Skip to Content

How to Tell if Lawn Mower Crankshaft is Bent: A DIY Guide

Are you experiencing difficulty maneuvering your lawn mower? A bent crankshaft could be the cause. With our guide, you’ll be able to diagnose this issue accurately. Let’s work together to improve your lawn mowing experience.

How to tell if a Lawnmower Crankshaft is Bent:

Inspect the crankshaft by checking for gaps; bolt wobble when pulling the cord, and blade swing while running. Use a dial indicator on the shaft. Professionals can straighten with a straightener or sledgehammer. If it is too bent, replace it with a matching new crankshaft.

Discover the secrets of diagnosing a bent crankshaft in your lawn mower. Difficulty maneuvering could be a sign of this issue. Our guide will walk you through identifying and fixing this problem. Let’s make lawn mowing enjoyable again!

Symptoms of a Bent Lawnmower Crankshaft:

There are two common symptoms associated with a bent crankshaft. You don’t need to be an expert mechanic to judge these symptoms. If the crankshaft on your mower is bent, you’ll likely observe these two symptoms. Just keep your eyes and ears open.

● Excessive Vibrations:

For moving objects, vibrations are inevitable. All moving objects vibrate at some frequency characteristic or specific to that object and an amplitude particular to the motion it is going through when an object is deformed.

If its motion characteristics are changed, its vibration amplitude will change. For example, when the force exerted on a moving object changes, it changes its amplitude of vibration- an increase in force can cause an increase or decrease in the amplitude.

Over two decades of experience, I can tell you that the most common signs of a bent lawn mower crankshaft are excessive vibration, difficulty starting the mower, uneven grass cutting, and the blade not spinning correctly.

– The Role of Amplitude in Excessive Vibrations

An increase in amplitude means that the vibration becomes more rigorous and observable. Excessive vibrations are the most common symptom of a bent crankshaft because of its high rotational speed. The vibrations can be felt for some seconds after intervals of smooth running, but they are vigorous enough to know that there must be an issue.

Excessive vibrations are also accompanied by grinding noise. If you observe these, immediately stop mowing and turn the engine off. Inspect the crankshaft and get it fixed before cutting again.

● Transmission Problems:

Transmission problems are also commonly associated with a bent crankshaft. This is because of two reasons. The bent crankshaft can hit other parts, messing up the ignition system, or the impact that bends the crankshaft could also break the flywheel and mess up the ignition system. In any case, the mower will have trouble starting and not running smoothly.

Physical Inspection for a Bent Crankshaft:

You can also conduct a physical examination of the crankshaft to determine if it is in the correct shape or not. A physical exam must be done if you have observed signs associated with a bent crankshaft or as a maintenance routine.

Doing a physical exam now and then is recommended because this way, you’ll always stay updated and will know of any issues before they become too serious. Multiple methods could be used to examine the crankshaft of your lawnmower physically. Some of these methods are listed below.

● The Gap between the Crankshaft and the Deck:

In this method, you check if the gap between the crankshaft and the deck changes throughout one complete crankshaft rotation. First of all, disconnect the spark plug. The clamp a ruler to the deck using duct tape so that its other edge is touching or almost touching the crankshaft.

Then, rotate the shaft manually, observing the gap between the ruler and the shaft. If the gap changes as you turn the shaft, it is bent and needs to be fixed.

Remember to wear safety gloves during this procedure to avoid an injury to your hands. Once again, the spark plug should be disconnected when you perform this exam.

● Look for Bolt Wobble:

First of all, disconnect the spark plug. Then, ask your friend to pull the mower’s starter cord. As they do so, carefully observe the bolt that holds the blade to the crankshaft. Any wobble (sideways motion) of the bolt as the starter cord is pulled indicates that the crankshaft is bent and needs repair.

Preventing a bent crankshaft is all about regular inspection of your lawn for any hard objects before mowing and avoiding mowing on uneven terrain or near the base of trees where roots might be present.

● Use a Dial Indicator:

A dial indicator is an instrument with a dial that measures or detects minimal changes in distance and amplifies them to be visible to the observer. Professionals use dial indicators to measure small displacements in all structures, e.g., beams, flat plates, shafts, etc.

You can also use a dial indicator to check if your crankshaft is bent. For this purpose, the dial indicator should be placed at specific points on the crankshaft.

The dial’s placement depends on the make and model of the shaft, and you can look for more info in the manufacturer’s manual or get help from a professional.

● Look for Blade Wobble:

Just like a wobbling bolt, a wobbling blade can also be an indicator of a bent crankshaft. To observe the blade, park your mower on an open flat area and keep the engine running.

While the engine runs, carefully observe the blade for wobbling and shaking. Blade wobbling can be caused if the blade is broken, but it can also indicate a bent crankshaft.

Fixing a Bent Shaft:

Professionals commonly use a couple of techniques for straightening bent crankshafts. These include:

● Shaft Straighteners:

Professionals use shaft straighteners to straighten bent crankshafts. This method applies a mechanical press force at the shaft at the required locations.

The shaft may be hot or cold while the force is applied. This is a very efficient technique for straightening the shaft and can make your bent shaft suitable for use once again.

● Sledgehammers:

The second method is using a sledgehammer to straighten a bent shaft. The difference between this method and a shaft straightener is that the sledgehammer applies impact force, whereas the straightener applies a pressing force on the shaft.

These methods are not very difficult, and you can perform them yourself at home. Tutorial videos can help in this regard.

Replacing a Bent Shaft:

Sometimes, a shaft is too bent and can’t be repaired enough to make it suitable. Also, trying to straighten a shaft using the above methods can cancel the warranty.

In any of these cases, you must replace the bent shaft with a new one. Ensure the new shaft matches the replaced shaft and complies with your lawnmower engine.

Some challenges one might face when trying to fix a bent crankshaft include diagnosing the problem accurately, disassembling and reassembling the engine, and ensuring the new crankshaft is installed correctly. I’ve encountered and overcome these challenges in my 20 years in the field.

Related Questions:

● What causes a bent crankshaft?

The blade of your lawnmower rotates at very high rpm. An rpm of around or over 3000 is not uncommon. This means that the blade and crankshaft possess a lot of kinetic energy.

Which gets dissipated when the mower hits an immovable obstacle. It can result in a bent crankshaft, hitting a hard rock, tree stump, or fence.


Its crankshaft can bend when your lawn mower hits a challenging obstacle when running at high speed. A bent lawn mower crankshaft causes many problems, such as excessive vibrations and transmission issues. Furthermore, a bent mower blade can result in serious injury if you keep using your mower now.

Using the guidelines mentioned in this article, you can determine if your mower’s crankshaft is bent. First, disconnect the spark plug for safety. Then, tip it on its side and tie the brake handle down. Look at the center of the mower, where the center bolt is. 

Give the blade a spin. If you notice that the blade is a bit off as you spin it, getting your mower over this issue is hard – it’s a sign of a bent crankshaft. Once you determine it is bent, you must do something about it. 

The Consequences of Using a Lawn Mower with a Bent Crankshaft

Starting your mower with a bent crankshaft can cause grinding noises and uneven cut looks. It would be best to fix or replace it as soon as possible. Remember never to use your mower with a bent crankshaft. To fix it, tilt your mower, remove the center bolt, then remove it.

Remember, if you’re not confident in doing this yourself, it’s best to take your mower to a professional. Never risk your safety or further damaging your cut by trying to fix a bent lawn mower crankshaft yourself.

Leave a comment

We try to answer each comment, but please read our comment rules first here. Use an existing and correct email address as you will receive an verification email. Your email address will not be published. All fields are required.

  1. Mason Peterson says:

    I appreciate the detailed explanation on how to diagnose and fix a bent crankshaft.

    • Allard Vdhooft says:

      Thank you for your feedback, Mason! I’m glad you found the explanation helpful. Let me know if you have any other questions or need further assistance.

  2. Serenity Woods says:

    Can I use the mower after fixing the bent crankshaft or should I wait?

    • Allard Vdhooft says:

      I suggest waiting until you fix the bent crankshaft before using the mower. It could be dangerous to operate with a damaged crankshaft, leading to more severe issues. Stay safe!

  3. Marjorie Jennings says:

    Is there a way to prevent the crankshaft from bending in the first place?

    • Allard Vdhooft says:

      It is best to avoid hitting hard obstacles while mowing to prevent the crankshaft from bending. Always be cautious of hidden obstacles in your lawn.

  4. Carl Weaver says:

    I have never thought about checking the gap between the crankshaft and other parts, will do this next time.

    • Allard Vdhooft says:

      Glad to hear you found the information helpful! Checking that gap can make a big difference in preventing serious damage. Stay safe out there!

  5. Terry Sutton says:

    Can excessive vibrations also be caused by other issues besides a bent crankshaft?

    • Allard Vdhooft says:

      Excessive vibrations can indeed be caused by other issues besides a bent crankshaft, such as damage to the flywheel or other internal components. It’s important to conduct a thorough inspection to pinpoint the exact issue.

  6. Derrick Daniels says:

    Are there any safety precautions I need to take when checking my crankshaft?

    • Allard Vdhooft says:

      When checking your crankshaft, watch out for excessive vibrations and transmission issues. Stop mowing and inspect if you notice these symptoms. Safety is key to prevent further damage.

  7. Janice Mendoza says:

    How often should I conduct a physical inspection of my crankshaft?

    • Allard Vdhooft says:

      Janice, conducting a physical inspection of your crankshaft is recommended as a maintenance routine to prevent serious issues. Look for excessive vibrations and transmission problems. Be safe!

  8. Darrell Wade says:

    Thank you for including a section on replacing a bent shaft, very helpful.

    • Allard Vdhooft says:

      Thank you for the positive feedback, Darrell! I’m glad you found the section on replacing a bent shaft helpful. Make sure to stay safe while working on your lawnmower.

  9. Stephanie Lambert says:

    The section on fixing a bent shaft is informative and well-explained.

    • Allard Vdhooft says:

      Thank you, Stephanie! I’m glad you found the information helpful. Let me know if you have any questions about fixing mower maintenance issues.

  10. Billy Thompson says:

    What other physical examination methods can be used to check for a bent crankshaft?

    • Allard Vdhooft says:

      Physical examination methods to check for a bent crankshaft on a lawnmower include observing excessive vibrations and transmission problems. You can also inspect the gap between the crankshaft and the deck, look for bolt wobble, use a dial indicator, and observe blade wobbling.

  11. Tina Jordan says:

    Great explanation on how to use a dial indicator to check for a bent crankshaft.

    • Allard Vdhooft says:

      Thank you, Tina! Excessive vibrations and transmission troubles are definite signs of a bent crankshaft – be sure to check for these symptoms before mowing again.

  12. Jeff Frazier says:

    Are there any signs to tell if the crankshaft is close to breaking completely?

    • Allard Vdhooft says:

      Yes, excessive vibrations and transmission problems can be signs of a bent crankshaft about to break completely. Be sure to inspect and fix it promptly to avoid further damage.

  13. Tanya Reyes says:

    The physical inspection methods provided are easy to understand and follow, thank you.

    • Allard Vdhooft says:

      Thank you, Tanya! I’m glad you found the physical inspection methods easy to understand. Let me know if you have any questions.

  14. Sara Watkins says:

    I had no idea a bent crankshaft could cause transmission issues, this is eye-opening.

    • Allard Vdhooft says:

      Thank you, Sara! It’s important to be aware of potential issues like a bent crankshaft. Excessive vibrations and transmission problems are key signs to look out for to ensure your mower runs smoothly.

  15. Renee Mckinney says:

    This article has been very helpful in understanding the symptoms of a bent crankshaft.

    • Allard Vdhooft says:

      Thank you, Renee! I’m glad you found the information helpful. Let me know if you have any more questions.

  16. Steven Grant says:

    I found the information on shaft straighteners and sledgehammers very useful.

    • Allard Vdhooft says:

      Thank you, Steven! I’m glad you found the information on shaft straighteners and sledgehammers helpful. Remember to stay safe and take care of your lawnmower’s crankshaft.

  17. Everett Newman says:

    Is it possible to replace the bent crankshaft with a more durable one?

    • Allard Vdhooft says:

      Yes, it is possible to replace a bent crankshaft with a more durable one for your lawnmower. It’s important to address this issue promptly to prevent further damage.

  18. Terrence Shaw says:

    I always feel the vibrations when using my mower, maybe my crankshaft is bent.

    • Allard Vdhooft says:

      It’s possible the bent crankshaft is causing the vibrations. Check for excessive vibrations and grinding noise, as well as issues with the transmission. Conduct a physical inspection to confirm. Stay safe!

  19. Vivan Mendoza says:

    This article is a great resource for anyone dealing with a bent crankshaft issue, thank you.

    • Allard Vdhooft says:

      Thank you for your kind words, Vivan. I’m glad you found the article helpful in dealing with crankshaft issues. Feel free to reach out if you have any more questions.

  20. Harold George says:

    Fixing the crankshaft yourself seems risky, is it better to get professional help?

    • Allard Vdhooft says:

      It’s safer to get professional help when dealing with a bent crankshaft due to the potential risks involved. Don’t hesitate to reach out to a lawn mower repair specialist for assistance.