Lawn Mower Blade Loose: What to Do + Why

Have you recently started hearing a rattling sound while you mow? Super annoying, right?! The good news is that there’s a solution – it’s not a sign that your mower is on its last legs. It’s likely a sign that your lawn mower blade is loose. In an ideal world, these things would fix themselves, but that’s not going to happen, so you need to know what to do. And that’s exactly what I’m going to cover in this post.

Signs that Your Lawn Mower Blade is Loose

It’s not like I’m obsessive about lawn mower maintenance (far from it…I’m quite lazy truth be told!), but I do have a fiddle around with the main components from time to time to make sure everything is in good condition and working properly. If that’s not you though, here’s how you might tell that your lawn mower blade is loose:

Rattling Sound

I mentioned this in the intro, and it’s worth repeating as it’s the thing you’ll probably notice first if you’re anything like me. Unusual sounds coming from a piece of equipment that I’ve used frequently is something I instantly pick up on. If the nut securing the mower blade has partially come undone, even just a small amount, the blade will vibrate/bang against other components and you’ll hear it. Unless you’re listening to music with your headphones in that is!

Uneven Cut

When a lawn mower blade is properly tightened up, it spins level on both sides. That’s the idea anyway. As a result, you get a nice even cut. But if you’ve started noticing that parts of the grass you’ve mowed are uneven – some parts high, other parts low – it could be because of a loose lawn mower blade. When it spins, the blade can move around on the spindle and that’s what causes the unevenness in the cut.

Gouging the Lawn

You might notice this if you like to cut your grass particularly short, and your lawn mower blade is particularly loose. In that situation, it’s possible that the blade could actually reach and impact the surface of your lawn, creating gouges. This is a clear sign something is wrong and should be very noticeable.

Grass Barely Being Cut

You might walk up and down your lawn a few times, not paying too much attention, before looking down and noticing that your mower has hardly taken anything off the grass. This is because your lawn mower blade is spinning but not cutting, which is obviously no good and this often happens when the blade is extremely loose.

How to Tighten a Lawn Mower Blade

Not addressing a loose lawn mower blade is potentially dangerous both for you and your lawn. And as I mentioned earlier, unfortunately, it’s not something that will fix itself. Maybe they will make a model one day that does, but for now, it’s going to require a few minutes of your time.

First Things First: Inspect the Blade

This needs mentioning as it’d be a bit dumb to simply tighten up a blade that is really past its best and needs replacing. One common cause of loose blades is actually running over a rock, trunk or anything solid. That kind of impact can do some serious damage to a blade, and if it’s bent it could end up ruining your crankshaft, which will basically KO your mower and make it useless.

So before you proceed to tighten the blade. Have a look at it.

• Is it bent?
• Is it dull – does it need sharpening?
• Is it corroded?
• Does it just look a million years old?
• Is the mower blade reverse thread or not?

If it does need changing, now would be a good time to remove your lawn mower blade and either sharpen or replace it. A lot of folks wonder…Do brand new lawn mower blades need to be sharpened? No, they don’t.

Disconnect the Spark Plug

If you’ve read a few of my posts, you might get sick of me saying this. But this is always the first thing you should do. It might save you your fingers someday!

Tip the Mower Up

If you have a workbench in your garage or some other raised surface, this is a lot easier. You can use a jack to raise the front end of the mower and keep it in place. You can also tip your mower on its side if it’s a walk-behind model. Just make sure you tip it the right way (yes there is a right way and a wrong way to tip a lawn mower). The air filter and the carburetor should be on the side that is pointing upwards when you tip it.

There’s no need to empty the gas to tighten the lawn mower blade!

Wedge a Small Block of Wood Between Blade and Deck

This is just to stop the blade from spinning around as you tighten it. The block will ensure it stays in one place and makes the job much, much easier.

You can also use a pair of vice grips to achieve this same result. Clamp them to the side of the deck tightly, and that’ll stop the blade from spinning round.

Grab Your Wrench and It’s Time to Tighten

A socket wrench with a ratchet is perfect for this as it allows you to get more leverage. There’s not much that can go wrong here. BUT it’s important to make sure the blade is in the right position before you start to tighten. Does your blade have 2 “outer holes” on either side of the hole that the blade is mounted onto the crankshaft through? If so, it’s super important that you line these up properly with the grooves on the shaft, as that’ll mean your blade is in the right position.

Most of the blades I’ve used have had a circular hole as the center hole, but it is possible that yours could have a different shape, like a 5-point star, a 6-point start, a bow tie, a 6-point spindle or a triangle spindle. With these different types of holes, it’s really important to make sure it’s properly seated onto the shaft before you begin to tighten. Continually tightening a blade that is not seated properly is a sure-fire way to round off the shaft end over time.

Now everything is lined up, tighten the bolt up until it feels solid and you should be good to go!

How Tight Should Lawn Mower Blades Be?

Every lawn mower blade has its own recommended amount of torque for installation. If you’re installing a new blade, that recommendation may be listed somewhere on the packaging that came with the blade. Or if you know what blade model you have, you could look this up online.

Between 38 and 50 foot pounds of torque is standard for walk-behind mowers.

Between 70 and 90 foot pounds of torque is the norm for riding mowers.

If you don’t have a torque wrench, you can just do it by feel. You want to tighten your blade to the point that it’s not able to wobble if you try to move it up and down, but you don’t want to tighten it to the point that it feels overly stiff, as it will have no give in it if you hit something. Over-torquing bolts is just as dangerous as under-torquing them and can lead to failure.

Recurring Problem: What Causes a Lawn Mower Blade to Keep Coming Loose?

If your lawn mower blade keeps coming loose, there’s something that you’re not doing right. Here are a few common mistakes.

Not Tightening It Up Enough

If you under-torque the nut/bolt, it’ll come loose again. If you’re not sure how tight it should be, buy yourself a torque wrench and tighten it to the foot poundage recommended by the manufacturer.

Blade is Not Seated Right

Remember how I said earlier that it’s SUPER important to seat the blade right before you start tightening. This is why. If it’s off-center or not on the shaft properly, it’ll loosen with use. And normally pretty quickly, even if it seemed tight!

Related Article: Which Side of the Lawn Mower Blade is Up

Your Blade is Unbalanced

This won’t usually cause a loose blade unless it is VERY unbalanced, but it is possible. This might be worth checking if you’re still scratching your head after investigating other potential causes.

A Loose Mower Blade is a Simple Problem with Complex Consequences

I say this because it’s quite easy to fix usually and it’s definitely not rocket science, but it can have so many negative consequences if just left. It turns your mower into a dangerously unpredictable machine to operate, can scalp your lawn, and ruin the crankshaft…A problem that must be fixed!