Originally Posted by Andy_K

That would require getting the remaining threaded piece of the headset off, which is looking like it may require a destructive method. Whoever thought it was a good idea to make this piece simply knurled didn't consider what would happen when the threads are damaged.

It turns out, the locknut won't screw down all the way even with the spacer removed. The threads are just too munged up part way down, which I suppose might suggest an alternate theory as to how the threads broke. I can thread the lock nut part way on, but then it sticks. The upper race piece threads off easily until it hits this point, a few millimeters from the top, and then it stops. I've applied as much force as I can manage with my bare hands and a cloth for grip, and it just will not pass. Any suggestions? Should I use the lock nut and a long wrench to try to force the threads open? Maybe apply vice grips or a pipe wrench to the knurled piece that's stuck? I'd hate to have to cut wrench flats into the stuck piece.