Use a 3/8" -24 tap to cut threads into the 5/16" pilot hole.
Use cutting oil (or WD40) and keep the tap threads clean.
If you have never used a tap before, the pilot hole is NOT the time to learn. I recommend you read up on how to correctly do this and practice on another piece of steel. The proper way to tap a hole is one full turn forward (clockwise) and one-half turn back (counter-clockwise).
Since the shaft is made of hardened steel, I cut it in one-half turn forward increments and cleaned the threads often. Better safe than sorry at this point.
