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How to Tell if Your Lawn Mower’s Crankshaft is Bent Full Guide of 2023

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how to tell if lawn mower crankshaft is bentA lawn mower that suddenly starts vibrating, making strange noises, or leaving an uneven cut across your grass likely has a bent crankshaft. Don’t ignore these red flags—a damaged crankshaft can lead to further issues and be dangerous to operate.

The good news is, with a few simple tests and inspections, you can easily diagnose a bent crankshaft yourself.

First, inspect the mower blade while it’s running. If the blade wobbles back and forth, that’s a clear sign of a bent crankshaft.

You can also stick a screwdriver into the spark plug hole and turn the mower over by hand.

Finally, check for physical damage like cracks or broken pieces around the crankshaft.

With this knowledge in your repair toolkit, you’ll be able to quickly identify and fix a bent crankshaft.

Key Takeaways

  • Listen for and feel any abnormal vibration or noise when running the mower.
  • Inspect the blade for signs of wobbling and uneven spinning as it turns.
  • Check for uneven or poor cutting performance across the width of the mower deck.
  • Test crankshaft rotation by inserting a screwdriver into the spark plug hole and turning it.

Signs of a Bent Crankshaft

Signs of a Bent Crankshaft
Has your lawn mower started making strange noises and vibrating excessively? This likely indicates you have a bent crankshaft. From grinding sounds and uneven blade rotation to poor cutting performance and stalling, key symptoms point to crankshaft damage that requires repair or replacement.

Act quickly once these issues arise, as continuing to mow with a compromised crankshaft invites catastrophic failure.

Strange Noises

You’re hearing those awful grinding noises and feeling the mower shake violently, aren’t you? Mowing tall grass or improper storage can loosen the chain, damage the flywheel, or bend the blade adapter.

Loud grinding noises while cutting indicate the crankshaft is strained, possibly from an unbalanced blade striking a metal pipe. Hairline cracks create an uneven cut. Regular maintenance prevents these issues.


The machine shimmies and shakes violently from the crooked crank. Blade imbalance and crank wobble cause strong vibrations. Mower hits put intense pressure on the crankshaft, damaging the flywheel. This interrupts smooth transmission of power, stalling the mower and causing uneven cutting.

Poor Cut Quality

When an improperly balanced blade wobbles from a bent crank, you’ll get an uneven cut. The blade hits high and low spots as it rotates, leaving your lawn looking ragged. An imbalanced cutting blade indicates potential crankshaft damage. Improper blade alignment or imbalance strains the crank over time, gradually bending the shaft.

Inspecting the Crankshaft

Inspecting the Crankshaft
You’ll need to perform a physical inspection and spin test to determine if your lawn mower’s crankshaft is bent. First, disconnect the spark plug and immobilize the mower’s blade for safety. Then, tip the mower on its side with the air filter facing up, spin the blade by hand, and watch the center bolt – if it wobbles excessively, you likely have a bent crankshaft.

Physical Inspection

Let’s inspect the crank by hand, partner, watching if the center bolt barely budges like a pin-up model, to reveal any wiggle that whispers of wicked bends. With the spark plug removed and blade immobilized, spin the blade and examine the center bolt’s motion; minimal movement indicates a sound crankshaft, while noticeable wobbling hints at damage from impacts, bent blades, or strained operation.

Spin Test

Partner, feel that wobble as you spin the blade, sending a shiver down your spine that signals the crankshaft’s bent.

  • Watch for uneven blade rotation and vibration
  • Wobble indicates misalignment and potential failure
  • Compare to a known good mower for reference
  • Have a professional inspect if concerned

Spin testing reveals issues before complete breakdown. Stay vigilant against damage from impacts, bent blades or strained operation.

Causes of a Bent Crankshaft

Causes of a Bent Crankshaft
Hitting an obstruction at high speed is the primary culprit behind a bent crankshaft. The sudden impact forces the spinning blade to collide with the housing, violently jerking the crankshaft.

Mower blades that are unbalanced or bent can also place undue strain.

Defective carburetors and problematic spark plugs strain the crankshaft over time too.

Carefully inspecting and balancing the crankshaft, using quality aftermarket parts, and replacing damaged keys and flywheels reduces the risk of recurrence.

Proper maintenance keeps your engine humming for years to come.

Fixing a Bent Crankshaft

Fixing a Bent Crankshaft
Dealing with a bent crankshaft can be frustrating, but there is still hope. You will need to assess the severity of the damage to determine if straightening or full replacement is required. With some effort and the proper tools, you can often repair slightly bent shafts, restoring performance and avoiding the expense of purchasing a new crank.


Fixing a slightly bent crankshaft is doable with a press or sledgehammer before you get more damage.

  • Disconnect the spark plug for safety.
  • Secure the mower on its side, with the filter facing up.
  • Spin the blade by hand and watch the crankshaft bolt.
  • A slight wobble means a small bend.
  • Use a press for minor bends.
  • Severe bends need professional replacement.

With early intervention, a mildly bent crank can often be straightened without full replacement. But take care – severe damage risks equipment failure. Regular maintenance keeps your mower humming smoothly for years.


Gotta swap that crankshaft completely by a professional when it’s badly bent. Your mower cries for help with nasty grinding, blade shaking, and coughing sounds. She’s begging you to realign bearings, balance blades, and massage starters before it’s too late.

But if your gal’s too far gone, let the mechanic replace her crankshaft so she hums smoothly again, cutting grass with ease.

Safely Checking the Crankshaft

Safely Checking the Crankshaft
Disconnecting that blade seems silly when inspecting the crankshaft, doesn’t it? But immobilizing the spinning steel before a physical inspection protects your hands as you check for problems.

Carefully tip the mower, watching the crank turn. Does it wobble? That shows it’s bent. Vibrations at high speed confirm a displaced crankshaft. Replace it to restore smooth performance and avoid future engine damage.

Regular maintenance like blade sharpening prevents strains that can bend the shaft over time.

Know the symptoms like hard starting, stalling and uneven cuts to catch problems early.

Frequently Asked Questions (FAQs)

What are some preventative measures I can take to avoid bending the crankshaft?

Regularly balance and sharpen blades to prevent imbalance and impact damage. Maintain engine timing, don’t over-rev, and avoid hitting objects. Lubricate crank bearings, check mower deck alignment, and replace worn parts early.

How much does it typically cost to repair or replace a bent crankshaft?

Repairing a bent crankshaft can cost $200-500 for parts and labor if you take it to a small engine shop. Replacing it will run $300-800 or more depending on the mower model. The best way to avoid this expense is to prevent damage in the first place.

Regularly inspect the blades, avoid obstacles when mowing, and maintain proper engine function.

Will running the mower with a slightly bent crankshaft cause more damage over time?

Yes, running the mower with even a slightly bent crankshaft will cause more damage over time. The imbalance and vibration will stress the crank and eventually lead to cracks or complete failure. You should replace the crankshaft or the entire mower before permanent damage occurs.

Can I straighten a bent crankshaft myself or do I need to take it to a professional mechanic?

You can likely straighten a slightly bent crankshaft yourself with a press or sledgehammer if you’re handy. However, for severely bent shafts, it is safest to take it to a professional mechanic who has the proper tools and experience to assess and repair the damage correctly.

Attempting do-it-yourself repairs risks further damaging the engine if not done properly.

If I buy a used lawn mower, what are some signs of a bent crankshaft I should look out for?

Excessive vibration while operating indicates potential crankshaft damage. Uneven grass cutting may also be a symptom. Crankshafts can bend over time from impacts or excessive strain, leading to an imbalanced spinning blade.

Hard starting requires progressively more pulls to engage the engine, which points to compression issues from a bent crank.

Visually inspect the blade for wobbling while manually rotating it. Any deviation or wobble around the center mounting bolt suggests the crank is out of alignment.

Avoid purchasing used mowers exhibiting these symptoms, as repairing bent cranks carries risks. The crankshaft is a critical component, and attempting DIY straightening often leads to further damage. Replacement crankshafts can be costly for older mowers. Carefully inspecting for signs of a bent crankshaft will help identify machines to pass on.


You know the sound – that horrible grinding noise when you start up your lawn mower. It’s like metal on metal, usually signaling something is bent or broken. Don’t ignore it and hope for the best. Take the time to properly diagnose the problem by checking the crankshaft. Look for wobbling and vibration as you spin the blades by hand.

Identifying and fixing a bent crankshaft early on will save you money and prevent further damage down the road. Inspecting your mower carefully and making needed repairs will keep it running smoothly all season long.

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Mutasim Sweileh

Mutasim is a published author and software engineer and agriculture expert from the US. To date, he has helped thousands of people make their yards lush and thick.