I am trying to dismantle a Shimano 600 headset (it's pitted or the bearings need replacing at least)
the nut and washer are off and unthreading the adjustable race SHOULD be trivial.

but about halfway up the threading, it jams. and I cannot figure out why. Have I left a second washer in there by mistake?

I am afraid that I have already powered it too far as I have started to damage the facets or whatever they are called. I am not even sure I can spin it back down on the tube.

I really don't want damage the fork as it is a Vitus aluminum fork, unique to this bike.

in the event that I have stripped the whatever you call the bit where you stick the wrench, does anyone have a suggestion for how I can handle it instead? wrap the headset in a rubber strip and use a pipe wrench? (ooohhh brutal.)

any suggestions on what I might be doing wrong. Or how I best proceed from here?

You need a thread chaser.
Bike shop should have one.
Yours is a common problem.
The spacer galled up some threads and they have to be repaired before the cone will come off smoothly.
You could try to re-tighten the cone, then spin the top nut back on without the spacer to see if that will clear up the damaged threads.
Works sometimes if the damage is not serious.


Enjoy

In all likelihood, the keyed washer rotated slightly dinging the threads at that height. Put some heavy oil on the threads and force the race up into the jammed area a bit, then back off and repeat, moving a bit farther each time until you can clear the problem.

After it's off oil the race, or better yet use a steel race, and run it back and forth over the damaged zone until the resistance is pretty low.

second

I would not recommend using an adjustable cup that now possibly itself has damaged threads and foreign pieces of metal in it as a thread chaser if you want the fork to remain in good shape. If you value the fork then put a few bucks into having a shop chase the threads. If you can manage to do so remove the bearings so that you can screw the cup as far down as possible. That way the fork die may be better able to reach the damaged threads.

This is all good in theory, but you have to work with the situation as it presents itself.

You could use a triangular file, or thread file to dress some of the damaged file, or if the damage is limited to only a few degrees of arc (not height) you might simply file the area as if creating a "D" key, but the one thing you can't do is use a die to save the threads. Dies have lead tapers and cannot thread to within 3 threads of a shoulder, which is where the damaged area is.

The idea of turning the top cup high enough to clear the balls won't work since by then it would have passed the damaged area, making this approach moot.

If the headcup is alloy, I'd make a decusion in advance as to whether I want to save it or not, If yes some file work in advance of removing it is the way to go. Otherwise, or if it's a steel cup, I'd work it by degrees as described in my prior post. With a bit of hand skill, common sense and some cutting oil to prevent gauling, this will get the cup off while chaisng the threads back close to form.

After the fork, use a steel cup as a chaser rather than a die to dress the threads, because you want to flow the metal nor remove it.

The first thing i'd do is get the proper tool to remove that. E.g

Park - Hw-2