The fit of the flex hex coupler to the motor shaft is area that is often overlooked but very important. If the motor shaft is smaller than the flex coupling (or any style coupling for that matter), then it won't be aligned along the center line after you tighten the grub screw. Now spin that off center load at 15K RPM and you will have a noticeable driveline vibration problem and be unnecessary loading the motors bearings.
So what to do if the fit is not hand in glove is you make it that way. You need to either find a better fitting coupling or add a sleeve to the motor shaft to increase its diameter. When I do this I open an area on the sleeve for the grub screw to pass through such that the grub screw is clamping on the motor shaft and not the sleeve.
The fit of the flex hex coupler to the motor shaft is area that is often overlooked but very important. If the motor shaft is smaller than the flex coupling (or any style coupling for that matter), then it won't be aligned along the center line after you tighten the grub screw. Now spin that off center load at 15K RPM and you will have a noticeable driveline vibration problem and be unnecessary loading the motors bearings.