Originally Posted by
100bikes
Done correctly, the mill will remove the same amount of material from both the top and bottom of the headtube.
If you are just facing the headtube to assure parallelism and not removing any significant amount of metal, this is fine. If you are intentionally shortening the headtube to allow a headset to match an existing steerer length, you want to remove material
only from the top. Removing any significant amount of metal from the bottom will change the bike's geometry by lowering the front end.