At the risk of further hijacking the retro-roadie thread and/or my own open mold disc road bike build thread, I'm going to start a separate thread to post some details about the threadless conversion headset I designed for my Trek 660:
Product shots below:
First, it is not a standard adapter in any way. You cannot install it without replacing at least the upper headset cup, though I made a new lower cup assembly as well. Mismatched parts just don't cut it!
Second, the way it works is quite simple. Instead of having an upper threaded race that merely adds bearing preload, the upper race of the conversion headset includes a 1 1/8" tube feature. The upper race is bonded to a quill which acts both as an anchor for the tube like a standard quill stem, as a means of locking out the headset adjustment, and completes the remainder of threadless tube. Installation/adjustment is as simple as threading on the upper assembly until desired bearing preload is achieved then using a 6mm hex key to tighten the quill. By using the quill in this manner, the standard locknut on a threaded headset is eliminated, and so is the fairly annoying process of adjusting a threaded headset. Here's a cross section of the CAD file for the upper assembly:
If you look right above the cartridge bearing you can see the internal threads on the upper race that grab the fork's steerer tube (and a dust seal). Above those threads is clearance for extra steerer tube. Of course, there is a limit to just how much extra can be accommodated. The quill is expanded by turning the top cap which has a long steel stud fixed to it (threads and Loctite). The lower threads on the steel stud draw the wedge into the quill's taper causing it to expand. Fully tightened, it won't turn or pull out and the concentric expansion of the quill doesn't disturb the headset's bearings. My original prototypes used a simple top cap and a standard bolt (like a typical threadless adapter) but I hated the look and moved to this updated design:
If you look super close at the above photo you'll notice a very slight difference in the design of the upper race. That's because my Trek still has one of the original prototypes on it, minus the old-style top cap. I have also cut my conversion down to almost the minimum such that the top cap is flush with the top of my stem with no spacers underneath. I did think about more than myself when creating this and designed spacers with an internal o-ring to keep them from rattling and complete the threadless conversion look for those who don't want to run their stems so low. The o-rings were necessary because this design doesn't put any preload force on the spacers (meaning you can swap them around without messing with headset preload, too).
Any further questions?