Bent crankshaft lawn mower symptoms are important to identify this problem from the get-go. Crankshaft is the mechanical component that connects the engine to the mower’s blade.

Bent Crankshaft Lawn Mower

Our lawn mower experts have compiled a list of all the possible signs and symptoms that indicate such a problem with the crankshaft.

Continue reading this article and you will know the reasons and fixes. 

What Are Some Bent Crankshaft Lawn Mower Symptoms To Look Out For?

Bent crankshaft lawn mower symptoms are vibrating too much, the poor transmission of energy, weird noise produced, and the grass cut would be poor. In addition, the blades are bent and wobbly when functioning, the carburetor is defective, or even the spark plug is problematic. 

– It Starts Vibrating Too Much

A bent lawnmower crankshaft will throw the cutting blade off balance while rotating. This, in turn, will start to produce some very strong vibrations while working the mower. Whether you have a riding-type mower or a push-type one, these vibrations are too strong to be missed. However, they might also go unnoticed in the case of automatic self-propelling mowers.

You do not have to worry so much, and this is because other associated symptoms will help identify the problem even in these mowers. When the mower’s speed increases, these excessive vibrations will only amplify in intensity. On the other hand, also remember that at times, these vibrations go unnoticed if the mower is used only at low speeds and the crankshaft is bent slightly.

– Poor Transmission of Energy

If the lawn mower crankshaft gets bent at particular angles, it can cause damage to the internal parts of the engine close to it. The issue caused by the latter can lead to several transmission problems in the engine’s running, and you would see the engine is taking much longer to start after pushing its start button. 

There might be a long lapse in turning the machine off or sideways than normal. Once some small part of the engine gets damaged or displaced, the smooth transmission of power through the engine naturally gets affected. In this case, the components most vulnerable to damage from a bent crankshaft are the flywheel and the ignition parts. 

– Weird Noises Are Produced

Most gas-powered mowers produce noise when cutting grass; now don’t worry, because, this is expected, especially when the mower is large and riding style. As a seasoned lawn maintainer, you can tell these normal noises from the grinding ones that indicate a broken or bent crankshaft. 

Once the crankshaft gets bent, the skewed blade starts grinding against the sides of the deck. The unmistakable grinding noise produced hence is loud and must not be ignored. Tip the mower on its side, and you will see scratch marks on the deck sides where the blade has been hitting.

– The Grass Is Cut Poorly

It is possible to diagnose a bent or displaced crankshaft even if you usually do not mow your lawn. All one needs is a keen eye and adequate knowledge of how the mower works. When the crankshaft is bent, the mower cannot cut grass evenly, and you will see this result after running it and seeing how there are patches that are not looking straight and well.

Bent Crankshaft Lawn Mower Symptoms

The key reason behind this is how the imbalanced cutting blade can no longer cut grass at the same height. Here, you should aim to look closely, and you will see that the grass blades are also not cut perpendicularly in a straight cut.

It is best to wait to cut grass until the problem is resolved fully. Grass cut poorly at an angle is at risk of catching infections and undergoing a growth slump. Cutting grass with a faulty mower is almost as bad as not cutting it.

– The Mower Blade Is Bent

We have discussed how mower blades get bent in response to bent lawn mower crankshafts; on the contrary, the reverse is also true in a lot of cases. Sometimes, the blades get bent and crooked after coming in contact with a hard object while cutting grass. Not fixing and continuing to use such a blade will quickly cause the crankshaft to bend. 

That is why a bent blade is another specific symptom that the shaft is bent. The crankshaft is not the strongest mower component and gets bent easily. Only a little can be done about bent mower cutting blades and crankshaft except fixing it.

– The Blade Is Wobbly

When the mower is turned off and its plug is disconnected, turn it over and spin the cutting blade. When the crankshaft is bent, the blade will not be able to balance itself and will wobble while spinning.

They are still moving and haven’t stopped, although the machine has been cut from its power source. 

– The Carburetor Is Defective

When the carburetor is defective or clogged with grime, this will result in a bent shaft. When the problem lies in the carburetor, the engine will take a long time to start and often produces black smoke as it starts. It will overheat rapidly and produces coughing sounds from the engine,

This is because it is the carburetor’s job to extend power from the engine to the shaft. If the carburetor has an issue in this case, undue strain is exerted on the crankshaft, causing bending out of shape. 

That is why it is imperative to use the highest quality fuel that does not clog the engine parts with debris. Getting the carburetor cleaned once every two years is another important preventative measure you can take in this. 

– Problematic Spark Plug

A spark plug is the most significant component of a mower engine because it is the source that gives power to the machine to run. In other words, it is responsible for the engine’s smooth running without straining the engine component. When something is wrong with this plug, the engine will not start quickly, and too much strain will be placed on the shaft. 

Consequently, the shaft eventually bends under this extra strain. It is common for the plug to become defective after five to six years of continuous use and must then be replaced. Often, the plug becomes problematic simply because it needs to be cleaner. Taking a dirty plug out and cleaning it will prevent it from bending the shaft.

How To Fix A Bent Crankshaft By Yourself?

To fix a bent crankshaft by yourself, you must check where the problem is coming from, then, test the wobble of the blades and the bolt. After doing so, you must straighten the shaft and give it a go, or you should replace the shaft altogether. 

Once you start noticing more than two to three symptoms of a bent-up shaft that we have discussed so far, it is time to inspect the engine with your own two eyes physically. Only then can you be sure fully and also begin to fix it. 

– Check Where The Problem Lies

You must identify exactly where the problem lies before fixing or replacing a problematic crankshaft. In this case, you need a set of good pairs of gloves, clamps, and a wrench for the spark plug.

Before fixing engine parts like the crankshaft, you need to make your safety a top priority. Turning the engine off is insufficient; the plug wire must be disconnected. Most engines designed in the US have their plugs located behind the air filter shroud that must be unscrewed first. Once you enter the plug cavity, disconnect its wire through a wiggling motion and then use a plug wrench to remove it, and keep the machine cool off.

A free-spinning crankshaft and blade will make inspection difficult and dangerous for you. Now, what you must do is pull the brake handle down and tie it there, so the shaft will stop moving altogether. Use anything available, be it a rope or holding clamps.

Fix A Bent Crankshaft By Yourself

Remember that your mower needs to be parked on a flat surface during this inspection. This will help ensure it doesn’t fall on top of you by accident. You must also wear insulating and thick rubber gloves and boots to keep yourself safe.

It is now time to tilt the lawn mower on its side with the engine facing upwards. The fuel tank and the carburetor must face upwards, so oil leakage does not occur. The deck and blade of the mower are now easily accessible as you have moved it up. Place a table or something similar on the other side to keep the mower from tipping completely backward.

– Test Wobble and The Bolt

Once you have clear access to the mower’s deck, there are two main tests. Push the mower blade via its back edge to spin it around. When everything is fine, the blade will not wobble on its axis. If it does, this is a clear-cut sign that the shaft has been bent or broken, and this is why you should consider either fixing it or buying a spare accessory to place it.

While rotating the blade, keep a keen eye on its central bolt. This bolt is not supposed to move as the blade is rotated. Even when the spinning blade is not wobbly, but the central bolt is rotating, this is a sign that your shaft has been compromised. 

– Straighten the Shaft

Once you figure out that the shaft in the mower engine has become bent, the next query is what can be done about it. When the mower is slightly bent, a shaft straightener is the right way. This is a very efficient method of repairing the shaft using mechanical press pressure.

Since not all of us can get a shaft straightener easily, a sledgehammer works just as well. Using a sledgehammer is tricky because, with too much force, you can bend the shaft in the opposite direction instead.

– Replace the Shaft

Sometimes, the shaft is bent too much and needs to be corrected using any method possible. In this case, the only option is to get your crankshaft replaced altogether.

This will cost a lot more and require professional help from a mechanic, which means that you should examine it well and aim to replace the part for the use of your machine.

Details of Bent Crankshaft Lawn Mower


You know all the symptoms of a faulty crankshaft in a mower, and here is a summary encapsulating all of them:

  • The mower will vibrate excessively whenever it is started if the shaft is bent.
  • The regular noise of the mower is overtaken by a new grinding sound indicative of a bent or damaged shaft.
  • Since the blade cannot rotate without wobbling, the grass cut is uneven and at an angle.
  • Tilt the mower on its side and spin the blade with your hand. If the central bolt wobbles during this, then this is also a sign of a shaft that has been bent.

After reading our guide, you can immediately figure out the symptoms of a bad crankshaft. Detecting a problem early means that you will be able to solve it early on as well.

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