A sheared flywheel key on a walk behind mower is most likely the result of what?

A sheared flywheel key on a walk-behind mower typically occurs as a result of a sudden stopping of the crankshaft. This situation happens when the engine is running at full speed and an external force abruptly halts either the mower or the engine. The flywheel is designed to keep the engine running smoothly by maintaining momentum, but if the crankshaft experiences a sudden stop—such as hitting a hard object or being forced to stop by excessive resistance—the flywheel can undergo significant stress. The key serves as a shear point to protect the engine from more severe damage in such scenarios. When the force exceeds the shear strength of the key, it will break, preventing further damage to the flywheel or crankshaft.

In contrast, overspeeding could lead to other types of mechanical failures or damage but is not specifically linked to the shearing of the key. Multiple starting attempts may put strain on the engine and starter but do not usually cause a key to shear. Thus, the situation that directly leads to a sheared flywheel key is indeed the sudden stopping of the crankshaft.