We’ve all been there and you are comfortably mowing your lawn under the cozy sun when Bang! Your mower suddenly hit something. It could be a stone, root, tree stump, or any other obstacle and then you start wondering will this damage my machine?

Well, yes! One of the most common damages that such a collision can lead to is a bent Crankshaft. The crankshaft is one of the most important components of a lawn mower and the last thing you want is to ignore its damage. 

As such, every homeowner needs to be able to tell if the lawn mower crankshaft is bent. As you’d expect, it is dangerous to ignore a bent crankshaft as it can potentially damage other lawn mower parts, cause injury from accidentally flying metal fragments, or even cause your mower not to start at all.

So, knowing if the lawn mower crankshaft is bent is key to avoiding a lot of damage and repair expenses.

What is Lawn Mower Crankshaft?

A Crankshaft is one of the most vital parts of a lawn mower that allows it to operate smoothly and offer a smooth cut to your turf. It is a rotating shaft located at the center of the mower’s engine. The function of the crankshaft is to transmit torque to the flywheel. 

In layman’s language, we can say that the crankshaft is a component that connects the spinning blades to your lawn mower.  

How to Tell if Lawn Mower Crankshaft is Bent? – Explained

How to Tell if Lawn Mower Crankshaft is Bent?

As we’ve already seen, taking the chance of running your mower with a bent crankshaft can potentially hurt you or damage your lawn care equipment. I’ll let you into a little secret, if you have any doubts that your mower’s crankshaft is bent, look out for the symptoms. If the indicators are there, you’ll need to tip your machine over and test for a bent crankshaft.

Some of the most common issues that you will observe if your lawn mower crankshaft is bent include:

1. Uneven Mowing

This is one of the initial signs that you will observe. Lawnmowers are designed in a way that the blades will offer a uniform horizontal cut to your grass. Remember that the blades of your lawn mower are connected to the crankshaft.

So, if the crankshaft is bent, your lawn mower blade will be unbalanced and as such, it will cut your lawn at an angle. In short, you will observe a saw-toothed lawn surface – some grass blades are taller than others after mowing. 

We all wish for that smooth even cut, it is what gives a lawn its enticing and beautiful look. So, whenever your mower starts to cut unevenly, it is possible that the crankshaft is bent and needs immediate servicing.

2. Too Many Vibrations

 It’s normal for a lawn mower to produce some vibrations but when the crankshaft is bent, the vibrations are way too loud. Trust me, with a bent crankshaft, you’ll be able to tell a difference. As we said, the blades rotate around the crankshaft in a balanced manner.

So, even the slightest bend can tip off this balance making the mower struggle while rotating. Also, because the blade spins fast, it is no doubt that the struggling blade will produce excessive vibrations.

3. Loud Grinding Noises

It’s pretty obvious for lawnmowers to make noise and especially if you use a gas-powered one. Nevertheless, if your mower produces a loud grinding sound, that is something to worry about! Grinding noises are produced when there is an unusual contact between mower components.

You’ll not miss it, it is an irritating sound, like that of metal against metal. Such noises from your lawn mower can be due to many reasons including excessive mowing and poor maintenance of your machine but the most common is due to a bent crankshaft. 

4. Stalling Mower Issues

So, you are ready to start working on your turf of green but your lawn mower simply won’t start or it starts but cuts out shortly after you start mowing. And you start wondering what the problem is. Well, it could be that the crankshaft is bent.

Besides, it could be that the flywheel or the flywheel key got damaged maybe after you collided with something. And, for damage to happen, the impact must have been severe enough to bend the crankshaft too.

If your lawn mower has the above symptoms, it is most likely that the crankshaft is bent but then, you’ll need to perform a test to verify that. There are different ways in which you can identify a bent crankshaft. 

Pro tip: Always disconnect the spark plug for safety purposes before examining your mower.

  • Method 1: Look for bolt Wobble

The cutting blade is connected to the crankshaft using a bolt. Tip the mower on its right side – the carburetor and the air filters should be on the side facing up. In this method, it is good to have an assistant. Besides, you need to be very careful.

Let the assistant pull the starter cord and observe the bolt. If the bolt wobbles upon pulling the starter cord, then you can be sure that the crankshaft is bent.

  • Method 2: Dial Indicator

If you have this precise measuring tool, it is time to get it out of your toolbox. You need to ensure that the crankshaft is horizontally in its place. To use the dial indicator, just place it at various points along the crankshaft while checking for any discrepancies. 

  • Method 3: Examine the Gap between the Crankshaft and the Deck

Lastly, the gap between the crankshaft and the deck can also help determine whether your mower has a bent crankshaft. Normally, the gap should be consistent. You can rotate the manually while paying close attention to the gap. A ruler can help in this method.

Also Read: Why Does the Mower Bog Down When Blades are Engaged?

What Causes Crankshaft to Bend?

The most common cause of a bent crankshaft is a collision between your lawn mower and a strong obstacle. Technically, the cutting blades can spin to as high a speed as 200 MPH.

If this fast spinning blade hits a stone, branch, tree stump, or any other object, all that energy can result in a bent crankshaft. Other less common causes are a bent blade, debris, damaged belt, bad fuel filters, and a faulty carburetor among other mechanical issues.

How to Fix a Bent Crankshaft?

Now that you know how to tell if a lawn mower crankshaft is bent, what do you do next? While others may opt to purchase another mower, the DIY warriors out there can opt to fix the bent crankshaft. Come to think of it, it is an easier way to solve your problem while saving you a few dollars at the same time. 

Step 1: Prep Your Lawnmower 

First, your mower should be turned off for hours, this is to ensure that the engine system is cool enough when you are working on the mower components. Drain the oil and disconnect the fuel hose.

Step 2: Disconnect the Air Filter

Next, you’ll need to remove the air filter. Besides, you can remove the oil tube and the clutch. 

Step 3: Remove the Flywheel

 Check for the flywheel below the blower tube. For most push mowers, what you are looking for is a plastic circle or a small metal. After locating it, use a socket wrench and a screwdriver to carefully loosen the bolt and pry it out respectively. You’ll also need to remove the blade and the motor.

Step 4: Remove the Valve Springs

Start by removing the valve cover and then the two valve springs underneath. You can use a spring compressor to make the procedure easier and faster. 

Step 5: Remove and Straighten the Crankshaft

Push on the connecting rod to access the crankshaft and remove it. Now, this is the major step and the primary point is to straighten the bent crankshaft. You can either use a shaft straightening tool or straighten it manually.

Place the bent crankshaft on a level surface and position a heavy metal pipe on top of it. You can repeat the same processes over and over again until you are satisfied that the crankshaft is straight. Then reassemble the mower and voila! You are done.

Also Read: How to Fix the Tire Back on the Rim?

What Does A Bent Crankshaft Sound Like?

If the crankshaft is bent, your mower will produce loud strange noises when running. Particularly, you will hear loud grinding noises almost the irritating sound of metal against metal.

How Do I Know If My Lawn Mower Spindle Is Bent?

You can easily tell if your mower deck spindle is bent. To start with, you’ll start feeling excessive vibrations coming from the mower’s deck. It’s normal for your mower to produce some vibrations but in this case, it will be a lot more than usual. Secondly, you will notice an uneven cut on the grass.

Final Thoughts:

A bent crankshaft is a serious problem that you have to address immediately. The mower produces irritating grinding sounds, excessive vibrations, and an uneven cut. What’s more, starting the engine becomes a problem and you might eventually end up damaging the whole machine if not hurting yourself. So, try to straighten the crankshaft if you are good at handy work or call in a professional to handle the task. 


Victoria Peterson

I am a passionate gardener who wants to help you create and maintain your dream yard. I know that it can be daunting to take on a project like this, but I am here to help. I have been gardening for years and have learned a lot along the way. I want to share my knowledge with you and help you create the perfect yard for your home.

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