I did a quick search and all I could see were adapters with a quill bottom and a cylindrical top so you can attach a modern stem to an old bike with a threaded steerer.

Do there exist adapters that go the other way?

I am trying to assemble a franken(mountain)bike for a kid, and I have a bike with a complete frame/fork/quill stem/handlebar where the frame is too big; but I also have a smaller frame and a threadless fork, but no stem/bar.

Maybe the best solution is just to get a cheap stem, either that grabs the small bar, or also get a shim?

I don't want to really 'invest' in this bike, as the investment could VERY quickly surpass the value of the whole bike...

never mind, now that the though occurred to me, I see many cheap (sub $10) options on eBay for threadless 1 1/8 / 25.4 stems

You could also check your local bike shop as they usually have a supply of cheap stems exchanged from new bike buyers.

\

Essentially its how Bike Friday makes their bikes now.. threadless fork incorporates its own preload of a threadless headset

with 2 clamping bands around the fork steerer.. Upper one blocks solidly, lower one is a cone-ring, collar .. combination gets wider, as you tighten the bolt.
outer conic forces inner cone downward, being blocked from above, that inner ring presses down on the upper headset..

and above that the handle bar mast, is removable, for packing to travel ...
using a scheme like that, you can use a 1" quill stem inside such a combination, if you wish..

I Got one of the Lower conic band camps and upgraded the original Tikit threadless headset adjustment Scheme, on my bike,
they used a 'problem solvers' adjustable spacer which gets wider by threaded parts...


maybe that and one of their locking spacers
can work for your application...

thx guys. I have a couple threadless stems laying around, and I ordered 31.2-->25.4 shims. Like $7.50, more than they're actually worth, but for this application I don't have time to wait for shipping from Asia, this little girl needs a bike by xmas!

Cool, that would grab the steerer tube like a stem, and allow the quill to grab it from the inside, right?

I found in use, the threaded 'problem solvers' adjustable spacer, whose lower part grips the threads, with a pinch bolt ,
to still unscrew inward and feels loose.. as you rode..

that still uses threads.. widening the parts, road shocks and gravity push them narrower, again..

maybe that larger pinch bolt applies greater force, and threads into steel, not aluminum, for more force before stripping..






.....

Since 1" forks are predominantly for threaded headsets , this is not a problem..

Sheldon certainly has stopped worrying..

that looks nice because it is shiny, but I have always thought threadless was a better system; particularly because of how easy it is to swap stems, or get bars on/off.