Walk-Behind Lawn Mower: Blade damaged after hitting an object

Several symptoms indicate that the mower blade is damaged, including an uneven or imprecise cut; irregular patterns in the lawn; excessive motor vibrations; ragged grass tips; and scalping. Inspect the blade and replace it if it's bent, broken or chipped.

Sometimes hitting an object damages more parts than just the blade. If your mower doesn't start after hitting an object, the flywheel key could have sheared off, disrupting the spark timing.

View this troubleshooting video to diagnose engine problems after hitting an object: Lawn Mower Won't Start Troubleshooting Video: Spark Plug and Ignition Problems

These repairs may help solve your Walk-Behind Lawn Mower problem:

Replace a lawn mower blade

Blade

For safety and for a cleaner cut, replace a mower blade that's bent, chipped, broken or otherwise damaged.

For manuals, repair guides, and specific part recommendations, enter your model number.

Replace a lawn mower flywheel key

Flywheel key

The flywheel key is a small metal rectangle that keeps the crankshaft and flywheel aligned when you tighten the flywheel nut. To protect more expensive components from damage, the flywheel key is designed to shear if the mower blade hits an object like a rock or tree stump hard enough to make the flywheel slip out of alignment with the crankshaft. If the flywheel shear slightly, the engine runs rough or misfires. If the flywheel key breaks or shears severely, your lawn mower stops running and won’t start. If the flywheel key breaks or shears, replace it.

For manuals, repair guides, and specific part recommendations, enter your model number.