Originally Posted by bartiny
MMM makes you wonder how they break in the first place, mine had sheared in half
Main cause is not having the flywheel nut torqued up enough or wrong profile on the key .
Used to be common on some outboard engines when points ignition was the day , especially with someone (DiYer) that may spend hours trying to remove the flywheel without a proper puller only to be then wary of re tightening up the nut too much for fear of having the same difficulty next time .
Even though it may seem tight ,when the engine fires the shock load can cause a rattle effect that will cause the key to fracture then snap .