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How Can You Tell If A Lawn Mower Crankshaft Is Bent? Find Out Here!

Written by Jess Da Costa
Last Updated on March 30, 2021

Is There Something Wrong with Your Lawn Mower Crank Shaft?

If you don’t know much about all the inner workings of a lawn mower, it can be a bit panic-inducing if your lawn mower suddenly starts making strange noises, vibrating erratically, or acting in other ways that it usually doesn’t.

If you’ve experienced unusual lawn mower symptoms, there could be any number of causes and it’s likely that a quick Google search will bring up some suggestions. But just as googling your own health symptoms can lead to misinformation and lead you down incorrect paths, googling your lawn mower symptoms isn’t always that helpful either.

Majority of lawn mower issues will stem from faults or damage within the engine, and the crankshaft can be a common culprit. But how do you know if it’s the crankshaft causing the problem?

We’ve outlined some key indicators that your crankshaft could be bent so that after reading this article, you should know exactly what to look (and listen) for!

Signs Your Lawn Mower Crankshaft is Bent

The first thing to watch out for when considering whether your crankshaft is bent is noise. If you mow your lawn regularly, or even if you’ve seen and heard someone else using a lawn mower a few times, you’ll know more or less what a healthy one should sound like.

There should be a steady, continuous hum that might change slightly if a patch of thicker grass is mown over or if there’s a particularly gritty section of lawn. Small and transient changes in your lawn mower’s sounds are normal.

If, however, you start hearing spluttering, clanking, or excessively violent vibrations, this is often a tell-tale sign that your crankshaft is bent or damaged. These kinds of sounds are usually easy to tell apart from normal lawn mower noises as they will often come on very suddenly for no apparent reason.

The other most commonly noticed indicator of a bent lawn mower crankshaft is if you have trouble starting your lawn mower. If your lawn mower usually starts in one go without any hassle, but suddenly it seems like nothing you do will get it to fire up properly, the crankshaft should be your first point of call.

Aside from issues with starting your lawn mower up, if you notice it keeps cutting out randomly, this could also be a sign that there’s an issue with the crankshaft.

Noise and transmission problems are the most common problems that will help you to identify a bent crankshaft, so if you notice anything like this, stop your mower immediately and investigate further.

How to Confirm that Your Lawn Mower Crankshaft is Bent

There are several ways to approach checking your crankshaft to ensure it’s not bent but the simplest and most fool proof method is by manually inspecting the lawn mower engine. There are a few key steps to follow that will make this process speedy and easy:

  • Firstly, locate your lawn mower’s spark plug wire and take it off. This should leave the spark plug more exposed.
  • You then need to remove the spark plug itself which you can do with a deep socket wrench or similar tool.
  • These two first steps are vital as they will ensure that your lawn mower engine has no compression and there is therefore no risk of it turning on accidentally and causing a safety hazard.
  • Secure the brake handle so that the brake is essentially turned off, as this will allow the engine to spin in order for you to see where the issue is. You can do this using a clamp, cable tie, bit of rope, or you can ask someone else to hold it down for you while you inspect.
  • Next you need to tip your lawn mower over to gain access to the underside, and subsequently, to the crankshaft. It is vital here that you tip it so that the air filter is on the top rather than upside down as this will ensure oil doesn’t leak into places it shouldn’t.
  • Once your lawn mower is tipped onto its side and you can see the blades and crankshaft, it’s time to test it by spinning the engine gently so that the blades turn, keeping your eye on the bolt that holds the blades to the crankshaft.
  • If the bolt appears to sway or swivel from side to side as the blades spin, this is a sure-fire sign that the crankshaft is bent.

This method will tell you without any shadow of a doubt, whether your crankshaft is bent or not. A straight crankshaft will not wobble or sway when the blades spin, so there will be no unevenness or scraping.

Are There Any Alternatives?

The other method involves measuring the position of the blades which can be a little more time consuming and less direct.

  • Remove the spark plug wire and spark plug just as you would in the method above and tip your lawn mower in the same way so that you can see the underside.
  • With the blades standing still, use a marker or other utensil to mark the position of the blades on the deck skirt.
  • Give the engine a half-turn and see if the other blade lines up with the mark that you drew. If it does, then your crankshaft is likely straight and the issues you’re experiencing are being caused by something else.
  • If the second blade doesn’t line up with the mark, then it’s most likely because your crankshaft is bent and throwing the blades off balance.

What Causes a Lawn Mower Crankshaft to Bend?

As you probably already know, lawn mower engines spin at incredibly fast rates and subsequently require a lot of kinetic energy to operate properly. Most of the time while you’re mowing, your lawn mower engine is spinning away largely unimpeded.

If you hit a rock, bit of metal, tree stump, or large root, the spinning of the lawn mower blades is abruptly interrupted which causes a lot of shock to be absorbed and passed through the engine, travelling through the blades and up the crankshaft.

Because mower engines spin so fast, the energy passed on by the collision can be enough to bend or even break the crankshaft. Unfortunately, this is not an issue that we can help most of the time – after all, no one is purposefully pushing their mower over large, immovable obstacles!

The only suggestion to avoid this would be to give your lawn a visual once-over before you start mowing to try and spot and eliminate any potential hindrances that could cause damage to your mower.

Should You Attempt to Fix Your Bent Crankshaft Yourself?

It is certainly possible to do so as long as you have the right tools and the necessary know-how, but there are disadvantages that come with attempting the fix yourself.

Of course, most people will not know enough about lawn mower mechanics to automatically know how to fix a bent crankshaft, so some proper research will be required. If you haven’t got all the right tools, you’ll also need to invest in buying some which might not be the best way forward if you don’t think you’ll use these tools much otherwise.

Whilst many people will be able to execute the fix successfully, you run the risk of causing further damage to the engine if you don’t know exactly what you’re doing, and this can lead to more problems down the line.

(image of a stop sign)

Fixing your lawn mower engine yourself will also void your warranty in a lot of cases, as many manufacturers stipulate that any repairs should be undertaken by a licensed agent. This could be a problem for you if your mower requires repairs at another stage that you are not capable of making yourself.

Locate the Problem, Contact a Professional

At the end of the day, the best way to approach a suspected bent crankshaft is to listen for undue or strange noise, watch out for transmission issues, and visually inspect the crankshaft to confirm your suspicions.

These are steps that virtually anyone can take, and noticing the signs early on will stop you from using your lawn mower in a subpar condition and potentially worsening the damage.

Once you know that the crankshaft is to blame for the troubles, you can then contact a manufacturer-approved technician who will be able to help you repair the crankshaft without affecting your warranty.

About the author
Written by Jess Da Costa
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