You wouldn't use a lathe to do a job like this, you'd use a milling machineRICSPEED wrote:syd the biggest problem for a machinest would be the requirement of a jig in which to hold the headset square in the chuck and also assuming the lathe was big enough in the first place to clear the bar ends ,as said above i would use a joiners marking gauge to scribe a line arround the bottom edge then remove carefully and frequently trial fitting to check the gap as you go
