The threads on the prop shaft stop about a 1/2 inch from the spur gear.
This pushes the prop further forward, accentuating the prop vibration.
While measuring the threads to cut them closer to the spur gear I found that the stock prop will press fit on the threadless part without the need to use a retaining bolt. To press it on, use the washer and nut on the front only to work the prop back. Do not tighten it all the way as that will bind the shaft bearings.
Prop vibration was reduced considerably.