Originally Posted by
saturnhr
Although some forum members might disagree (see numerous discussions on these threaded unthreaded headset issues in this forum), it is not absolutely necessary to find a threadless headset. You can make the threaded headset work for an unthreaded steerer: the most significant modification is to file out the threads of the upper lock nut so it fits over the new steerer tube (some washers (separators) might need a 'nose' removal too).
You must file out the threads of the top bearing race, not the locknut which is unnecessary for a threadless headset. However, for decent bearing life and proper steering, these threads must be removed uniformly and have a smooth, close tolerance slip fit over the threadless steerer. If they are off center or a sloppy fit there will be steering problems and fork play. Remember, there will be no split cone centering ring in this arrangement. This is a job for a lathe and an operator who knows how to use one, not a guy with a file.
Pro mechanics used to do this on Dura Ace headsets back when Shimano sponsored teams had to use all-Shimano components, even though they never made threadless headsets. It's a stop-gap at best and a kludge at worst. Get the correct purpose-built headset.