No single part of your lawn mower has a tougher job than the blade. Your lawn mower blade spins around 3,000 times per minute, and makes short work of grass, weeds, leaves, and even (perhaps more often than you’d like) the occasional stick, rock, or debris that finds its way onto your lawn. These blades are built to last, but at some point, every blade reaches the end of its useful life. Knowing how to change a lawn mower blade is a skill every owner needs in their back pocket, so below we’ll break down the simple step-by-step procedure for installing a new blade.
How Do I Know When To Replace My Lawn Mower Blade?
Regardless of the type of mower you’re using, there’s always one dead giveaway that your blade needs replacing. Take a look at your lawn the day after you cut it: If the tips of your grass are green and uniform, your blade is fine. If they appear brown and/or uneven, however, your blade isn’t doing its job.
Your average blade cuts for around 200 hours before losing its cutting ability (especially with regular sharpening). That figure depends on the size and condition of the area you’re mowing, however, so if you often mow rough areas with lots of sticks and debris, your blade wears down much faster.
Step 1: Prepare Your Lawn Mower
Before you go diving under your mower with wrench in hand, you’ll need to do some quick prep work.
Your first step should be to disconnect the rubber spark plug cap from the front of the mower. Disconnecting the spark plug is always the first thing you do when working on a mower if you value your fingers. That’s because the simple turning of your lawn mower blade by hand can cause the motor to inadvertently fire, and by disconnecting the plug, you’re effectively preventing any and all combustion inside the engine.
Once you’ve disconnected the plug, it’s time to seal off the gas cap. Lawn mowers use “vented” gas caps, which means even when they’re screwed on completely, they still don’t form an air-tight seal. To do this, simply remove your gas cap, place a plain plastic sandwich bag flat over the opening, and then reinstall the gas cap over the bag. The plastic bag will fill in the spaces between the threads of the gas cap, blocking any fuel from escaping as you work.
Finally, go ahead and turn your lawn mower over on its side so that the blade is exposed and easily accessible. Ideally you’ll want to lean the mower so that the “exhaust” side faces down and the “air box” side faces up to prevent oil from seeping into your air filter.
Step 2: Immobilize The Lawn Mower Blade
At the center of the blade, you’ll see a single nut holding it in place. This is what we’ll need to remove to get the old blade off, but before we can do that, we have to lock the blade in place so we can put pressure on the nut.
There are two main ways to go about this: The first is to take a block of wood (a 6″ section of scrap lumber works great) and wedge it between the blade and the chute opening (where grass clippings leave the mower). No need to get fancy here, as long as the blade stays put under pressure, you’re good to go.
The second popular method is to securely attach either a C-clamp or a pair of vise grips to the deck of the mower at the end of the blade. Again, so long as the tool prevents the blade from spinning in the desired direction, you’re good to go.
Step 3: Remove The Old Blade
Once you’ve immobilized the blade, you’ll be able to get enough leverage on the retaining nut to break it loose. We recommend using something more substantial than a standard wrench here, and find that either a sizable ratchet or breaker bar provides the ideal amount of leverage for the job.
As you remove this nut, take care to note the position of any additional washers/spacers sitting underneath before setting it aside. Once the nut is removed, take note of the position of the blade and the direction of its cutting edges, then remove it from the mower.
Step 4: Install The New Blade
Once you’ve removed the worn blade, it’s time to install the new one in its place. Not all replacement blades share the exact same shape, (mulching blades, for example) but they all share the same one-directional cutting surface. Install your new blade in the same orientation as the old blade, then install the nut and any washers/spacers you removed by hand.
Once you’ve got the nut “finger tight” on the new blade, remove your wood block or clamping tool, and move it to the opposite side of the blade. This step is important because we’ll be applying pressure in the opposite direction we used when removing the old blade.
Once the blade is immobilized again, you’ll be able to tighten the bolt down on the new blade to lock it in place. To do this correctly (re: safely) you’ll need to find the torque spec listed in your owner’s manual for the blade nut. Once you’ve got this number, use a torque wrench to tighten the nut to the required spec.
Step 5: Get Cutting!
Once you’ve got your new blade installed and properly torqued down, the job is done! Put your mower back onto its wheels, remove the plastic bag from your gas cap, and reinstall the plug wire over the spark plug.
Note: When restarting the mower for the first time, it’s common to see some white smoke coming out of the exhaust. This is perfectly natural, as fluids have a habit of creeping into your exhaust while the mower is on its side. The smoke should stop after a moment or two, and the mower will run as normal.
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