Step (3)
(A) Run the 3/8"-16 tap through the existing 15 MM hole in the knobs.
(B) Cut off all but about 1/2" of the threads on the 2 bolts. Taper the threads to thread the knobs on EZ, apply red, or blue locktite to the threads of your bolt and tighten up the knobs firmly.
(C) Now cut off the heads of your bolts leaving no more than 1/2" of shank. Chamfer your new end.
(D) Using the 12MM, or 31/64 drill bit, drill out one side of the set-screw coupling 1/2" deep.
(E) Put the 3/8" end of the knob into the 3/8" end of the coupling, and snug up the set-screw.

Step (4)
Remove your handle-bars. Install the knob/coupling as a unit. Snug up the set-screw over your pre-load adjustment shafts. Re-install your handle-bars.