How to Tell If Lawn Mower Crankshaft is Bent?

Nora Held
Nora Held
Research Writer
Apart from being our main writer, Nora also works for a number of gardening-related sites and magazines, sharing her experience as an owner of a small farm in North Carolina read more
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Bruce Williams
Bruce Williams
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Bruce owns a small farm and loves to grill since his early childhood. He’s a professional cook, but hiking and enjoying the great outdoors is his no less favorite part of read more
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Did you hear sharp noises coming from your lawnmower Trusted Source Lawn mower - Wikipedia lawn mower (also known as mowergrass cutter or lawnmower) is a machine utilizing one or more revolving blades to cut a grass surface to an even height. ? You might have recently hit a bump or a rock, and it caused damage to the machine. That might have led to a bent crankshaft, which is a common issue with lawnmowers. The problem is that you have no idea how to fix it.

Before you resort to repairing the unit, it’s important to determine the issue. But how to tell if the lawnmower crankshaft is bent? That’s what we are trying to discover in this article.

You might suspect something is wrong because your machine doesn’t act the same as before. For example, it might be vibrating while starting or during an operation, or you hear loud noises. The problem is that a bent crankshaft can lead to a variety of issues that could destroy your machine but also cause injuries. Here is what you should know about this damage and how to repair it quickly!

Can you find any signs of deformation?

How to Tell If Lawn Mower Crankshaft is Bent?

Is there a way to positively identify that a bent crankshaft is causing problems with your machine? You have two approaches to determining the issue. The first one is to look for potential symptoms, and the alternative is to check the crankshaft yourself. Here is a detailed explanation of the lawnmower bent crankshaft symptoms.

Typical symptoms

The problems you can notice come down to the following:

  • Unusual sounds and vibrations – it’s normal for lawnmowers to emit some vibration and sounds. The problem is when the sounds are unusual and not the same ones as before. For example, the machine might produce harsh sounds once turned on, but only for a couple of seconds. It might vibrate excessively when you are trying to move it. Anything that seems out of the ordinary can be an indicator of a bent crankshaft.
  • Transmission problems – these occur when you try to ignite the lawnmower. It’s similar to starting a car – you might have trouble turning it on, and it could require several tries to succeed. It’s a common symptom of a bent crankshaft.

Physical inspection

The next is a physical inspection, which means checking the crankshaft by yourself. Now, there are multiple ways to do this, and we’ll explain them below.

While you are assessing the unit, you might encounter other issues, too. If you have a problem with your blades, find a replacement among the best lawn mower blades to ensure the maximum longevity of the new component.

How does it work?

If you want to determine a bent crankshaft, it’s best to ask a neighbor or family member to help with assessing the damage. For starters, take off the spark plug from the machine. Now, ask the other person to initiate the cord to start the machine. While they do that, you focus on the blade and the bolt that holds it. If you notice any shakes or wobbles, it’s an indicator something is wrong.

Now, there is another method on how to perform a physical inspection yourself. Here are the steps to follow:

  1. Remove the spark plug and turn your machine over.
  2. Identify the cutting deck skirt on one end. Use a ruler and clamp it from the deck’s edge to the place where the shaft touches it.
  3. Use your hands to turn the shaft. Check out the gap and see what happens during a single rotation. Did it expand and contract after that? If that’s the case, there’s a deformity in your crankshaft.

To avoid these issues in the future, you can also get an electric model, such as the Greenworks G-MAX 25322. It can mow 400m2 on a single charge and is suitable for various grass types.

What causes a bent crankshaft?

Your mower’s blade operates at over 200mph when in action. Those are huge speeds, which is why the machine uses plenty of kinetic energy. Once the unit encounters an obstacle, such as a blade, it dissipates energy through the shaft and the blade. It’s that energy that causes the bending.

Here are some causes of a bent crankshaft:

  • You hit a rock, sprinkler, pipe, or another immovable object that’s not too light.
  • You tried to do something with the machine and dropped the shaft accidentally.
  • Something went wrong during the manufacturing, installation, or maintenance process.

It’s always smart to pick reliable and high-quality mowers to reduce the risk of any issues. The selection of the best corded electric lawn mowers can help to identify the right machine in that category.

How can I repair a bent crankshaft?

The first thing is to secure the right tools.

Here is what you’ll require:

  • Wrench
  • Screwdriver
  • Nut drivers (depends on the machine)
  • Socket set
  • Heavy pipe
  • Valve spring compressor tool
  • Rubber hammers
  • Vise grips

The first step is assessing the shaft. Here are the steps for doing that:

  1. Make sure your lawnmower is cool and turned off. Make sure to drain the oil by removing the oil plug. Now, drain the fuel by disconnecting the hose and opening the gas cap.
  2. Remove the air filter and then take off the engine. You want to remove the oil tubes and throttle cables, too.
  3. Now, take off the clutch, but use a wooden piece to place between the housing and the flywheel.
  4. Pry loose the flywheel bolt and remove the shear pin from the shaft. Remove the bolt from the blade, too, and take off the bolts that hold the engine and the deck.
  5. Place the motor in a safe position and use a screwdriver for the screws that keep the valve spring cover. Use the compressor tool to push down the spring. Allow it to compress and then slowly let go. Now, do the same with the other valve.
  6. Check out the blade and find the camshaft gear. Remove it and unscrew the bolts to access and disconnect the rod end cap.
  7. Slowly turn the crankshaft and take it off the engine.

Now, let’s do the straightening process. Position the shaft on a solid surface and place a heavy pipe on top of the bent. Use a hammer over the pipe several times. Now, see if the shaft is straight by using a level. If necessary, do some more straightening with the pipe and the hammer.

Once everything is straight again, assemble the machine back. You can follow the same steps above but in reverse. If you did everything right, the machine should work correctly again.

Do you need a replacement Trusted Source Buying a lawnmower: a guide | Family finances | The Guardian Whatever the size of your green sward, it’s time to consider how you are going to keep it under control. ? Check the CRAFTSMAN M105 lawnmower, an affordable battery-powered unit that’s an excellent choice for residential landscapes. The maneuvering process is easy with the bigger rear wheels, and you can pick from five different cutting heights.

Extra tips

Here are some other crucial things to note when dealing with a damaged crankshaft:

  • Check the warranty – is your product still under warranty? If the answer is yes, you’ll need to find a licensed technician. If you try to repair the issue yourself, it will void the warranty. If it happens that you fail during the repair, you will have to pay someone to fix the unit.
  • Is the repair worth it? – although fixing a bent crankshaft doesn’t have to be expensive, replacing this part is. If the replacement is necessary, make sure that’s the right move. Perhaps it’s better to check out the best lawn mowers under $400 and invest in a new unit.
  • Pay attention while mowing – the reason why the crankshaft gets bent is that it hits an obstacle. It’s impossible to notice every rock, but paying attention while mowing might decrease the risk of bending it.


Q: What’s the price of replacing a bent crankshaft?

A: The total price shouldn’t go over $100. If you decide to pay a professional to handle the repair, that could increase the cost to $150.

Q: Is it possible to continue using the lawnmower when the crankshaft is bent?

A: While it might theoretically be possible to use the machine, the experts advise stopping immediately. If you notice any issue with the crankshaft, it’s imperative to stop it from damaging the engine and destroying the unit. It’s easier and more affordable to fix a bent crankshaft than a damaged engine.

Q: Is it smart to fix a bent crankshaft by yourself?

A: You should only do this if you know what you’re doing. If you are unsure about any step of the process, call a professional. It helps if they are a licensed agent for that mower brand. Another thing to consider is the warranty – if it’s active, you shouldn’t handle the repair yourself.

Final thoughts

A lawnmower is an incredibly useful machine. It makes maintaining residential and commercial landscapes easy. While using these devices isn’t challenging, it’s vital to recognize any problem as soon as it appears. A bent crankshaft is a common issue with mowers, so pay attention and make sure to recognize the signs.

We hope our guide helped with how to tell if the lawnmower crankshaft is bent. You should rely on your eyes and ears when determining the issue. Once you establish the bending of the crankshaft, you have the option to repair it yourself. However, make sure that you know what you are doing during each step. You don’t want to risk causing even bigger damage. If you feel like you could use help, don’t hesitate to contact a professional.


Lawn mower - Wikipedia
lawn mower (also known as mowergrass cutter or lawnmower) is a machine utilizing one or more revolving blades to cut a grass surface to an even height. 
Buying a lawnmower: a guide | Family finances | The Guardian
Whatever the size of your green sward, it’s time to consider how you are going to keep it under control.

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