[philso]

Any side forces on the shaft will be applied at the front end of the motor, by definition. Securing the motor by the front end will allow the mounting system to resist these side forces better than securing the motor by the rear end. Another way of looking at it is that the lever from where the forces are applied on the shaft is very short if the motor is secured at the front. If the motor is secured at the rear, the same force has a much longer lever to cause the body of the motor to deflect. This could result in your pinion gear not meshing. You may get more vibration, especially under load. The higher torque on the two mounting screws at the back could cause the rear plate of the motor or the mounting plate to deform.