All of this drilling is unlikely to result in the accuracy expected from a collet chuck.
The closest solution that I can figure is to have a lathe with a 4-jaw chuck.? Insert a short piece of 3/8" rod and indicate until you get the best accuracy.? Now take your collet chuck and tighten well onto the 3/8.? Use a boring tool until you get the diameter that you want for the M14 tap.? Then put the collet chuck body in the 4-jaw, indicate, then tap it.
Or you could buy an M14 collet chuck and sell the M12 one to someone here.
