Even in the best of times, lawn mowers are noisy machines. The loud sound of the engine and the hum of the blades can make it difficult to hear something that is out of the ordinary. Well, that is unless the mower blade is hitting the deck. The sound of a blade hitting the deck is a sound you won’t miss. Most importantly, it can be very damaging to the lawn mower. So, let’s figure out what is going on and how to fix it.
Why Is Your Mower Blade Hitting The Deck? (The Short Answer)
If you have the wrong-sized blade installed on your lawn mower, then the mower blade will make contact with the cutting deck. In addition to this, damage to items such as the drive shaft, pulleys, deck shroud, and the deck itself can result in the blade making contact with the lawn mower structure.
5 Possible Reasons Your Blade Keeps Touching The Deck
A mower blade hitting the deck isn’t just limited to one design of lawn mower. This is a problem that you might face if you have walk-behind or various types of ride-on lawn mowers. So, let’s take a closer look at the different problems that could cause your lawn mower blade to hit the cutting deck.
- Wrong-Sized Blade
- Damaged Blade
- Bent Driveshaft
- Damaged or Loose Pulley
- Damaged Deck
- Damaged Deck Shroud
Trying to Use the Wrong-Sized Mower Blade
Lawn mower blades are not universal. Therefore, it’s important that you use the specific cutting blade designed for your lawn mower. The two main items to match with your lawn mower when picking a blade are the mounting system and the length of the blade. If you pick a blade that is too small, you have problems with speed, but if you pick a blade that is too long, you’ll have an issue with the blade striking the cutting deck.
The Lawn Mower Blade is Damaged
Next on the list of potential causes for your mower blade hitting the deck is that you are using a damaged blade. If you have a blade that has struck an object in the yard, such as a rock or even the edge of your driveway, then it’s possible that the blade has suffered enough damage causing it to now hit the cutting deck.
The Engines Driveshaft is Bent
One of the problems associated with damaging a cutting blade is damage to the driveshaft that the blade attaches to. Even though the driveshaft is made of a very strong metal, they are still able to bend if you hit something big and hard enough.
One of the Pulleys is Either Loose or Damaged
Once you move from a walk-behind lawn mower to a riding lawn mower, the design of the cutting deck changes. The most obvious changes are that the deck is separate from the mower and that pulleys and belts are introduced to transfer power from the engine to the blades.
So, there is no longer a direct connection of the blade to the engine and, therefore, no bent drive shaft. But with this type of deck design, the blade attaches to the pulleys. So, the pulleys can become damaged if the blade comes into contact with objects such as rocks, stumps, and hard surfaces.
Another problem that pulleys face is that their bearing and mounting bolts can become damaged and loose. All of these issues related to pulleys can cause the blade to hit the deck.
The Cutting Deck is Out of Shape
Cutting decks are made using two main methods: fabricated and pressed metal. Fabricated decks are welded together using a thick gauge metal that you’ll find on more expensive lawn mowers. These decks are extremely strong and very difficult to damage.
On the other hand, pressed decks are cutting decks that are pressed into a mold using a much thinner gauge of metal. Pressed decks are more affordable designs, but they lack strength. So, if you own a pressed deck and you hit a few trees or bash it into a wall or two, then your deck could be damaged.
The Shroud in the Deck is Damaged
A shroud on a lawn mower deck is designed to tunnel the airflow and clippings in a particular direction around the deck. Without a shroud of some type, the clippings would collect inside the deck. So, manufacturers use a thin gauge metal to form that path in the deck for the grass and air to flow.
If you have ever installed a mulching deck shroud on your lawn mower, then you’ll know what I mean. This is an additional shroud that you bolt into the deck to improve mulching. These shrouds are, unfortunately, very flimsy. So, just like the deck can get bent out of shape, so can the shrouds.
Diagnosing Where the Blade is Hitting the Deck
The easiest way to figure out where the blade is hitting the deck is to rotate the blade by hand. So, if you have a push lawn mower, remove the spark plug, then tilt over the lawn mower the correct way. Next, take hold of the blade and give it a spin.
For you guys with riding lawn mowers, you’ll have to either jack up the mower or remove the cutting deck. I find removing the deck to be the easier method for finding the root of the problem, as it’s easier to diagnose when the belts are removed.
So, spin the blade and figure out where it’s hitting the deck. Check to see if anything is bent, damaged, or loose and if the blade is the wrong size.
How to Stop Your Blade Making Contact With The Deck (5 Possible Fixes)
Now that you’ve figured out where the blade is hitting the deck and what’s causing the problem, we can dive into the fixes. So, get out your tools, and let’s go over how to stop your blade from hitting the deck.
Selecting the Correct Blade
Picking up the wrong blade is very easy to do, especially when you have a deck with more than one blade. So, to avoid any confusion, take a look in your user manual or look up your lawn mower’s model number and confirm the length of the blade you need to use with your mower.
Replacing your Lawn Mower Blade
If your lawn mower blade has suffered significant damage, then you’ll need to remove it and replace it with a new blade. I don’t recommend that you try to straighten out the blade, as it’s nearly impossible to get the blade perfectly straight.
So, brace the blade in place with either a blade brace or something like a 4×2 timber. Then remove the center bolt/nut using a socket or impact wrench and remove the old blade. Next, install your new blade onto the lawn mower making sure you tighten the bolt with the correct torque.
Finally, remove the blade brace and give the blade a spin. Hopefully, by replacing the blade, there will no longer be any contact with the deck.
Tools & Parts to Replace a Mower Blade
- Blade Brace
- Socket Wrench
- Torque Wrench
- New Blade
Straightening the Driveshaft
To straighten the drive shaft, you’ll need to start by tilting over the mower and removing the blade. First, follow the previous steps from replacing the lawn mower blade from above.
Next, you need to figure out how much the shaft is bent and also have a way of telling how much of an improvement you are making when you begin to straighten. So, I use a strong magnet placed on the underside of the deck with a metal screwdriver fixed to it.
Then, I position the tip of the screwdriver up close to the driveshaft so that it’s nearly touching. When you slowly pull the starter cord, the driveshaft will turn, and you will be able to see how much the shaft moves back and forth from the tip of the screwdriver.
Pull the cord so that the shaft is closest to the tip of the screwdriver, and mark it with a marker pen. This mark identifies the direction in which the shaft is bent and shows that you will have to pull the drive shaft in the opposite direction.
Now, it’s time to straighten the driveshaft. For this, I use a long steel pipe that’s about 3 feet long. If it were any shorter, I wouldn’t have enough leverage to straighten the driveshaft. So, the longer, the better.
To straighten the drive shaft, place the pipe over the shaft and pull it in the opposite direction of your mark. Now, you need to be fairly forceful when pulling, but not too much. Give it a try, and then recheck using your screwdriver and magnet.
Repeat pulling and testing until you find that the shaft no longer moves back and forth from the screwdriver tip during your testing. Now, this can be difficult to achieve, and you may find you can’t get it quite right.
If it’s still bent, you might be able to stop the blade from hitting the deck, but it will create a terrible vibration and an uneven cut to your lawn. If this is the case, then you might need to either get a replacement shaft or replace the engine/mower.
Tools to Straighten the Driveshaft
- Marker pen
- Metal Pipe
Replacing and Tightening a Deck Pulley
If your blade is hitting the deck because of a bent or loose pulley, you’ll want to start by removing the deck from the lawn mower. Then, if it’s a bent pulley, you’ll need to use a socket wrench and remove the pulley from the lawn mower.
Now, depending on how much work you want to do, you can purchase a complete pulley unit that includes the top pulley and the spindle. Or you can just purchase the individual components that you need. Usually, it’s just going to be the spindle that is causing the problem.
So, let’s go over the steps to replace the spindle.
Steps to Replace a Deck Pulley
- Remove the blade.
- Remove the top pulley.
- Remove the pulley deck mounting bolts.
- Lift the spindle from the deck.
- Position the new pulley onto the cutting deck.
- Install the pulley mounting bolts.
- Attach the original top pulley.
- Attach the blade.
- Tighten all bolts/nuts to the correct torque settings.
If you found that your pulley and spindle unit was loose, then you can just skip straight to the last step and tighten the bolts to the correct torque.
Tools & Parts to Replace a Pulley/Spindle
- Socket Wrench Set
- Torque Wrench
- Replacement Deck Spindle
Removing Dents from the Cutting Deck
If you have a dent in your cutting deck, you’ll have to grab a hammer. This isn’t a technical fix, and there aren’t any special tools and procedures required. All you need is your hammer and access to the underside of the cutting deck.
So, take your hammer and try to knock out the dents. Just be careful not to go too hard, just in case you have some rust and end up making a hole in the deck. If you do, then you’ll have extra work, and you’ll need to repair the hole in the deck.
Tools to Fix a Dent in a Cutting Deck
Fixing the Cutting Deck’s Shrouds
The last item on my list of fixes is the blade touching the deck because of a damaged shroud. Again, this isn’t a technical fix, it’s just about getting the shroud out of the way so that it doesn’t hit the blade.
So, turn the deck over by either removing it from the mower or tilting the mower over. Then, take whatever tool works best, like a pair of pliers or a hammer, and move the damaged shroud. If the shroud is badly damaged and straightening it doesn’t look like an option, then I’d recommend using an angle grinder and removing the part of the shroud that is causing the issue.
Just be aware that without the shroud in its correct position or with bits missing, you might have an issue with grass clumping under the mower.
Tools to Repair a Damaged Shroud
- Angle Grinder