In general, three threads are enough for the 3rd adapter nut that accepts the center bolt to hold on spinners such as Tur-truns. I.E., if the nut threads on with three revolutions, it will be fine. My 160DZ 4-stroke is set this way. 500+ flights, no prop ejections, spinner stays put, and it probably kicks harder than any other 4S. But please read this carefully, the two jamb nuts are fully on the shaft. I'm only talking about the third nut which only holds the spinner and is not critical for securing the prop to the thrust washer/backplate.

I've had some setups where there just wasn't enough crank to get everything on. Often you can play "games" with washer thickness, nut sets, etc. such that you can get a safe jamb nut setup and still have enough threads for the third adapter nut.

Cheers.