Are Lawn Mower Blades Meant to be Loose?

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Have you noticed your lawn mower blades move freely on their own? Maybe this concerned you and it should, because that is not supposed to be the case. Mower blades have to be fastened securely so they do not fly off when in use. In this post you will learn how to tighten loose lawn mower blades and how to prevent it from happening.

Lawn mower blades should not spin freely, but you should be able to move it by applying a bit of force. A push mower has a tightness level between 35-60 torque foot pounds, and for rider mowers it is 70-90 foot pounds.

How Tight Should Lawn Mower Blades Be?

Mower blade tightness is measured in torque foot pounds. There is no standard as the torque varies depending on the manufacturer, make and model. Check your mower operating manual or the company website for information.

This table is meant to give you an idea of the typical tightness range. You should check the instructions that came with your lawn mower to find the exact specifications required.

Table. Lawn mower blade tightness comparison
Lawn Mower Type
Reel lawn mower As tight as possible
Push / walk behind lawn mower 35-60 ft. lb.
Self-propelled lawn mower 35-60 ft. lb.
Rider lawn mower 70-90 ft. lb.
Zero turn /garden tractor lawn mower65-100 ft. lb.

As you can see, the range is pretty wide. The reason is brands have their own specifications, designs and mechanisms that affect blade tightness. Depending on the mower, blade and design, you may have to tighten or loosen it. So if you have a Craftsman M105 for instance, the blade replacement must be specific for that model.

Some blades have more mobility even when tightened, while others are more rigid. It comes down to the design which is why you should consult the owner’s manual for the recommended torque.

Signs of Loose Lawn Mower Blades

Given how dangerous lawn mower blades can be when loose, it is important that you are aware if the blades are coming loose. While you are at it, remember to follow lawn mower safety guidelines.

If you inspect your mower regularly, loose blades are easy to spot. But even without examining the blade closely there are other signs the blades are not tightened properly.

Grooves in your lawn. You will definitely notice this if you keep your lawn short. If the blades have come loose they could make contact with the lawn and create grooves on the surface.

Ragged cuts. Loose blades will move all over the spindle, so some cuts are low and others high. If the blades are tight this should not happen, so if it does you know there is a problem.

Noisy. If the nuts securing the blade come loose even by just a bit, it will create a noise. Every time you us the lawn mower, the blade will make contact with the other parts, producing a loud sound. There could also be a problem with the alternator if your lawn mower has one.

If you notice some or all of these signs, you have to tighten the lawn mower blades. In the next sections you will learn how to tighten the blades on push and rider mowers.

The following is for general information purposes only. You should follow the instructions particularly for the tightness level required.

How to Tighten Push Lawn Mower Blades

You will need:

  • Torque wrench
  • Torque needed by your lawn mower (can be found on the manual)
  • 2 x 4 wood block

Step 1. Turn the mower off and empty the gas tank. Let the mower run if there is less than 1/4 gas remaining in the tank. The gas will dissipate. If there is more than a fourth, use a hand siphon to empty it.

Step 2. Turn the mower onto its side so the oil fill tube points up.

Step 3. Position the 2 x 4 wood between the back of the blade and the mower deck bottom. Doing this will keep the blade steady when you make adjustments with the wrench.

Step 4. Put the proper socket size onto the torque wrench. Majority of mowers use a 9/16 inch socket but yours may be different. Check the owner’s manual to be sure.

Step 5. Adjust the torque setting to whatever is recommended by the manufacturer. Also follow whatever instructions are provided with the wrench on making adjustments.

Step 6. Grab one end of the blade. Use the socket wrench to put the socket over the bolt. Tighten the nut until it clicks.

Step 7. Take the block of wood and restart the mower. The blades should be tight now and work properly.

How to Tighten Rider Lawn Mower Blades

If you have a rider mower like the 48V Ryobi, the process will be a bit different. This is a general overview only as riders have different specifications.

You will need:

  • Torque wrench
  • Torque setting (can be found on the manual)

Step 1. Drive your mower onto even grounds in your driveway or yard.

Step 2. Set the mower deck to its highest possible position. Turn off the ignition and remove the key.

Step 3. Hold one end of the blade you want to tighten with your hand. Set the socket over the bolt. Tighten with the torque wrench until it clicks.

Step 4. Repeat the step above for the other blade if it is also loose.

Why are My Lawn Mower Blades Always Coming Loose?

There are many reasons why this can happen. If you just tightened the blades and it came loose again, any of the following could be the culprit.

Bolt is Too Loose

This may sound obvious, but it is possible that you did not tighten the blade enough. This can happen with new lawn mower owners who are afraid of damaging the blade. While over tightening should be avoided, a loose bolt is not good either.

The solution is to tighten the blade again. Refer to the manufacturer instructions and use their recommend torque foot pound setting. Be careful when you tighten the bolt. If you put too much pressure it can cause damage. The best way to do is incrementally until the bolt is securely fastened.

Image Credit: Kalerna

Bolt is Too Tight

This is the exact opposite: you can over tighten and damage the bolt, the blade or both. When this happens the blade will eventually come loose. If the blade is too tight it will snap off when it runs into hard materials.

Even if the bolt does not break, over tightening leads to deformity and loss of threads. When this happens the only solution is to replace the bolts. So to prevent this you have to apply just the right amount of pressure when tightening the blade.

Blade is Not Centered

Lawn mower blades will come loose if they are not centered. If the bolt is not a perfect match for the holes, the blade will veer off to one side or another.

The main cause of this is incompatibility. You should only use blades that are compatible with your lawn mower. Not all blades are the same even though they look similar. The construction and the way it fits into the mower will depend on the compatibility. Bottom line: never use blades that your mower brand does not recommend.

Damaged Blade

If the blade runs into something hard it could get damaged or deformed. Either way this can cause the blade to loosen up.

Deformed mower blades can also lead to imbalance. An unbalanced bade will spin freely and loosen the bolt. Any number of things can happen next, such as the blade suddenly coming off or it could wear out.

Either way the fix is the same: buy and install a new blade. There is no sense in using broken blades because one, it is dangerous and two, the results will be unacceptable.

Uneven Sharpening

Sharpening blades helps performance, but if it is not done properly will cause balance problems, and an unbalanced blade will eventually loosen.

The key here of course is to sharpen the blades evenly. You should also note if it is time to replace the blades. However this is probably the last thing you should consider if the blade keeps coming loose. The other reasons given above are more likely to make them loose.