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A pinion or pinion gear is a small gear attached to the output shaft of a motor or engine that engages the larger main gear. The ratio of the number of pinion teeth to the number of teeth on the main gear determine the model's gear ratio and so to a large degree the model's head speed.

The coarseness of the teeth on a pinion and the main gear is described by its module number, and it is essential to get the correct module pinion for your helicopter to avoid damaging the main gear. It is also important that the gear mesh be set correctly, with just a tiny bit of backlash between the pinion and main gear.

The easiest way to install a press-fit pinion on a motor shaft is to push it on to the motor shaft using a vice. The easiest way to remove the pinion is with a pinion puller.

Common pinion shaft sizes for electric motors are:

  • 3.17mm (0.1248")
  • 2.3mm (0.09")
  • 5mm (0.197")

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