
This step-by-step lawn mower repair guide explains how to replace the flywheel key on a lawn mower with an overhead valve (OHV) engine. The flywheel key is a rectangular metal peg that keeps the crankshaft and flywheel aligned when you tighten the flywheel nut. If the mower blade hits an object with enough force to knock the flywheel out of alignment, the flywheel key breaks, protecting expensive parts such as the crankshaft from damage.
If the flywheel key shears slightly, the engine runs rough or misfires. If the key breaks, the lawn mower stops running and won't start. When you try to start the engine, the starter rope might jerk back. Replace the flywheel key with the manufacturer-approved lawn mower part.
This repair guide and video shows how to replace a sheared flywheel key in common Briggs & Stratton OHV engines that power Craftsman, MTD, Troybilt, Toro, Husqvarna, Poulan, Murray, Weed Eater and Snapper walk-behind lawn mowers.
If your lawn mower is an older model with a flathead engine, see our repair guide How to Replace a Lawn Mower Flywheel Key on a Flathead Engine.