threadless steerer. This was done by teams whose equipment supplier was Shimano so they could use Shimano headsets.