ORIGINAL: cwrr5

Yup. Probably the culprit. The motor needs to be TIGHT and in the same plane (very punny!) as the gearbox, or the gears will mesh at an angle and strip out like yours did. After everything's aligned, use a piece of plastic from the package the gears or gearbox came in to establish the gear mesh by feeding it in between the gears as you rotate by hand. Tighten everything with the plastic still between the gears, and don't forget to dab a small drop of locktite or something( NOT CA! ) on the screw threads and set screws. Then after everything is set, you can remove the plastic by rotating the prop again by hand. Grease on the bearings and gears will help prolong the life of all the moving parts, but as with anything, they only have a certain life expectancy(especially the 'el cheapo EFlite gears)