How to replace a lawn mower flywheel key on a flathead engine

This step-by-step lawn mower repair guide explains how to replace the flywheel key on a lawn mower with a flathead 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.

Use these steps to replace the flywheel key in common Briggs & Stratton flathead engines that power Craftsman, Husqvarna, MTD, Toro, Poulan, Troybilt, Cub Cadet, Murray and Ariens walk-behind lawn mowers.

If your lawn mower is a newer model with an overhead valve (OHV) engine, see How to Replace a Lawn Mower Flywheel Key on an OHV Engine.

Instructions

1. Disconnect the spark plug wire

Disconnect the spark wire so there's no chance that the mower could accidentally start.

2. Remove the starter rope from the guide

Remove the starter rope from the rope guide. If the rope won't slip past the guide and the handle, loosen the guide.

3. Remove the engine cover

Remove the screws securing the engine cover to the engine. Slide the rope through the hole in the engine cover. 

4. Remove the fuel tank

Remove the screws that secure the fuel tank to the engine. Lift the tank off, being careful to keep the fuel tank upright so it doesn't leak.

5. Remove the dipstick

Remove the screw that secures the dipstick tube to the housing. Remove the dipstick from the engine and plug the hole with a paper towel to keep debris from falling into the engine.

6. Remove engine blower housing

Remove the bolts securing the blower housing to the engine and remove the housing from the engine.

7. Immobilize the brake pad and crankshaft

Pull the engine control bail against the handle and secure it with a piece of rope to move the engine brake pad away from the flywheel. 

Tip the mower onto its side with the air filter up. Wedge a block of wood between the mower blade and the housing to keep the crankshaft from turning when you loosen the flywheel nut. 

8. Free the flywheel

Remove the flywheel nut and starter cup. To protect the crankshaft thread, rethread the nut so it's flush with the end of the crankshaft.

Hook the jaws of the 2-jaw puller under each side of the flywheel and tighten the puller bolt. Tap the end of the bolt with a hammer to pop the flywheel loose.

9. Remove the flywheel key

Remove the flywheel nut and flywheel. Use needle-nose pliers to remove the flywheel key. If it's sheared in 2 pieces, you might need to use a small screwdriver to remove the pieces.

10. Install the new flywheel key

Install the new key into the slot of the crankshaft. Align the slot in the flywheel with the key and push it on to the crankshaft. 

Reinstall the starter cup and flywheel nut. Make sure the block of wood is in position so the engine doesn't rotate while you tighten the flywheel nut. 

11. Reassemble the mower

Position the engine blower housing on the engine and bolt it into place.

Remove the paper towel from the dipstick opening, slide the dipstick into the engine and secure it with the screw.

Feed the starter rope handle through the engine cover up to the rope guide.

Reinstall the fuel tank.

Position the engine cover on the engine and screw it into place.

12. Reconnect the spark plug wire

Attach the spark plug wire and make sure the mower works properly.