I use a tap
then blow out the threads and plastisize by running thin super glue into it, and shaking out, like Hightower recommends. Then schase with the tap when dry
if you use a router, then you will have to custom make a single edge cutter from a 4 inch long solid carbide rod, 1/4 thick, and add a cutting tip by brazing, only way to have enbough room to enter the hole and cut the tip by backing out with the lathe. The tap works fine for me. I will braze up some cutting tools when i have the spare time, to try it. I have a cutter with a hs steel multi flute head that i use for outside cuts on male pieces of wood. It works great, but way to big to insert into a shaft hole which you will drill out first at 5/16 priobably! Good luck !
