9/16" x 20 TPI.. how far off are they (other than 18 being a bit coarser than 20.. with a Pedal tap set You could CHASE the threads ..

Cutting just a Little bit of difference ... is re threading 18tpi out to 20tpi going to work? IDK.

You ever used a Tap and Die set? You Need Oil . there is Cutting oil made for machinists and mechanics ..



Buying a conversion BB and then putting in a shorter arm square taper crank would have been more straight forward an approach, IMO.

In short Wrong Nuts ? Yes wrong pitch..