Thanks. Just to not misslead anyone, on the fourth photo you can see the aluminum prop nut. As a matter of fact there are two nuts for the prop. There is a high-grade steel hex nut to hold a prop and the aluminum prop-nut is used just to lock the steel one. Aluminum prop-nut was turned inside for the outside diameter of the hex nut, and the depth of just a bit less than the height of the hex-nut. So the steel-hex nut is kind of hidden inside the aluminum one.

The steel threaded part of the prop-shaft is made by cutting off the head from the high grade machine screw and set in the hub with green loctite.