How To Sharpen Lawn Mower Blade

Before you buy a new blade, try this!

February 15, 2019household
How To Sharpen Lawn Mower Blade

Lawnmowers are the essential tool for keeping your yard looking civilized, and a well-groomed yard adds to the look and value of your home.

A quality lawnmower can be a reasonably good-sized investment, so it's important to keep it running well.

A well-maintained mower with sharp blades not only prevents the breakdown of the mower and keeps it running for longer, but it saves time on your lawn care and makes all your garden chores faster and more pleasant to do.

The Importance of Sharp Lawnmower Blades

Dull blades on a lawnmower tear at the tips of your grass, rather than cutting them cleanly.

Those ragged edges come with many aesthetic and health problems for your lawn. The torn grass tips quickly fray and brown, making your lawn look less green and healthy.

The frayed edges are also unhealthy for the grass, making it more vulnerable to pests and diseases.

When grass is cleanly cut, it heals cleanly, stays green, looks better, and is more resistant to disease. Even better, sharp blades cut grass faster, saving you time.

How Often to Sharpen Lawnmower Blades

Generally speaking, you should sharpen your lawnmower blades 2-4 times a year, or every 8th mow.

However, many factors can dull blades earlier, making it necessary to sharpen more often.

The presence of yard debris

If your lawn also includes not just grass, but also tough, woody weeds, leaf litter, twigs and branches, and other debris, and you routinely run over it with your mower, the blades will dull faster than if they were simply cutting grass.

Keep in mind that all those tough plant and wood fibers take a toll on your blades.

The condition of your soil

If your hard has lots of pebbles, rocks, and other soil elements that are coming in contact with your lawnmower blades, they will quickly chip and dull the blade.

Sandy soil is known for quickly dulling blades.

The kind of grass you have

Some creeping greens are simply tougher and more fibrous than others.

If you have tougher grass like Bermuda or St. Augustine, the chances are good that your blades need to be sharpened more often.

Cutting the grass too low

If you cut the grass too low to the ground, your blades have a greater chance of encountering not just grass, but also soil, rocks, and other debris.

It may seem like cutting the grass lower reduces the number of times you need to mow your yard, particularly in dry weather, but cutting grass too low prevents healthy thatch from forming and allows patches of bare dirt in the lawn.

Low blades are more likely to encounter rocks, sand, and soil, and dull more quickly.

How to Tell if Your Lawnmower Blades Need Sharpening

The easiest way to tell if your blades are is to look at some of your grass trimmings after a mow. The grass should look as though it's been cleanly sheared off, as though it were cut with scissors. If the edges look torn and ragged, with plant fibers showing, your blades should be sharpened.

When to Sharpen Lawnmower Blades

Sharpening the blades is an excellent part of your spring maintenance routine for your lawnmower. Because winter cleaning involves removing the blades, you could choose to clean and store them separately from the mower over the winter, ensuring that they stayed dry and protected from the worst fluctuations of temperature and humidity. If you remove the blades, you can protect them from rust and damage during the offseason. This habit makes it easy to check, clean, and sharpen them again in the springtime, before your first lawn mow of the year.

While many people sharpen their blades in winter, before putting the mower away for the season, if they are exposed to dampness and humidity over the winter, they will need to at least be checked, and probably sharpened again, in the springtime.

Aside from these seasonal variations of spring and fall, the blades will also need to be sharpened as needed, every few weeks, throughout the year.

How to Sharpen Lawnmower Blades

Before You Begin

Here are a few things to remember.

Some lawnmowers may spill a little gas when they are tipped up

Be sure to keep a rag on hand to clean up any drips.

Don't allow lawnmower fluids to leak from your mower into the soil or a storm drain; it is harmful to the water supply.

You may find that your blade is damaged and needs replacing

It is always a good idea to keep a spare lawnmower blade on hand in the case of extensive damage so that you don't have to stop mid-task and go to the store.

Having the second blade can also save time; if you need to mow the lawn before a party or a weather change, just replace it with the new blade and sharpen the old one later.

‘Mulching’ blades are much more difficult to sharpen because they have different cutting edges

It may take several different types of files to sharpen all the surfaces, and it may be better to take a mulching blade to a professional sharpening service.

Be safe

Blades are sharp, even when they are dull, and it's important to work slowly and attentively, being mindful of your safety at all times.

What You Will Need

To sharpen your lawnmower blade, you will need:

  • A pair of leather work gloves
  • A long-handled socket wrench
  • A vise
  • A file
  • A sharpening stone

The Procedure

How to Sharpen a Lawn Mower Blade
  1. Disconnect the spark plug wire, or remove the spark plug altogether.

    This will prevent the lawnmower from accidentally engaging and spinning the blade unintentionally.

  2. Note which side of your lawnmower has the carburetor and air filter.

    Keep this side upward when tipping your mower, to prevent oil leaks and smoke clouds the next time you start the mower.

  3. Tip the mower, keeping the carburetor on the upward side.

  4. Use paint or a wax pencil to mark your blade before you remove it.

    The most common mistake homeowners make is to reinstall their lawnmower blade upside down. Marking it before you take it off ensures that you will reinstall it correctly.

  5. Examine the blade to determine whether it is damaged and needs to be replaced.

    If the blade has large nicks or dents or is bent in any way, it should be replaced rather than sharpened. Brush the blade free of any dirt or debris, cleaning it before working with it.

  6. Remove the blade with the socket wrench. It is usually held on with a single nut, and you may need extra leverage to remove it.

    Do not use your foot for leverage. If necessary, use a clamp or wedge a 2x4 against the blade to stabilize it, and use a long-handled socket wrench to keep your (gloved) hands-free of the blades.

  7. Once the blade is removed, examine and clean the underdeck of the lawnmower.

    It's a good time to remove any dirt and debris that may be beneath your mower and make sure everything looks in good order.

    Once it's clean, place the mower back down on all four wheels while you work with the blade.

  8. Use a vise to hold the blade securely in place while you sharpen it.

    No matter what tool you use to sharpen the blades, they need to be held stably and securely.

  9. Use a file to sharpen the blade. Most lawnmower blades are made from fairly pliable steel that can be sharpened with 40-50 strokes of a 10-inch file.

    You don't need to achieve razor-sharpness but may want to follow filing with a sharpening stone for a cleaner edge.

    Unless you have experience with a grinder, do not use it to sharpen the blade, because you can easily damage it with excess heat.

  10. Sharpen the blade from the top side of the cutting edge, pushing with the file until you feel it “bite” into the blade.

    If you don't feel the bite/cut, your file may be dull, or you aren't pushing hard enough. Flip the blade over and sharpen the other side as well.

  11. Balance the blade. An unbalanced blade will introduce vibration into the engine and may damage the mower.

    To balance the blade, drive a nail into a wall and hang the blade from it horizontally, like a plane propeller.

    If the blade rotates and one side falls, filing that side to lighten it may help. Work with the blade until it stays horizontal and balanced from the center.

  12. Reinstall the blade, making sure that it is installed right-side-up.

    Hand tightens the nut before using a clamp or 2x4 to stabilize the blade and tighten it further with the wrench.

    Tighten it as firmly as possible; under-tightening this bolt is a common mistake, and bad for your mower.

  13. Reattach the spark plug wire or replace the spark plug.

How to Sharpen the Blades on a Reel Lawnmower

Reel lawn mowers are designed to be self-sharpening, and, when adjusted and appropriately used, need sharpening very rarely.

Before ever using a reel lawnmower, ensure that the blades are properly spaced.

When properly spaced, they will sharpen themselves, and the mower will work better and more efficiently overall.

Often people think they need to sharpen their reel mower blades, when in fact they just need to adjust the spacing.

To check the spacing on a reel mower:

  1. Place the mower on a workbench, so you have easy access to the mechanism. Clamp or chock the wheels to prevent them from turning accidentally.

  2. Spin the reel with your hand and listen to the sound of the mechanism. If you hear a metal scraping sound, use a screwdriver to adjust the blades away from the cutting bar.

    If it spins freely with no contact, adjust the blades closer to the cutting bar. These screws need only a very slight turn to adjust the position of the bar, so make small adjustments.

  3. Slide a strip of paper between a blade and the cutting bar and gently turn the reel by hand.

    If the blade cleanly cuts the paper with no scraping or grinding, that blade is correctly positioned.

  4. Make the same adjustment for all the blades on the reel lawnmower.

A properly adjusted reel lawnmower can go for years without requiring manual sharpening, depending on your grass and soil conditions.

After every use, clean a reel mower and visually examine the blades for nicks and signs that they need sharpening.

If your reel lawnmower blades need sharpening, the easiest and best way to do it is to purchase a back lapping kit and follow the instructions.

Generally speaking, a back lapping kit includes a sharpening compound that you apply to the mower blades.

You then adjust the mower so that the wheels rotate backward instead of forwards, and push the mower or rotate it by hand, allowing the blades to scrape against each other and the compound to sharpen the blades.

Note that this requires some strength and some patience; the blades are grinding and resisting the rotation.

When this reverse rotation becomes free and easy, the blades are sharp. When the blades are sharp, you rinse off the compound and reset the mower to run forward again.


Dull blades make the task of mowing your lawn more difficult and time-consuming, and less effective overall.

Dull blades are bad for your grass, and the general health and appearance of your lawn, so taking care of your lawnmower blades is an integral aspect of your overall lawn health.

Sharpening a lawnmower blade is a routine part of your home and lawn maintenance, and is a relatively simple task when done often enough.