How to Sharpen Lawn Mower Blades

It's easy when you know what you're doing!

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Time

An hour or less

Complexity

Beginner

Cost

Less than $20

Introduction

Your lawn mower blade is dull. Sharpen the blade twice each season to help maintain a green, healthy lawn. A sharp blade not only cuts blades clean so grass plants recover quickly, it helps reduce your lawn mowing time.

Tools Required

  • File
  • Rags
  • Socket/ratchet set
  • Vise

Materials Required

  • Gloves
  • WD-40 lubricant

Why It’s Important to Sharpen Your Lawn Mower Blades

A dull blade rips and pulls the grass blades, leaving ragged tears that both weaken the plant and promote fungal growth and other grass diseases. A sharp blade, on the other hand, cuts cleanly, allowing the plant to heal and recover quickly. A lawn mower blade sharpening also lets you complete your lawn-cutting chore faster and with less stress on the mower.

These steps will work for just about any walk-behind mower. Riding mowers require different blade removal techniques, which we don’t show here. If you’re having trouble with your lawnmower find out why your lawnmower won’t start.

Video tutorial: How to sharpen a lawn mower blade yourself

Project step-by-step (5)

Step 1

First, Remove the Lawn Mower Blade

  • Remove the spark plug
    • Pull the spark plug wire from the spark plug to prevent the motor from accidentally starting. Tape or tie it back so it doesn't flop back into contact with the plug.
      • Pro Tip:We recommend always removing the spark plug when you're working on the blade. If the piston happens to be at the top of the compression stroke, a little bump to the blade might force the piston over the “hump” and into the power stroke. If that happens, the blade will lurch around and could break your hand!
  • Tip the mower on its side
    • Turn the mower onto its side with the air filter and carburetor side up. This keeps oil and gas from dripping into the air filter.
      • The carburetor is usually easy to recognize because it has throttle cables running to it. If you keep this side up when you tip your mower over to get at the blade, you won't get a smoke cloud from leaking oil the next time you start it. Some mowers have gas caps with air holes that could leak a little gas onto your garage floor, so work outside or keep a rag handy to clean up drips.
  • Mark Your Blade
    • Mark your blade with spray paint before you remove it so you know which way to reinstall it.
      • Note: Mower repair pros say that the biggest mistake homeowners make is installing a blade upside down after sharpening it. The blade won't cut—and they'll go nuts trying to figure out why!
        • Pro Tip: Keep a second blade on hand. The store will probably be closed when you need one!
  • Remove the Blade
    • Wedge a short 2x4 between the blade and the deck to clamp the blade.
    • Loosen the bolt (or nut) with a long-handled wrench. Turn counterclockwise to remove the bolt and blade counterclockwise.
      • Note: You'll usually find a single bolt or nut holding the blade on. It's usually very tight and you'll need to clamp the blade to loosen it. The 2x4 method we show is simple, quick and safe. Don't use your foot!
    • Once the blade is off, set the mower back onto all four wheels until you're ready to reinstall your blade.

Step 2

Sharpen the Blade With a File

  • Clamp the blade in a vise and sharpen the cutting edge with a mill bastard file, held at the same cutting angle as before.
  • File until the blade is “butter-knife” sharp.
    • Once you remove the blade, examine it to determine whether to sharpen it, or replace it.
      • Pro Tip: Sharpen your blade with a hand file. Mower blades are made from fairly soft steel. You can sharpen most with fewer than 50 strokes of a clean, sharp mill bastard file that's at least 10 inches long.
        • Note: Grinders also work, and much more quickly (pros use them). But they're more difficult to control and you might overheat and ruin the blade.

chalk file for filing metal

Step 3

Check for the Proper Filing Angle

  • File in the direction of the original cutting angle.
  • Always sharpen from the top side of the cutting edge; this will give you the longest-lasting edge on the blade.
    • Note: The file cuts in one direction only, on the push stroke; you'll feel it bite into the steel on the blade. If you don't feel that cutting action, your file is probably dull or you're not pressing down hard enough.
      • Don't try to make your blade razor sharp; it'll dull more quickly. “Butter-knife” sharp will do.
    • Note: Sharpening mulching blades is sometimes more difficult. Mulching blades may have longer or curved cutting edges, and you may need several types of files to sharpen them. 

sharpening lawn mower blade

Step 4

Check Blade Balance

  • Before reinstalling the blade, hang it on a nail to check the balance.
  • If one side dips, file a bit more off that side until the blade remains horizontal.
    • Pro Tip: An unbalanced blade will cause vibration and possibly ruin the blade shaft or bearings. 

 

Step 5

Reinstall the Blade

  • Reinstall the blade and screw in the bolt.
  • Wedge the 2x4 back in and tighten the bolt firmly with your socket and breaker bar.
    • Pro Tip: Insert the 2x4 in the reverse direction so you can bear down on the breaker bar to tighten the bolt. Mower sharpening pros say that the second most common mistake they see is under-tightening the bolt. A loose blade throws off the engine timing and sometimes makes the mower hard to start.