Do most lawnmowers engines have a built in mechanism designed to interrupt service in order to prevent further damage to the engine if the lawnmower blade impacts something very hard such as a steel pipe or concrete?

I know there is a device connecting the crankshaft to the flywheel but is that really designed to be a shear pin or is it solely a key?

On many small lawn mower engines the flywheel key is designed to shear to help prevent damage to the crankshaft, when you strike a solid object with the blade. The blade adapter on some mowers is also designed this way. Not all engines are set up this way, most Honda engines have a steel key and although it may shear, there is usually damage to the shaft by the time it does.

So exactly how does the shearing happen? If the flywheel is spinning and the blade suddenly stops what causes the key to shear? Is it just the sudden change of momentum of the flywheel or is there more to it?

Also what percentage of mower engines would you say have a flywheel key designed to shear? Just looking for a rough estimate.

If the engine stops abruptly such as the blade striking a solid object, the inertia of the spinning flywheel generally causes the key to shear. This can also happen if the engine kicks back when starting. Engines that use a "soft" key will shear most all Briggs and Tecumseh engines use a "soft" key, although the newer keys for Briggs with Magnetron ignitions are harder then they used to be and do not shear as easily as the older keys that were used with point ignition systems.

IMHO, the reason for changing from soft flywheel key and soft adapter key is the older B&S and Tec engines last too long under normal homeowner use. If it doesn't fail they sell no new product. Under normal use and with scheduled mantenience both engines will outlast the owner. Have a good one. Geo

IMHO, the reason for changing from soft flywheel key and soft adapter key is the older B&S and Tec engines last too long under normal homeowner use. If it doesn't fail they sell no new product. Under normal use and with scheduled mantenience both engines will outlast the owner. Have a good one. Geo

I think Geo hit the nail on the head on that one!! :thumbsup:

I think Geo hit the nail on the head on that one!! :thumbsup:

can't really argue that, thats for sure