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How to replace lawn mower blades

Replacing lawn mower blades isn’t difficult, but you need to know how to do it correctly and safely. We’ll take you through key steps to doing it right. Sharp mower blades cut grass more efficiently, and you won’t have to mow as often.

Replacement is the best option especially if the blade is:

  • Bent
  • Cracked
  • Worn down

How often should I replace my mower blades?

Annually. Sooner, if they’re damaged, or your yard has tough turf grass or rocky soil. Many homeowners and pro landscapers change blades after 25 hours of use, or before their first mow of the season. Refer to your owner’s manual for specific recommendations for your mower. Sharper blades cut grass better and reduce mowing time.

Hobby farmers and gardeners will know if a blade is dull by looking at the grass after it’s cut. Frayed ends to grass blades give the yard a brownish tone and subject it to disease because they can’t easily retain water. Dull blades also can cause bare spots when they cut.

Pro tip: Keep 2 blades so you can replace one and have one on the mower.


Replacing lawn mower blades, step by step

1. Get the serial number

It’s on a sticker on your mower. Get the entire number, including dashes and spaces, for a proper match to a new blade. A match is essential. Mowers are designed to have a specific blade for efficiency and safety. 

2. Get the right tools

What you need to change a mower blade:

  • Block of wood, 4x4 inches (optional)
  • Thick mechanical gloves
  • Plastic bag
  • Ratchet wrench with correct sized socket
  • Safety glasses
  • Shop towels
  • Small putty knife
  • Torque wrench

3. Disconnect the spark plug wire

Tuck the wire end safely. Disconnecting this will prevent accidental startup and electrical flare-up that could occur if gas or oil comes in contact with a connected spark plug.

4. Cover the gas tank

Remove the gas cap, and place the plastic bag over the opening. Replace the gas cap with the plastic under it.

Pro tip: If possible, change a blade when the mower is out of or low on gas. You can also self-siphon gas from the tank with a pump available at auto parts and hardware stores or a turkey baster.

5. Tilt the mower on its side

Ensure the air-filter side of the mower is up. If the blade bolt is too tight or rusted, stick the block of wood between the mower deck and blade to keep the blade in place as you loosen the bolt with the wrench (counterclockwise!). You can clean the bolt first with a wire brush. Hold the blade with one hand as you loosen the bolt.

Pro tip: Tip your mower on the side that doesn’t have the carburetor to help prevent a fuel leak. 

6. Remove the blade

Remove grass buildup under the mower deck with the putty knife. Wrap the blade in shop towels to help prevent injury. Look for instructions stamped on the blade to determine which side goes up. Most mower models are designed to allow a blade installation the correct way only. Installing a blade upside down could result in your lawn not being cut properly, and could damage your mower.

Pro tip: Keep a shallow cup to store washers and bolts. Replace worn hardware. Take a picture of your blade assembly for reference. Check your owner’s guide for an illustration.

7. Apply new blade

Hold the blade against the blade adapter on the mower with one hand. Tighten the blade bolt with the other. Use a gloved hand to hold the blade in place as you tighten it with the torque wrench. It should be between 450 inch-pounds minimum and 600 inch-pounds max. Reinsert the woodblock for leverage.

See the owner’s guide for a recommendation of how tightly to turn the bolt. The new blade should have no wobble when you move it up and down firmly.

Pro tip: A torque wrench is preferred. A 12-15-inch long wrench will suffice if you tighten the bolt as tight as you can.

Pro tip II: Overtightening the bolt can inflict damage, or cause the blade to break if it hits a hard object while mowing. Under-loosened bolts could allow the blade to fly off during use.

8. Turn the mower over

With the blade tightened, tip the mower back on its wheels. Reconnect the spark plug wire. Remove the bag under the gas cap, and replace the cap. Wait 30-60 minutes for the oil to return to the motor before mowing, to prevent motor damage. Check oil before use. Check the air filter to ensure oil didn’t get on it during mower blade replacement.

Pro tip: Apply a light coating of silicone spray to the deck to help prevent clipping buildup that could compromise the airflow.

Whether you mow lawns for work, are a rural homeowner with a big yard, or a rancher, staying on schedule to replace mower blades can keep your lawn lush and healthy. Fill all your lawn needs with help from AgriSupply.

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