[Kevin Sevcik]

The problem with a bench vise, and especially a C-Clamp, is the slop and play in the clamping surfaces. Too much slop makes it easy for the shaft or gear to get out of alignment. This just encourages the application of more force, which can pretty quickly result in a bent shaft.

Since it hasn't been mentioned, you need to make sure you're pushing only against the back part of the shaft when you press this on. If you set it up so you're pushing against any other part of the motor, you're going to destroy bearings, the motor frame, etc.