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@Zebra Most quill-ahead adapters just take a plastic bung in the top but there are some that are set up to take a regular top cap. I had to find one in order to fit one of the top caps I laser engraved to Mrs M_V's bike when I put it together.
Sure you could use it to preload the bearings, no idea what brand I went with or anything though. Maybe just look for one with a bolt on top cap instead of the usual bung?
Edit: first hit on google,
M_V-
I still don't get how that would work. If you use the wedge of the adaptor then you don't have access to the star nut to preload the headset bearings; if you don't use the wedge then you're bolting the adaptor downwards into the steerer and relying in the interface of the flat top of the steerer walls and the tapered section of the adaptor to hold the two together. That's not how adaptors are meant to be secured, so I wouldn't trust it, especially since the tension required to properly preload the headset bearing is very low and so wouldn't force them together very firmly.
ffm
It's possible that it won't fit as threadless steerers might have a different internal diameter to threaded ones in order to handle the compression force of the stem (on the other hand may threaded steerers are thicker to have enough thickness for the threads to be cut into, so take that with a pinch of salt).
How do you plan to attach it and preload the headset though? If you leave the star nut in you wont be able to put the extender in as the wedge would foul it. If you remove the star nut and fit the extender you've got nothing to screw the headset preload bolt into.
Presumably you want something like this, but for a 1" steerer. I have no idea whether it would be safe to shim that.