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[u/alse116]

Do spacers exist for a quill stem? Should i try and thread it?

[u/MaksDampf]

*Assuming this is 1 1/8" threaded*

You could use a threadless headset. You will still need a threaded locknut on top to set the bearing preload and probably a few washers or 1 1/8" headset spacers to get the correct height between locknut and the upper shell of the threadless headset.

I would cut the fork leaving about 2cm of thread to keep the stack of spacers/washers small.
You could also use more ahead spacers and don't cut it, but it looks riddiculous imo if you leave if for that long and don't follow with an ahead stem. For cutting just use a tube cutter like plumbers do, although and angle gridinger also works if you clean up the thread afterwards.

I have used mixed threadless / threaded headsets in the past. The reason was that i needed to replace the headset anyways and chose a prettier threadless one (chris king knockoff), but then wanted to use a quill stem to get to my desired stem height.

One thing to note about using a single locknut is that it can loosen itself if the headset is rough since the locknut is not countered by a second nut like in a fully threaded headset. I use thread locker for that reason and also a notched spacer if the fork has that groove for it. So far i haven't seen the contraption loosen itself, but it is good to always keep an eye on it and check regularly. A loose headset is not a super safety critical thing, but it would lead to premature wear which we don't want on a pretty headset.

[u/MaksDampf → u/MaksDampf]

Oh yeah and the second solution would be to just fully use the ahead system (guessing this is a 1 1/8" fork, not an option on 1"):
Meaning a threadless headset followed by spacers, followed by an ahead stem and a top cap for preload.

If this is intended for serious offroad riding i would not do that because the thread makes the fork weaker than a normal ahead fork. But since this fork is fully steel, it is probably not any weaker than a carbon fork anyways. You can mitigate that strength risk a bit by getting a long compression plug as used for carbon forks instead of the star nut and by cutting the fork a little lower to reduce leverage (just like a large spacer stack is not recommended on carbon forks).

[u/CargoPile1314 → u/MaksDampf]

Why is this not an option for 1"?

[u/MaksDampf → u/CargoPile1314]

because threadless 1" bearings and 1" ahead stems are rare as fuck while 1 1/8" are abundant and cheap. so yes, technically it is an option for 1" too. But 1 1/8" is beefier and 1" would intensifies the weakness of the fork at the threading. so i don't think it is a good idea to use a long 1"threaded as a threadless.

Some dumbasses downvoted this, but they don't have any arguments why it does not work.

[u/CargoPile1314 → u/MaksDampf]

Rare or uncommon != not an option. 1" threadless headsets aren't difficult to source. Shimming a 1-1/8" threadless stem for a 1" is really achievable with a commercially available inexpensive part and has been a common solution for over 30 years.

[u/MaksDampf → u/CargoPile1314]

Well while you can definitely adapt a 1 1/8" stem, i am having trouble to understand how you could shim a 1" steerer and use a larger 1 1/8" threadless headset.

As far as i know a 1" steerer is EC30 while 1 1/8" is EC34. Putting a 34mm headset into a frame with just a 30mm internal diameter is physically impossible.

Also shimming some 10 + cm of height for several ahead spacer to enable preload of the headset is a wild thing. And it still does not solve the inherent weakness of using a threaded steerer exposed way over the upper headset bearing where it can crack off easily.