The reason it stripped is likely due to a) it was meshed wrong, or b) you are using a metal spur. The problem with metal spur gears is that ultimately either the pinion or the spur is going to be made out of a slightly harder metal (just like how from the factory the plastic spur is meant to be softer than the metal pinion), meaning that if it isn't meshed just right, the harder gear will begin to eat the softer one, slowly stripping it. This is why in most cases metal spurs are a waste of money. Good luck with it, whatever the issue.