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S.E.R. FAQ
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NotTaR of small Gasoline Engines and Rotary Lawn Mowers : Why soft metal keys must be used
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1994-2007, Samuel M. Goldwasser. All Rights Reserved. Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied: 1. This notice is included in its entirety at the beginning. 2. There is no charge except to cover the costs of copying.
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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.