You are stripping the one way bearing, or the shaft, or both...Those damn rotostarts are too strong to be used on engines that are made for pullstarts, especially when the engine is new.... The oils and fuel are making it spin when its supposed to catch the Bearing..so when you clean it, it grabs..as soon as it gets lubricated again, it slips....The Losi Mach 427 engine is made for a rotostart....get a used starter box......you are going to keep stripping One ways