What Are Lawn Mower Bent Crankshaft Symptoms?

It’s possible to hit a rock, a toy, or an unseen object while mowing your grass. It could physically damage the mower, and the flywheel may even shatter due to the event. When a collision occurs at high speed, the likelihood and degree of damage almost doubles. One of the most common examples of physical damage due to such collisions is a bent crankshaft.

Having a lawn mower with a bent crankshaft is as dangerous as hammering dynamite. In addition, it can cause significant hazards if not inspected on time. But don’t worry! This guide will take you through every lawn mower bent crankshaft symptoms, reasons, and how you can fix the problem.

The symptoms listed are not proof that your crankshaft is bent. Instead, they’re indicators that it might be the case, so you should flip your mower over to inspect further.

2 Main Lawn Mower Bent Crankshaft Symptoms

A bent crankshaft has three common symptoms. Fortunately, to judge these signs, you don’t need to be an expert mechanic. If your mower’s crankshaft is bent, these signs will be quite visible. All you have to do is keep your eyes and ears open.

Uneven Cutting

When a crankshaft is bent, the mower blade may fluctuate, causing it to cut in a disorderly manner. That being said, it will almost surely cut your grass at an angle. As a result, you may notice that some grass blades are significantly higher than others. However, there are many issues that cause a lawn mower to cut uneven, so this doesn’t ensure that you have a bent crankshaft.

Excessive Vibrations

Vibrations are unavoidable for items in motion. All moving objects vibrate at a unique frequency and an amplitude proportional to the motion they are undergoing.

However, the vibration amplitude of an object changes when it is deformed or alters motion characteristics. For example, when the force applied to a moving object changes, the amplitude of vibration changes as well. That’s because an increase in power can induce an increase or decrease in amplitude. Therefore, the vibration gets more rigorous and noticeable as the amplitude increases.

Because of its high rotating speed, excessive vibrations are the most prevalent indication of a bent crankshaft. You can feel the vibration for a few seconds following smooth-running intervals, but they are strong enough to indicate a problem. Along with that, you may also notice a grinding noise.

Transmission Problems

A bent crankshaft has frequent transmission issues due to two factors. The bent crankshaft may collide with other components, causing the ignition system to malfunction. Or, the impact that bent the crankshaft may break the flywheel, forcing the ignition system to malfunction.

In any event, the mower will have difficulty starting and running smoothly. If you notice any of that, stop mowing and turn off the motor right away. Also, inspect the crankshaft and repair it before cutting again.

Physical Inspection of a Bent Lawn Mower Crankshaft

Lawn Mower Bent Crankshaft Symptoms

Generally, before using a mower, it’s good to inspect the crankshaft to see if it is in proper working order. Having a physical inspection now and then keeps you up to date and informs you of any problems beforehand. Apart from periodic maintenance, if you’ve seen indicators of a bent crankshaft, you should get a physical test right away.

Physically inspecting the crankshaft of your lawn mower includes a variety of ways. The following are a few of those.

Method 1: Disconnect the Crankshaft from the Deck

This method involves checking if the space between the crankshaft and the deck changes during a crankshaft rotation. Here’s how you can do it.

Remove the spark plug.

Using duct tape, secure a ruler to the deck such that its opposite edge touches or almost touches the crankshaft.

After that, manually twist the shaft while keeping a close eye on the distance between the ruler and the shaft. Remember to wear safety gloves. If the gap changes while the shaft is rotated, it’s bent and needs to be repaired.

Method 2: Inspect the Bolt Wobble

This method involves checking that the bolt connecting the blade and shaft doesn’t wobble. Here’s how to do it.

Unplug the spark plug first.

Then have someone pull the starter cord slowly on the mower.

Observe the bolt that secures the blade to the crankshaft as they do so. If the bolt wobbles (moves sideways) while pushing the starter cord, the crankshaft is deformed.

Method 3: Inspect the Blade Wobble

A wobbling blade, like a wobbling bolt, might be a symptom of a bent crankshaft. Here’s how to inspect it.

Keep your mower running and parked in an open flat area to view the blade.

While the engine is running, keep an eye on the blade wobbling or shaking. A broken blade can cause wobbling of the blade, but it can also be a sign of a bent crankshaft.

Method 4: Use a Dial Indicator

A dial indicator is a device with a dial that monitors and detects slight changes in distance and amplifies them for the observer. Professionals use dial indicators to measure minor displacements in various constructions, such as beams, flat plates, shafts, and so on. To check for a crankshaft bent with a dial indicator, follow the steps below. 

You should place the dial indicator at specific positions on the crankshaft for this purpose.

The shaft’s make and model determine the dial’s location. You can find additional information in the manufacturer’s manual or seek professional assistance.

Method 5: Measure the Blade’s Location

The other way entails measuring the location of the blades, which takes a little longer and is less direct. Check out the steps below.

Remove the spark plug and tip your lawn mower to see the underside.

With the edges standing still, mark the position of the blades on the deck skirt using a marker.

Check to see if the other blade lines up with the mark you drew by turning the engine half a turn. If it does, your crankshaft is most likely straight, and the recurring problems are most likely due to some other reason.

If the second blade does not align with the mark, your crankshaft is perhaps bent, throwing the blades off balance.

Reasons for Lawn Mower Bent Crankshaft

A typical reason for a crankshaft deformation is colliding with an immovable obstruction when mowing. These obstructions can be a large rock, pipes, sprinklers, or anything of the sort.

The crankshaft connects the mower blade to the motor, which rotates at a very high rpm. Thus, when the mower collides with a hard object, the crankshaft can flex slightly, causing issues. The reason behind this flexing is energy dissipated by the blade and the shaft when the mower stops after the collision.

How To Fix a Bent Crankshaft On a Lawn Mower?

You can’t continue to mow with a bent crankshaft and thus need to fix it right ASAP. Here are some methods to repair a bent crankshaft.

Method 1. Use a Shaft Straightener

You can quickly repair a bent crankshaft using a shaft straightener. It applies a pressing force on the crankshaft, bringing it back to its original position. The tool is quite common among professional service providers as it instantly returns the mower to its regular operations and significantly reduces vibration. Plus, it’s quite easy to use and can help you complete the job safely at home.

Method 2. Sledgehammers

You can also use a sledgehammer to straighten the crankshaft. This method includes fixing the mower on a firm surface and hitting it to reduce deformation. Although it will not make your mower as good as new, it will ensure that rotating blades don’t touch other machine components, causing further damage.

Needless to say, having an OPE repair shop replace the bent crankshaft is the easiest way to solve the problem. In fact, they may also mount new crankshaft oil seals and piston rings. However, if you’re on a tight budget, we recommend investing in a tiny engine crankshaft straightener.

In Conclusion

A deformed crankshaft is a safety problem and must be remedied promptly. If you think your cutting machine may have distorted the crankshaft, stop mowing. Now that you know the lawn mower bent crankshaft symptoms, you should be able to confirm your suspicions easily. 

However, before beginning to mow, constantly tour the proposed mowing area and clear obstructions such as rocks and tree roots. You can also erect a perimeter wire/boundary if necessary. This way, you’ll reduce the chances of hitting hard objects and avoid lawn mower bent crankshaft symptoms.