Not to be snippy, you want the minimum free play that doesn't slow the motor down when you fly maneuvers.  A ball bearing shaft should have a few thousandths more clearance the the clearance in the bearing.
With the bearings taking up all the load the clearance between the case and the prop driver doesn't really matter, as long as the prop driver can't drag on the case.

Did we ever come up with a turnkey 1/2a morning expensive electric setup?  My kids are flying my old 1/2a cox engines but they are finicky even with some new parts on them. If I could build a few electrics I could get the kids from their school flying these. Even though my kids are average 11-14 year olds and spend too much time on electronics, they do enjoy control line, more than RC actually.

I bet we could get kids in model airplanes with these ideas. Right now is a great time because you can fly these almost anywhere while we are pretty well locked down.

I’ll try and figure this out with a readily available motor, esc  and battery from somewhere. The key is you need to know parents of young kids. I have young kids so no problem but other need to talk to your grown kids if you have them and have them recruit kids from their schools, scouts, sports teams.

Great.  Thanks for sharing.