The easiest way to learn that you have the
correct gear mesh is to use a piece of paper and fit it between the spur
gear and pinion gear as you tighten the motor mount screws. After the
motor is completely tightened, the piece of paper should be impossible
to remove except by turning the spur gear to rotate the paper out of
place. With the paper gone, turn the spur gear with a finger and feel
the amount of movement or "play" there is between the spur gear and
pinion gear. There should be very little play, but you should feel a
little bit. This is the correct gear mesh. With practice, you will no
longer strip any spur gears.
Make sure the motor mount screws are very tight, a big crash could move
the motor towards or away from the spur gear, making the spur gear strip
out.
Motor mount screws should be checked as part of your regular maintenance on your model.