Originally Posted by
nfmisso
If the threads will not fit over the steerer tube; just thread the threadless steerer tube to get a tight fit for the upper cone, then but standard threadless stuff above it. The threads on the aluminum steerer tube will not be strong enough to clamp everything together, but will be more than sufficient to hold things correctly until you clamp everything up. This is approach is a lot simpler and cheaper than the others proposed.
Unless I'm misunderstanding, this is a recipe for disaster. Any threading above the upper bearing is a stress riser (especially in aluminum) and will snap under the stress coming from a threadless stem.
Threaded forks, without exception call for a quill stem that extends below the last thread to buttress the threaded area, and carry stresses down to the non threaded section. Even non-threaded steerers have been known to snap from excess stem stress, and a threaded one with an external stem is doomed from the start.