RE: RC Tank F.A.Q.
Q: How do I properly break in a motor?
A: First, mark the motor and its rotation, then remove the motor from the tank. You would want to run the motor without any load at about 1/3 to 1/2 it's rated voltage on the same direction of rotation for an hour or two. This should be long enough to wear the brushes down without arcing. A good voltage to break in a stock 380, 385, or 540 motor is 3 volts. You could tape a couple of Alkaline D cell batteries together in series and tape the wires from the motor to the batteries' terminals and let the motor run until the batteries are dead. I find it easier to clamp the wires from the motor to a single shorter 3V lithium cell, which doesn't cost much more than a couple of D cells.
If you don't want to use an electric drill to break in your metal gears, then you can break in your motor first using the method above, then use the motor to break in the metal gears.