How to replace a sheared flywheel key on a Briggs & Stratton walk behind lawn mower

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*Always follow the instructions in your repair manual when doing repair or maintenance work on Outdoor Power Equipment. Manuals can be found at the manufacturers website.*

This video shows how to replace the flywheel key on a Briggs & Stratton walk behind lawn mower. The engine is a Briggs & Stratton single cylinder, 650 series, L-Head engine. The procedure is similar on other brands of engines.

Sheared flywheel keys are a common problem on lawn mowers that have the cutting blade connected directly to the crankshaft. The problem happens when the cutting blade hits an object of large mass. This impact usually causes the engine to stop turning. The sudden stopping of the engine causes the rotational energy in the rotating flywheel to be transferred to the flywheel key and the key shears. The flywheel key is designed to shear when this occurs to prevent the energy in the flywheel from being transferred to the crankshaft and causing serious damage.

The method I used to loosen the flywheel from the crankshaft is not recommended by the manufacturer, but it works if you know how to do it. You need to be careful, because using the pry bar can damage the crankcase and hitting the crankshaft with a hammer can also cause damage. My first choice would be to use a flywheel puller, but the holes on this flywheel, used to attach the puller, were not threaded, so using a puller was not an option. Also, most people don't have the specialized tools recommended by the manufacturer.

Tools used to change the flywheel key:

15/16" Socket for the flywheel nut
3/8" Socket
5/16" Socket
Strap Wrench
Torque Wrench
Pry bar
Phillip Screw Driver
Small Ball Peen Hammer

Specification:

Engine Model Number: 124T05
Tighten flywheel nut to 55 ft.-lbs

Flywheel keys can be purchased at your local small engine shop and online.