The Tongsheng motor comes standard with a nylon gear that allows the motor to be quiet in operation. Around 2000-3000km there is a service interval that requires changing out the nylon gear and replacing it with another. You can also opt to run this brass gear, and it will last the lifetime of the motor. It will be a little bit noisier than the nylon gear but some people like the peace of mind knowing that they will not have to do any maintenance to the motor.
Also, for those who modify their motor to handle more power with things such as a heat sink plate internally, upgraded firmware, etc. The brass gear is able to handle much more power before stripping.
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