What To Do If You Hit A Rock With Your Lawn Mower

Doing things around the house and yard for maintenance, like mowing the lawn, isn't just cost-effective — it's also pretty rewarding. While cutting the grass is a pretty straightforward job, there are a few rules of thumb to keep in mind, like skimming the grass for objects or rocks beforehand to make sure nothing gets in the way while you mow. Still, some rocks could slip past your eyes and end up colliding with the mower. When that happens, the damage can be serious.

Hitting a rock with a lawn mower is usually pretty obvious. You'll hear a loud, jarring sound as it clanks against the blade. Sometimes it's more of a bang or thud, and other times, it's a scraping noise, kind of like the sound you hear when a metal utensil is caught in the kitchen sink grinder, but on a much larger scale. In some cases, the mower may shut off immediately after impact.

Beyond the noise, the damage can range from a bent or dulled blade to a cracked crankshaft, a vital component that makes the mower blade spin. The engine block is also at risk of damage — that's the heart of your mower, which houses and protects all the vital moving parts of the machine. If the block cracks or gets damaged, it can lead to oil leaks, lost compression, or, in the worst cases, a mower that won't run at all. Here's how to handle hitting a rock with your mower.

When you hit a rock with your lawn mower, the force can cause it to become lodged inside the machine, creating chaos under the hood or even causing the mower to shut off immediately. Sometimes, hitting a rock will break the shear key, also known as the flywheel key, and that's actually intentional. The shear key is meant to absorb the brunt of the impact, protecting your mower from more serious damage. That's why the machine sometimes turns off suddenly after striking a hard object.

If your mower is still running after hitting a rock, the safest thing to do is turn it off immediately. This prevents the rock from being flung out at high speed or bouncing around inside the mower, which could further damage vital parts like the engine block or crankshaft. Riding over a rock can also bend the blade. While this isn't the worst-case scenario, a loose or bent blade can continue moving inside the mower and strike other components, potentially damaging the drive shaft. Always inspect the blade for bends or cracks and replace it if needed. 

The good news is that bent blades are relatively easy and affordable to replace — much cheaper than replacing your whole machine, especially an expensive zero turn lawn mower. Mower blades typically start around $10 and can go up to $100 at stores like Home Depot, depending on the brand, size, and model. Checking the shear key and drive shaft is more complex. If you're not comfortable working on small engines, it's best to have a professional inspect and repair your mower.

Anytime your lawn mower runs over a rock, turn it off and let it cool down before inspecting it for potential damage. Once the mower is cool, disconnect both the spark plug and the battery to prevent the engine from accidentally turning on. If you need to tilt the mower on its side to check the blade and engine, first drain the oil, and then tip it to the side opposite the carburetor.

Start by inspecting the blade for damage. If it's bent or broken, replace it by loosening the nut in the center of the blade. That will free it so you can remove and install a new one (make sure to follow your manual's recommendations). Once the new blade is in place, tighten the nut securely, then reconnect the spark plug and battery. If your mower completely stopped running, the flywheel may have been knocked out of alignment. To check, carefully remove the shroud and, if applicable, the flywheel clutch and brake. Look for a misaligned or damaged flywheel key, which may appear broken or deformed.

Now, replacing the flywheel key is a straightforward process if you're comfortable working on a lawn mower. All you need to do is pop the old one out and install a new key. If neither of these fixes solves the problem, the issue may be with the drive shaft, which calls for a more in-depth inspection. If you still can't get it running, it may be time to replace the mower, and our guide to the best time for buying a lawn mower can help you save money when you do.