ISO 9/16" x 20tpi LH and RH thread taps are available on Amazon for about $15. A drill press and way to keep or clamp them PERFECTLY true is paramount. Mine had a wide flat for the original bore and that was my index.

The 175's had a LOT of extra room. Each MFG crank arm is different, yet this took advantage of a SPARE Shimano 175mm set I had (way too big for me) and it worked like a champ.

This is 155mm length.