If you fly the micro WW-1 aircraft (and their Cub or Playmate) you might encounter a full-on nose-impact crash right on the propeller boss. This leaves the prop very stiff to turn by hand and not free wheeling. If you power it up you will not get full RPM and you may burn up the brick due to the current the motor is drawing. The "obvious" conclusion is that you have bent the gearbox and ruined it, maybe even ruined the motor. However the usual case is that the big plastic gear toothed hub had been jammed into the brass bearing of the gearbox. The cure is to put an x-acto blade into the jam and gently wiggle them apart. This is pretty quick to do. I have thrown away at least one motor and gearbox, maybe two, after crashes. I finally figured this out after the second engine change on the pup. I have experimented with a drop of ACC on the brass bearing at the joint with the gearbox. Of course you would need to put this on with a toothpick because if you get ANY ACC on the shaft, you are done with that gearbox. Once this has set up you may be able to fix the jam by simply pulling the prop forward until you hear or feel a click when the jam lets go. This is why in my stick and tissue conversions I like removable panels and nacelles to work on motors and mounts.