The thickness of the shaft is no problem. I attach an exel-sheet, where you
can see this.
The difference in cross-square between a 0.5cm shaft and a 4.5cm shaft is
more or less the same as the difference between a rotor-diameter between
19.5 and 20.0cm. If you need a bigger central hole because and the rotor
would not work (which I don't believe), make the diameter of the rotor a
_little bit_ bigger.