"Jim Wilkins" wrote:

5/8 - 16 is a standard drill chuck mounting thread.
https://www.tenaquip.com/category/cl...nt-drill-chuck

Thanks for the citation. Under very close inspection, it measures
PERFECTLY at 16 TPI.

Given the fact a 1/2 inch drive socket drives the pedal hub, the
selection of interface sizes is large and precise enough so that the
spindle itself can plug into a socket, with a cotter key (I guess).

I would like to drill a hole through the socket and the spindle, but
not sure I have that capability. Notching each end is probably
sufficient.

Simply screwing a nut onto the drill spindle is a neat way to do it.
That sticks into the working side of a 1/2" drive socket to drive
the 1/2" square taper bicycle bottom bracket (on the other side).
Modular and quite cool IMO, but oh well!