How to Tell If Your Lawn Mower Crankshaft is Bent? The Symptoms

We’ve all done it. You were mowing your lawn, minding your own business, when BANG! – suddenly you hit something. Was it a rock, a tree stump or a metal pipe? Who cares?! Well your lawn mower might, as the biggest danger is that your crankshaft is bent. And that means trouble, as it’s often prohibitively expensive to fix, and most of the time the best option is just wheel it to the end of your driveway and start shopping for a new one. But this article is about how you tell if your crankshaft is bent.

Some of the main symptoms of a bent lawn mower crankshaft include excessive vibration, grinding noises, issues with the mower starting up, stalling and an uneven cut.

I’ll talk more about these tell-tale signs in this article, as well as talk about how to tell if your lawn mower crankshaft is bent for sure. I’ll outline my #1 method for checking.

Symptoms of a Bent Lawn Mower Crankshaft

The things that I’m going to tell you to watch out for here are pretty much the same as the symptoms of a bent mower blade, so they’re not definitive proof that your crankshaft is bent; they are indicators that it could be, and that you absolutely need to tip your mower over to investigate further.

Trust me on this one – you do not want to take the chance of running a mower with a potentially damaged crankshaft. As well as being bent, it could have hairline cracks and the blade could come flying out from under the deck and do YOU some serious harm.

Excessive Vibration

Even if your crankshaft is just a tiny bit bent, you will notice some pretty serious vibration. This is because a lawn mower blade needs to be balanced properly, and a bent crankshaft with throw that balance off. And considering the speed at which that unbalanced blade will be spinning, you’re sure to feel extra vibration.

Strange Noises When Running

Mowers make noise, particularly gas mowers. That much is obvious. But is your mower now making loud grinding noises? This is a sign that something has been bent out of shape (either your blade, blade adaptor or crankshaft most likely), and that there’s contact between different components that shouldn’t be happening. In short, strange noises are never good!

Your Cut Looks Uneven

This goes back to the first symptom. If your crankshaft has been bent out of shape, your blade will be unbalanced. And an unbalanced blade spinning at high speed is almost certainly going to cut your grass at an angle. So you’ll see some blades of grass that are left much higher than others.

Trouble Starting Your Mower/Stalling Issues

This isn’t technically a direct symptom of a bent crankshaft, but if you find it hard to get your mower going or it cuts out shortly after firing up, it could be that the flywheel or the flywheel key was damaged when you ran over whatever you ran over. And if the impact was severe enough to damage those components, it’s very probable that it bent the crankshaft too.

How to Tell If Your Lawn Mower Crankshaft is Bent

You have your suspicions, but now it’s time to confirm whether the cause of the symptoms you’ve observed is in fact a bent crankshaft. And there’s a very simple (and quick) test that I like to do to confirm this. Here’s what you’ll need to do:

Disconnect Spark Plug + Then Remove It

Safety is the number one priority whenever you do any lawn mower maintenance or repairs. So the first step is to disconnect your spark plug to ensure the lawn mower has no possibility of starting.

That’s usually all you need to do, but on this occasion, you’re also going to want to remove the spark plug completely. Why? Well, you want to ensure that the engine has no compression at all, so the engine spins as fast as it can.

Tie the Brake Handle Down

Another part of your mower that will prevent the engine/crankshaft from spinning freely is the brake. So you’ll also need to tie that down. You can use whatever you have handy. Some rope or clamps are perfect for this.

Tilt Your Mower

Now let’s get to the important part. You’re going to need to flip your mower over. If you’re not sure how to tilt your lawn mower properly, I’ll give you a few pointers. You can tip it back on its back wheels, especially if there are two of you and one can hold it in position. I don’t love this way of tipping the mower though, as the mower is angled at 45 degrees, and it’s hard to get a good look underneath without hunching over.

I prefer to tip it on its side. Just make sure you tip it on the right side. You just need to make sure the carburetor and the air filter is on the side that’s facing upward. Do that and you’ll be ok.

Spin the Blade

Here’s the actual “test” to see if a mower crankshaft is bent or not. Now be careful with your fingers and thumbs here, and if you’re worried about catching yourself, put a leather glove on one hand.

All you need to do is give the blade a push on the back edge, so that it spins around. Obviously quickly get your hand out of the way once you’ve pushed it.

Because you removed the spark plug and tied down the brake, it’ll spin very freely.

Look at the Center Bolt

The trick here is not to look at the blade, but to keep your eye on what the center bolt is doing. On a mower that doesn’t have a bent crankshaft, you will hardly notice that center bolt move at all. However, if you spin the blade and that center bolt is wobbling all over the place, it’s a sure sign that your lawn mower crankshaft is bent.

What Do You Do Next?

Honestly…in most cases, I’d recommend getting a new mower. It really does get quite expensive to just replace the engine most of the time, and attempting to straighten a crankshaft is a bit risky in most cases. As I mentioned earlier, it could have hairline cracks, and given the intense pressure this component is put under every time you use your mower, I wouldn’t want to tempt fate. Better to be safe than sorry!