The shaft to be threaded should be turned down to dims re the formula:
Nominal diameter - (.10825 /
# thds per inch) For a 1/2-13 that would be: 1/2 - (.10825 / 13 ) =
0.491 in.

I learned something today. Don't happen often for an old goat like me. Is
this just some sort of emperical formula or is there a logical basis?

Karl