[Al Skierkiewicz]

In almost every case of drilling into the motor case, shavings will be attracted by the internal magnet and find their way between the magnet and the armature. When you apply current, those shavings will then stand up and align with the magnetic field between the armature and the magnet usually causing some kind of mechanical friction. It is also likely that you have dislodged the magnet or cracked a piece off internal to the motor. Either way that will also produce significant internal friction when current is applied. The robot rules prevent modifying the motors (among other things), for just this kind of damage.