How To Tell If Lawn Mower Crankshaft is Bent

What is that strange grinding sound coming from the mower? It could be nothing, or it could be something important, like a bent crankshaft. The crankshaft is the piece that connects to the rapidly spinning blade of your lawnmower. If the blade suffers a collision with a rock or root, the crankshaft could bend.

Learn how to identify the signs of a bent crankshaft. It is dangerous to ignore crankshaft damage! There are a few symptoms to watch for, several simple tests to verify your suspicions, and, for the DIY warriors out there, instructions on how to straighten the crankshaft yourself.

The dangers of a bent crankshaft

Do not continue to use a mower with a bent crankshaft! Doing so could result in injury, damage to property, or even death.

The blade under the mower is spinning extremely fast. If it dislodges due to a bent crankshaft, it could fly out from the mower. Obviously, this is extremely dangerous to anyone or anything that is near the mower.

The symptoms of a bent crankshaft

Unusual vibrations and sounds

It is normal for your lawn mower to vibrate. After all, there are a lot of moving parts! You want to look for abnormal vibrations that come with a grinding noise. The vibrations and noise may not be continuous.

If your mower is vibrating excessively and you hear a grinding noise, turn it off immediately! Do not use it again until you have a chance to examine the crankshaft.

Transmission issues

The bent crankshaft may hit other internal parts of the mower and cause further problems. If the mower will not start or runs really rough, you may have a bent crankshaft.

How to identify a bent crankshaft

The best way to identify a bent crankshaft is to perform a physical inspection of the mower.

How to perform a physical inspection

There are several ways to identify a bent crankshaft through a physical inspection of the mower. Always disconnect the spark plug before examining the mower!

Check the gap

The gap between the crankshaft and the deck should be consistent all the way around. Any inconsistencies point to a bent crankshaft.

Turn the mower over. Attach a ruler to the deck with either a clamp or duct tape with one end barely touching the crankshaft. Manually rotate the shaft. Pay close attention to the gap between the deck and the crankshaft. If there are any variances, the crankshaft is bent.

Look for bolt wobble

Your mower has a bolt that connects the crankshaft and the blade. Turn the mower over. You will need some assistance with this test! Have a friend pull the starter cord. If the bolt wobbles when the cord is pulled, the crankshaft is bent.

Use a dial indicator

If you have a dial indicator in your toolbox, it is time to use it! A dial indicator is a very precise measuring tool. Place the dial indicator at various points along the crankshaft to check for any discrepancies.

How to fix a bent crankshaft

Disassemble the mower

Materials needed

  • Socket set
  • Nut driver set, including the correct size for your mower
  • Screwdriver
  • Heavy pipe or crankshaft straightener
  • Rubber mallet
  • Vise grips
  • Wrench
  • Valve spring compressor

Disassemble the mower

You will need to access and remove the crankshaft in order to straighten it. Make sure the mower is completely cool before you start any work.

1. Drain the oil

Remove the oil plug and completely drain all the oil.

2. Disconnect the fuel hose

Close the fuel hose, disconnect it, then drain all the fuel from the mower.

3. Disconnect the air filter assembly

4. Unhook the throttle cables and the engine

5. Remove the oil tube

6. Remove the clutch

7. Remove the flywheel

Before removing the flywheel, place a small piece of wood between the flywheel and the housing. Then, loosen the bolt on the flywheel, and pry it out with the screwdriver.

8. Remove the blade

There is a pin that connects the blade to the crankshaft. Pull the pin, then turn the bolt on the blade. Once the blade is free, remove it from the mower.

9. Remove the motor

10. Remove the valve springs

First, remove all the bolts on the valve spring cover. There will be two valve springs underneath. Use the valve spring compressor to compress the valves one at a time. Release the springs carefully.

11. Remove the camshaft gear

12. Remove the crankshaft

Push on the connecting rod to move the cylinder piston out of the way. Then you should be able to access the crankshaft.

Straighten the crankshaft

Place the crankshaft on a sturdy surface. Place the heavy metal pipe on top of it. Use the mallet to hammer the pipe. Check the crankshaft often with a level. Repeat the process until the crankshaft is completely straight.

Or, you could use a lawn mower shaft straightening tool to easily straighten the crankshaft.

Reassemble the mower

Follow the disassembly instructions above in reverse to reassemble the mower. It may be helpful to lay aside the parts in the order they are removed. Labeling the pieces is also beneficial when it is time to put the mower back together!

When to replace a bent crankshaft

If you are unable to straighten the crankshaft completely or the crankshaft is damaged, it is time to buy a replacement. Buyer beware! It is expensive to replace a crankshaft. It may be worth it to purchase a new mower, instead

When to replace a bent crankshaft

Frequently Asked Questions

What causes a bent crankshaft?

A mower blade can spin at a speed up to 200 miles per hour. If that spinning blade hits an immovable object, like a rock, a branch, or a sprinkler head, all that speed and energy must be transferred somewhere. The most common cause of a bent crankshaft is hitting an object with the mower.

Less frequently, a crankshaft could be bent due to a manufacturing defect or an accident while shipping the lawnmower.

Can a bent crankshaft be fixed?

It is possible to straighten a bent crankshaft with the right tools and a little grunt work.

How much does it cost to fix a crankshaft?

If you decide to take your mower into a shop to be repaired, be prepared to pay. It can take three hours or more for the shop to disassemble the mower and repair the crankshaft. If they charge $50-$100 per hour for labor, that will add up quickly.

For a mower crankshaft repair, be prepared to pay $150-$500.

How do you test a lawn mower crankshaft?

The best way to test a lawnmower crankshaft is to perform a physical inspection on the mower. See the instructions above to inspect your mower safely.

Final Thoughts

Don’t ignore a bent crankshaft! Stop mowing and check it as soon as possible. It may be possible to save some money and straighten it yourself.


Author: Matt Hagens

Hi, I’m Matt the owner of Obsessed Lawn. I love to be outside working on my lawn, planning my next project. I created this website to help people like you find the best products for yard care and great advice. Learn more about me and find me on Facebook.