FYI:
Be sure to check for MISSING TEETH on the main gear, when the brass motor pinion hits stripped-out teeth on the main gear while under-load it just strips out more teeth due to mis-engagement, so 4 missing teeth will turn into 6 to 8 missing teeth & so on.

There should be a slight amount of play between the brass pinion & the main gear, too much engagement results in excess friction, faster wear on gear, and reduced flight-time. Too little engagement will result in stripped out main-gear with any stoppage of rotor.

Rick