Here is something every racer should have as a spare if you are using an aircraft starter motor to fire up your blown hot rod or nitro injected monster. This could be the difference of getting to make it to another round at the races.
It is a heat treated steel armature woodruff key. This key fits the most popular RCD aircraft starter motors. These typically can sheer if the motor pops while it starts or if the batteries are low.
This key also fits the RCD angled mag drive coupler.