Scott,
A 9/16-20 thread could be rolled into pre-threaded aluminum crank arm if it were reamed out about .010" to .011", maybe less, trial and error. You will probably get about 50% threads. Drilling isn't that accurate so reaming would work better.
9/16-12 and 9/16-18 while not common threads sizes are the US standards. One each of a special RH and LH 9/16-20 Form Tap is going to cost somewhere between $60 and $100 each. How many do you want?
Chas. Colerich Oakland, CA USA
> I wonder what a threadforming tap (as opposed to a threadcutting tap) would
> do in this application? Not that anybody probably makes them in this
> oddball size (especially the left-handed one).
>
> Scott Minneman
> San Francisco, CA USA