Lawnmower Makes Loud Clanking Sound? (Possible Causes & Fixes)

Upgraded Home Team
by Upgraded Home Team
Lawnmowers can run into many problems, and they are often easy to identify based on uncommon sounds. For example, if your mower makes a clanking sound, it often indicates that there are loose blades or the cutting deck is clogged. Whether it be a problem with the cutting deck or oil, let’s explore why your lawnmower is making a clanking sound.

When using a gas lawnmower, you become accustomed to the loud revving the engine makes. However, what happens if you notice a sound that isn’t supposed to be there? In some cases, you might hear a loud, clanking sound while mowing. If you do, it could indicate one of a few different problems.

The most common reasons why lawnmowers make a loud clanking sound include loose blades, old engine oil, or a clogged cutting deck. To fix the problem, you may have to tighten your blades, change the oil, or clean out your deck.

Because there are multiple culprits for this noise, we’ll break down each option and its solution so that you can get your mower back up and running in no time.

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Top Reasons Why Your Lawnmower Makes a Loud Clanking Sound

The Problem: A Loose Blade

Your lawnmower blades take a lot of abuse. Not only are they spinning at such high RPMs all the time, but if they knock against anything besides grass (i.e., tree roots, rocks, etc.), they can get dull and damaged relatively easily.

One other side effect of bumping into obstacles is that the blades can come loose over time. When that happens, you’ll hear some knocking or scraping as the piece begins to wobble. Over time, this problem will only worsen until they can shear the bolt holding them in place.

The Solution: Tighten the Blade

Fortunately, this fix is quick and easy, so you should be able to do it and continue mowing right after. If you’re using a push mower, all you have to do is lift it up to access the blades. You can learn more about how to do that here.

Once the mower is up, a single bolt and nut are holding the blades in place. Tighten the nut, and you shouldn’t have the problem anymore. If it doesn’t feel like the bolt is tightening, you might have to replace it. However, that is rare.

If you are using a riding lawnmower, you will have to either drive your machine on blocks to access the cutting deck’s underside, or you can use a jungle jack to raise the whole thing high enough to access the underside. Before working on the blades, be sure to disconnect the spark plug and raise the deck height all the way. A single bolt also holds the blades on a riding mower, so all you have to do is tighten it.

The Problem: Old Engine Oil

Much like your car, oil is critical for your lawnmower’s engine. Without it, the various components would scrape and rub against each other, causing them to overheat and potentially break. Ideally, you should change your engine oil at the beginning of each season, but if it’s been awhile, you could be using old oil.

When old oil is in the system, it can create a knocking sound within the motor itself. Because the fluid isn’t lubricating as well as it should, the piston doesn’t flow as smoothly as it should, which results in a knocking or clanking noise.

The Solution: Change the Oil

Fortunately, most automotive oil should work well in a small engine. We recommend SAE-5W 30, if possible. The W stands for “winter,” and the number before it refers to the oil’s viscosity in freezing temperatures. The higher the number, the better the oil works in cold weather. Unless you’re cutting your grass during the winter, 5W should suffice, but you can always use 10W instead if you’re worried.

Follow these steps to change the oil in a push lawnmower:

  • Step One: Run the Machine – Doing this warms the oil so that it flows out easier.
  • Step Two: Cover the Gas Cap – You can use a sandwich bag or some other plastic to insulate the gas cap. Doing this ensures that no fuel leaks out.
  • Step Three: Tilt the Mower – Make sure that the gas tank is facing up. Place an oil drip pan underneath the oil tank opening to catch the fluid. Wait for it to stop flowing out.
  • Step Four: Add New Oil – Tilt the mower back and start pouring oil into the tank. Most push mowers need about 14 ounces, but it differs between models. Refer to your owner’s manual before starting. If you put too much oil in the system, you will have to drain the excess. Use the dipstick to tell when you’re at the right amount.

If you are using a riding lawnmower, follow these steps to change the oil.

  • Step One: Run the Mower
  • Step Two: Locate the Drain Valve – This piece may be on the right or left side. Refer to your owner’s manual if you can’t find it. You will likely have to open the hood to access it.
  • Step Three: Place a Drip Pan and Towel – Some oil will likely drip out once you open the valve, so you want to be ready with a towel.
  • Step Four: Open the Valve – On some models, there is a valve and a stopper. If you only have the valve, attach the hose first (see next step).
  • Step Five: Attach a Drain Hose – Because of the valve’s location, the oil will drip over your lawnmower if you didn’t use a hose. You can buy an official drain hose from the manufacturer, or you can make one yourself. Just make sure that it fits over the valve and that you have some way of keeping it in place (unless you want to hold it manually the whole time). One way to allow the oil to flow faster is to open the dipstick while draining.
  • Step Six: Close the Valve – Once the oil is drained, you can close the valve and the stopper (if applicable). Remove the drain hose.
  • Step Seven: Add New Oil – Riding lawnmowers require about a gallon of oil, but again, refer to your owner’s manual for a precise amount.

Bonus: Change the Oil Filter

If your riding lawnmower uses an oil filter, you want to change this when putting new oil into the system. You will need a strap wrench to dislodge the filter and be sure to place a drip pan or towel underneath when removing it, as some oil will dribble out.

When placing the new filter onto the engine, we recommend rubbing a little bit of oil on the o-ring. Doing this will create a tighter seal. Put the new filter on before adding more oil so that any dirty oil from the old filter won’t contaminate the new fluid.

After installing this piece, be sure to run the mower for a few minutes so that oil can flow through the filter.

The Problem: Clogged Cutting Deck

When was the last time you cleaned your mower’s cutting deck? If you’re like most owners, it has likely been a couple of seasons or more. Grass, dirt, and other debris will collect on the deck’s underside, which can bump against the blades as they spin.

Cleaning your deck is always recommended, even if you don’t notice any clunking noises. Grass clippings are full of water, which can cause the metal to rust over time. If you don’t clean the deck, you will have to replace your mower sooner rather than later.

The Solution: Clean Your Cutting Deck

If you have a push lawnmower, you can either tilt the machine on its side or lift it from the front. We always recommend the latter option because tilting the mower can potentially cause fluids to leak out into the engine. If that happens, you could be looking at some expensive repairs.

Depending on how dirty your deck is, you should be able to get most of the debris out with a hose and a scraping tool. In extreme cases, you might have to scrub the deck with sandpaper or something rough to get it clean. Be sure to dry everything thoroughly afterward so that it doesn’t rust.

If you are using a riding lawnmower, it should have a wash port on the deck’s top side. Simply attach a hose with a special coupling, and that will work wonders. Be sure to put the deck at its lowest setting before starting.

The Problem: A Bent Crankshaft

The crankshaft is the most central piece of your engine, as it is what spins the blades and the drive belt. Usually, the crankshaft gets bent if you hit something large with the mower. Once it’s bent, you’ll notice both shaking and loud noises.

The Solution: Replace the Crankshaft

Unfortunately, replacing a crankshaft is expensive, so it might be cheaper to buy a new machine altogether. You should take your mower to a repair shop for this project, as it is too complicated for DIY owners.

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Related Questions

My mower is making a rattling noise; what does that mean?

Generally speaking, any rattling noise means that some parts are coming loose. You will want to inspect the various pieces of your mower and tighten any loose nuts, bolts, or fasteners.


If my lawnmower still works with the clanking sound, do I have to fix it?

Yes, you should. The problem will only get worse, which means that it will be more expensive. The sooner you can fix your machine, the better.

Upgraded Home Team
Upgraded Home Team

We are a team of passionate homeowners, home improvement pros, and DIY enthusiasts who enjoy sharing home improvement, housekeeping, decorating, and more with other homeowners! Whether you're looking for a step-by-step guide on fixing an appliance or the cost of installing a fence, we've here to help.

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