It is important that gears (particularly the pinion and main gear) correctly meshed. If they are meshed too tight, then there will be unnecessary drag in the gear chain, and the gears will wear quickly and run noisily and inefficiently. If they are meshed too loose, then the main gear risks stripping when any power is used.
The gear mesh is often pre-set on nitro helicopters where the pinion sizes only rarely change, but on electric helicopters (which may use a wide range of pinions) the motor is usually mounted so that it can be moved back and forth to get the correct mesh with the pinion used.
The correct gear mesh is when there is enough of a gap between the pinion and main gear that a tiny amount of free play can be felt. One way that is often suggested to get this gap is to insert a piece of printer paper between the pinion and gear when installing the motor.
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