Having ruin one of these already, I have learnt my lesson for getting it onto the motor shaft. It may appear to be a little tight, which is good. What I do now is heat it on the gas stove (direct flames) - using some sort of grip to hold onto to it, course. When the alloy is nicely expanded then you will find it fits onto a cold motor shaft pretty smoothly. If it is still does not go on, then use a little wet n dry (sandpaper) onto the spinning shaft. I have only had to do this to one motor. Before the shaft has cooled, make sure you tighten up on the grub screw onto the flat of the motor shaft (so I heat with the grub screw on, but slightly loosen out!)
Also, note that this adaptor has a shorter section length where the grub screw is, work out which adaptor suits your rotor best.