[brijar]

Mine do that sometimes. I can usually get them off with a small squirt of off brand WD 40 into the pinion gear and gently wiggle the pinion free. Before I put another pinion on, I take a 1/8 drill bit, put it into the pinion, and run it back and forth on the table to help smooth up the inside of the pinion. I also plug the batteries in, take a file, and run the motor shaft very slowly while rubbing the file against it for a few seconds to remove the burrs. The new pinion slides on like a hot knife through butter:003:.
I try not to grind the shaft too often, because I don't want to make it too small or remove the flat spot.

Brijar!!!