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Ryobi RY40109 20" Mower Blade Replacement

Jim Keplinger and one other contributor

Last updated on Apr 13, 2026

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15 - 30 minutes

Easy

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Introduction

Go to step 1 ↓

Operating your mower with a dull blade can be hard on your lawn, leaving the grass tattered or torn rather than cut. This will cause it to look yellow and restrict healthy growth. Also, a damaged blade can be a danger to the operator or nearby people.

Replacing the blade isn't difficult and is a task that anyone should be able to complete in 15-30 minutes. There are some things to keep in mind as you start this project though. You will be working with a sharp object (the blade), so using a pair of heavy-duty gloves, removing the battery, and carefully blocking the blade are important safety measures to keep in mind as you start your work.

Once you've completed the replacement, I recommend mowing your yard right away and experience the satisfaction of your crisply mowed lawn!

What you need

Parts

Ryobi 20" Lawn Mower Replacement Blade-part #AC04020 For sale from another site

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Tools

1. Step 1 Blade
  Before working with the blade, remove the battery.
  
  Firmly grasp the battery, depressing the battery release and slide the battery out.
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2. Step 2
  Turn the mower upside down on a flat, stable surface.
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3. Step 3
  Preventing the blade from spinning is an important safety step!
  
  Use an 8" to 10" block of 2x4 to prevent the blade from spinning counterclockwise.
  
  You may have to experiment a bit to position it correctly.
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4. Step 4
  Protect your hands by putting on your heavy-duty canvas gloves while working around the blade.
  
  Using a 15 mm socket wrench, remove the blade nut by turning it counterclockwise.
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6. Step 5
  Remove the nut spacer and blade insulator.
  
  Remove the blade.
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7. Step 6
  Identify the alignment shapes on the blade hub.
  
  Install the new blade on the hub using the alignment posts.
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8. Step 7
  Replace the insulator, spacer, and nut.
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10. Step 8
  Re-block the blade to keep it from spinning clockwise this time.
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11. Step 9
  Set the torque wrench to between 310-360 in.lbs.
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12. Step 10
  Tighten the blade nut with the torque wrench by rotating the wrench clockwise.
  
  You have completed the replacement! Time to mow the grass!
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