Have a situation and need help.
I just bought a rigid fork to replace and old Quadra 5 Rock Shox fork. It is 1 1/8" threaded. I assumed that the steerer would be the same length, however it is about 1/4" shorter.
I assembled it and the threaded cup/nut threads on almost all the way (maybe one full thread does not,) and because of this I can't uses the top locknut.
I am using a quill adapter so I can use modern threadless type stems.
What are my options? If I use spacers between the stem and cup/nut will that be enough to hold it in place and keep it from loosening? Is there another type of headset I can use? Since it is 1 1/8" can I change it to a threadless headset set up?
And if it makes a difference, the frame is a '96 Gary Fisher Mamba.
Here is a pic.

You can't use a threadless headset because the fork steerer is too short for a stem so you still have no way to hold the bearing preload.

You might be able to find another 1 1/8" threaded headset with a shorter stack height, hard to say because threaded 1 1/8" is very old and was only produced for a short time. Alternatively, you could keep your current headset and attempt to use smaller ball bearings in place of whatever size is being used currently but that might not work with the headset cup and cone fit.

Seems to me you've caught plenty of threads; so all you're missing is room for the jam-nut? If it were me, I think I would just do this:

Oh sure, do it the easy way.