How to Replace a Lawn Mower Flywheel Key

If you run over something hard with your lawn mower, like a tree stump or the edge of a concrete driveway, you’ll likely end up breaking or sheering your lawn mower’s flywheel key. Thankfully, your lawn mower has a flywheel key, as, without one, these kinds of impacts could cause substantial damage inside your mower’s engine. If you hit something and your engine stopped dead, and you’re now having problems, it is a good idea to inspect the flywheel key and see if there is any damage.

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How to Replace a Lawn Mower Flywheel Key (The Overview)

To replace the flywheel key on your lawn mower, you’ll first need to remove the engine cover. Then, you need to remove the recoil start mechanism and flywheel cup if your lawn mower has them. Next, lift the flywheel off your lawn mower’s engine and remove the old flywheel key. Finally, install a new flywheel key and reassemble your lawn mower.

What Does the Flywheel Key Do?

The flywheel key locks the flywheel into position with the mower’s main driveshaft, ensuring that the ignition system and the engine position are always aligned so that ignition occurs at the correct time. The flywheel key is usually made of a weaker metal, such as zinc or aluminum, so that when the lawn mower blades strike an object and stop the engine dead, the flywheel key breaks instead of part of the engine, such as the driveshaft. The flywheels key is a safety feature to protect the engine in the event of a sudden impact.

Common Signs of a Sheered Lawn Mower Flywheel Key

If the flywheel key on your lawn mower is broken, then it is no longer aligned with the ignition system. Therefore, as the magnet on the flywheel passes the ignition coil/s, it makes the spark plug create a spark, and the engine will attempt to fire up. In other words, the engine attempts to start when the pistons and valves are in the wrong position, the fuel/air mixture hasn’t been pulled into the cylinder head, and the exhaust gases haven’t been released.

Here are some common signs of a sheered lawn mower flywheel key.

The engine may not start
Oil works its way out of the engine
The engine sounds rough
Pull cord rips out of your hand
Engine backfires
Blade becomes loose very quickly

How Do I Know if My Flywheel Key is Broken?

You can do a visual inspection to check if your lawn mower’s flywheel key is broken. All you need to do is gain access to the top main driveshaft, where the flywheel is located, and look for the slots. The slots are located where the flywheel sits over the driveshaft. With some lawn mowers, you can do this without having to take your lawn mower apart, but with some mowers, you’ll need to remove the engine cover and the recoil starter.

How To Inspect a Flywheel Key

When you find the flywheel key, look for two slots, one on the flywheel and one on the main driveshaft. These two slots should be perfectly aligned with the flywheel key. If your flywheel key is broken, these two slots will no longer be aligned, and you’ll either find the flywheel keep sheered or bent, depending on how bad of an impact your lawn mower suffered. In either case, the flywheel key should be replaced.

Are Flywheel Keys Universal?

Flywheel keys are not universal and are specific to individual lawn mower engines. You’ll need to purchase a replacement flywheel key specific to your mower engine. So, you’ll need to know the make and model of your lawn mower’s engine to match up the correct replacement key. I don’t recommend that you use the make and model of your lawn mower since some manufacturers use different engines on the same model. So when purchasing a new flywheel key at your local lawn mower repair store or ordering online, the engine model number is what’s important, not the make and model of the lawn mower.

How to Replace a Lawn Mower Flywheel Key (The Complete Guide)

If you inspected your lawn mower and discovered that your flywheel key is broken or sheered, you’ll need to install a replacement to get your mower running like it used to. The design of lawn mower flywheel setups can vary between lawn mowers. For example, a lawn mower that uses a pull cord has the recoil start mechanism on top of the flywheel. On the other hand, an electric start lawn mower won’t have this mechanism since it doesn’t use a pull cord. Either way, the actual flywheel and the flywheel key are the same. Here are the steps for both lawn mower designs.