What size threads on the shaft?
I have a mess of McCollochs and they are all 32's. One was a 28 but I converted it to a 32 also. I have never seen one smaller than a 28. They are all 3/8". All the other brands of converted weed whacker I have are 5/16-24. Why don't you get some nuts from the Hardware and see which one fits?
Jim