Did almost the same on my 3/8" BSW drawbar on my mill where the thread had just about stripped.
Turn the end down to 6 mm dia for about 40 mm (just about removed all of the old thread)
Cut off 40 mm from a high tensile caphead and discard the head.
Drill right through the threaded section at 6.1 mm dia.
You drill right through to prevent the piston effect when you subsequently bond the threaded sleeve to the drawbar using Loctite 638.
Job done.
Very effective.
Len. P.