NotTaR of small Gasoline Engines and Rotary Lawn Mowers : Why soft metal keys must be used        
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Why soft metal keys must be used

Normally, the soft metal keys lock the blade and flywheel to the crankshaft. However, should the blade strike an obstacle and stop suddenly, one or both key(s) will shear and reduce the likelihood that the very expensive crankshaft or other parts will be damaged. The reason is that the substantial inertia of the crankshaft and that of the flywheel will tend to try to keep them rotating. Something has to give and you want it to be the 25 cent key and not the $75 crankshaft! However, this isn't foolproof as explained in the section: Why you really don't want to attempt to move an immovable object.

Note that the soft metal flywheel key can also be damaged without totally shearing which may result in slightly incorrect timing. Symptoms may include a mower that is hard to start, runs rough or lacks power, or cannot be restarted when hot. Therefore, always replace the key if there are any signs of damage or wear.