How to Tell if Lawn Mower Crankshaft Is Bent

Is your lawnmower crankshaft bent? Learn how to tell if lawn mower crankshaft is bent. The most common cause for a bent crankshaft is when it’s dropped off of a car or truck, which can happen quickly because it’s so long and heavy. The next most likely cause for a bent crankshaft is when someone first tries to start the engine without releasing the clutch lever.

Either one can lead to bending in several places on the crankshaft, which leads to severe problems like damage to other parts of the mower, not getting started at all, or even worse an injury from flying metal fragments! So before you take any action towards repairing this issue, read this full blog to have a clear idea of lawn mower crankshaft.

How to Tell if Lawn Mower Crankshaft Is Bent

10 Common Causes for Bent Lawn Mower Crankshaft:

1. Debris in the Air Intake:

If your mower blade suddenly stops spinning, the first thing to check is the air intake for any rocks or other debris that may have been sucked up. Next, unplug the spark plug and inspect the area where the blade goes through the deck, looking for any obstructions. If necessary, clear them away.

2. Bent Mower Blade:

Remove the blade from your mower and examine it carefully to check for bends, dents, cracks, or other signs of damage that might have caused a resistance that the crankshaft could not overcome. If there is visible damage, replace the blades before going any further.

Remove the Blade From Your Mower

3. Bad Mower Belt:

A more complicated but far less likely problem is that the mower belt has worn out or snapped. This can cause your engine to work harder than usual, resulting in the crankshaft bending slightly. If this is the case, you will need to replace both blades and belts before testing for bent crankshafts.

4. Faulty Ignition Module:

If your mower has an electronic ignition module, the part may have overheated and locked up. This will cause enough of a resistance to bending the crankshaft over time.

5. Bad Spark Plug:

A faulty or worn-out spark plug can prevent your engine from running smoothly. A poorly running engine will put an increased load on the crankshaft, which can result in damage.

6. Bad Starter Solenoid:

If your lawnmower is an electric start, you may have a bad starter solenoid that does not allow enough amperage to generate to turn the engine over quickly. First, try unplugging the spark plug and using it to turn over the machine manually. If this attempt fails, you may need to take your mower in for service or replace your solenoid.

Turn Off the Mower Manually

7. Bad Fuel Filters:

Dirty fuel filters can cause a decrease in how much power gets from the carburetor to the crankshaft. This can cause your engine to strain, also leading to bent crankshafts.

8. Faulty Carburetor:

If the above options do not fix your crankshaft, you may have a problem with the carburetor. This will require complete disassembly and cleaning of the carburetor to ensure no debris or buildup in the fuel lines.

9. Faulty Muffler:

A faulty muffler can also cause bent crankshafts. The engine’s heat causes expansion in the mower’s metal, which, if not allowed to escape through the exhaust, creates too much pressure on specific areas of your engine.

Common Causes for Bent Lawn Mower

10. Other Mechanical Problems:

Bent crankshafts are also sometimes caused by issues with other mechanical parts of the engine, including the gears that transfer power from the flywheel to the blade.

Procedure: How to Tell if Lawn Mower Crankshaft Is Bent

Method 1: Stick a Screwdriver 

Stick a screwdriver into the spark plug hole and use it to turn your lawnmower over by hand. If you feel abnormal resistance, the crankshaft is bending under the load.

Method 2: Pull Back Slowly 

Take off the spark plug wire and stick a screwdriver into the plughole. Put one end of a sturdy metal ruler against the screwdriver, put your hand on top of it, and pull back slowly to feel any unusual tightness in your crankshaft. If there is abnormal resistance, this is also an indication that your crankshaft is bent.

Method 3: Check the Mower’s Blade

Check the mower’s blade while it’s still on the ground and running to see if it wobbles back and forth. If you notice any abnormal movement of your lawnmower blade, it could signify a bent crankshaft. This is a suitable method on how to tell if lawn mower crankshaft is bent.

Check Mower Crankshaft Is Bent

Method 4: Check for Any Unusual Resistance

Make sure to check the blade if you’ve experienced any issues with your lawnmower, such as unusual resistance when trying to start it or a sharp knocking sound. A bent crankshaft could cause this.

Method 5: Take Off the Mower’s Spark Plug

Take off the mower’s spark plug, stick a screwdriver into the end, and use it to turn your lawnmower over by hand. If you feel a lot of resistance when trying to start the mower, this may be caused by a bent crankshaft.

Method 6: Get a Piece of Masking Tape

Remove the blades from your lawnmower and place them on a workbench to stick straight up in the air. Next, place a piece of masking tape on the bottom of one of the blades.

Turn your lawnmower over so that it’s facing downward, and watch as you try to turn it on. If there is any wobbling or shaking, this could indicate that your crankshaft is bent and needs to be replaced.

Method 7: Turn Off the Lawnmower

Turn off the lawnmower and remove the spark plug so you can look down inside of it. Any rust or dirt in your crankshaft could be a sign that your crankshaft has bent or broken because of corrosion.

Method 8: Check if There’s Any Rust or Dirt 

If you have an older lawnmower, check to see if there’s any rust or dirt in your lawnmower. This could be a sign that your lawn mower is using more energy and requiring more and more of your strength to turn over.

Method 9: Put on Some Safety Glasses

Take off the mower’s air filter cover and wear some safety glasses to look closely at the blades inside. Make sure there’s no debris stuck in between the edges, which could indicate that there’s a bent crankshaft.

Method 10: Ask Your Neighbor 

Ask your neighbor if they use the same mower type and ask if they’ve experienced similar issues with their lawnmower. This will give you an idea of whether or not it is something normal to share with your lawnmower model.

Method 11: Check the Abnormal Sounds Coming From the Engine 

If you have a riding lawnmower, make sure to check for abnormal sounds coming from the engine while it’s running and check that there isn’t any rust or dirt in your crankshaft.

Checking for  abnormal sounds

This indicates that your crankshaft may be bent and requires immediate attention. Likewise, if you have a push mower, check for signs of rust or dirt in your blade while checking the signs of abnormal resistance when trying to turn it.

Method 12: Look for Any Signs of Damage

Inspect the lawnmower again and look for any signs of damage, such as cracks, dents, or bent blades. This can be another indication that your lawnmower’s crankshaft is bent.

Method 13: Look at the Oil Level

Look at the oil level in your lawnmower and ensure it isn’t low. If there’s not enough oil in your mower, this can lead to further damage to the crankshaft over time. This indicates that your crankshaft may be bent and requires immediate attention.

8 Symptoms of a Bent Lawn Mower Crankshaft:

Here are Eight symptoms on how to tell if lawn mower crankshaft is bent or not. Look at these symptoms carefully and see if there are any crankshaft problems with yours.

1. Engine makes a loud clanking noise while the lawnmower is in use

2. Engine vibrates excessively when in use

3. Engine wobbles or rocks excessively when in use

4. Mower blades wiggle or have intermittent contact with the ground when in use

5. Deck of mower rocks back and forth or vibrates excessively while in use

6. Engine bogs down or has trouble starting while the lawnmower is in use

7. Mower blades spin slower than usual while in use

8. Clutch engagement sound is inconsistent while the lawnmower is in use

How Do You Fix a Bent a Lawn Mower Crankshaft?

Now that you’ve learned how to tell if lawn mower crankshaft is bent or not. Here are some steps to try to fix your bent lawn mower crankshaft.

Step 1: Make Sure Your Lawnmower Is Turned Off

First, ensure your lawnmower cannot be started or has been turned off for at least twenty-four hours.

Step 2: Remove the Air Filter

Next, remove the air filter and locate the flywheel. The flywheel can usually be found just below the blower tube. This will be a small metal or plastic circle underneath the blower tube on most push-style lawn mowers. The flywheel has a key to allow the cutting blade to turn when you pull on the starter cord.

Step 3: Slide the Key With a Screwdriver 

Using a small screwdriver, try and slide that key out of the flywheel. Do not use pliers as this may break off inside of your engine.

Step 4: Inspect the Flywheel

Inspect the flywheel for any bends, gouges, or cracks. If the flywheel is bent or has deep scratches, you can replace it with a new one.  You should only need to get a new flywheel if there is significant damage to it.

Step 5: Turn the Flywheel Counterclockwise

The presence of any bent metal could be an indication of a bent crankshaft.  To check this, turn the flywheel counterclockwise with your fingers to see any resistance in the crankshaft.

Step 6: Remove the Flywheel and Starter Cord

To fix a bent lawn mower crankshaft, you may need to replace it altogether. This can prove difficult as you must remove the flywheel and starter cord to get to the crankshaft.  If this is not an option, you can replace your flywheel with a new one.

Steps to Fix a Bent a Lawn Mower

Step 7: Flush Against the Side of the Flywheel

To do this correctly, put the key back into the lawn mower’s flywheel. Slide it on so that it sits flush against the side of the flywheel and tighten it into place.

Step 8: Make Sure That You Line Up Both Ends

Reinsert the starter cord by sliding the metal ring back on and pushing it all the way to the end of its track. Make sure that you line up both ends so that they match perfectly. Then, snap them together.

Step 9: Take the Starter Cord Off Again

Take the starter cord off again by pulling it out of its track. Then, slide it back in so that is just past the point where you removed it from. Snap this end into place by pushing down on both sides until you hear a click.

Step 10: Reinsert the Metal Ring

Reinsert the metal ring onto the flywheel and start your lawnmower. Repeat these steps if it does not start and ensure everything is connected properly.

Precautions and Safety Measures:

1. Be sure to disconnect the spark plug wire before working on the lawnmower.

2. Do not attempt to take apart, modify, or fix a gas engine without proper knowledge and training.

3. Do not drop metal parts in grass clippings and dirt; they will rust and/or scratch internal parts of the lawnmower. 

4. Wear safety goggles and gloves to protect your eyes/hands from any metal shavings or particles produced during the repairing process.

5. Never smoke while working on a lawnmower; it has highly flammable gas inside of it (gasoline).

6. Work outside, preferably in an open shed or garage, to avoid spreading harmful fumes throughout your entire home.

7. Keep children and animals away from the repairing area so they do not get injured by sharp metal edges or shavings flying into the air while working on the lawnmower.

8. Do not operate a gas-powered lawnmower in an enclosed room; this can lead to suffocation or carbon monoxide poisoning.

Is It Good to Fix a Bent Crankshaft by Yourself?

The first thing to consider when deciding if it’s a good idea for you to repair the bent crankshaft yourself is if you’re capable of doing it. For example, if the engine has been submerged in water for a long time, then there’s no way that you can fix it even with help from an expert mechanic.

There are usually warning signs like if the engine doesn’t make any strange noises before it fails. See the steps above to determine if the lawn mower crankshaft is bent. If you notice that nothing seems wrong with the engine before it dies, you probably can’t repair it yourself.

Frequently Asked Questions

Can you use a lawnmower with a bent crankshaft?

Yes, you can use a lawnmower with a bent crankshaft. However, it is important to ensure that the engine is in good condition and that the blades are sharpened properly. If the engine is not in good condition, it may not be able to handle the increased torque and could break. Additionally, if the blades are not sharpened properly, they could damage the engine.

What causes a crankshaft to bend?

A crankshaft can bend if it is not properly lubricated. The main cause of crankshaft bending is the lack of oil in the crankcase. This can be caused by several factors, including dirty oil filters, worn-out oil seals, and low oil levels.

If you notice that your engine is making a weird noise or your crankshaft seems to be wobbling, it is important to have it checked out by a mechanic. A crankshaft that is not properly lubricated can cause serious damage to your engine and may even require replacement.

Can you fix a bent mower shaft?

Yes, you can fix a bent mower shaft. If the shaft is bent enough, it may not be able to rotate the blades and will need to be replaced. There are a few things that you can do to try and fix the shaft:

1. Check to see if there is any debris or sand inside the engine. If so, remove it using a plunger or vacuum cleaner.

2. Replace the belt if it’s broken or worn out.

3. Try tightening all of the bolts on the engine using a wrench.

What causes a bent crankshaft on a lawn mower?

A bent crankshaft is a common problem on lawnmowers. It is caused by improper maintenance or by wear and tears from use.

One of the main reasons a crankshaft becomes bent is due to improper maintenance. This can include forgetting to regularly replace the oil and filter, not adjusting the carburetor or throttle properly, or overloading the engine.

Wear and tear from use can also cause a bent crankshaft. This includes using the lawnmower in wet conditions, using the wrong type of grass, or pushing the lawnmower too hard.


This article provides information on how to tell if a lawn mower crankshaft is bent. It will teach you about what a bent crankshaft looks like and give tips for determining whether or not it needs to be fixed. 

We hope that this article has helped you learn about lawnmower crankshafts and how to identify bent ones. If you need any help with your lawnmowers, please contact us.

Jennifer Branett
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