I had one once upon a time. I want to say it was 10-32. Take a known 6-32, 8-32 or 10-32 screw and use it as thread gauge. If it will mesh with the crank threads, then you know it's 32 TPI. 10-32 nuts are a dime a dozen at the hardware store. If it's not 10-32, my next guess would be 5mm, like Rossi or Supertigre. Have you tried nuts from any of your other engines to see what fits?