Do I Need To Sharpen New Mower Blades?

Agh…I love the smell of fresh cut grass, but joy can quickly turn to frustration when using a dull lawn mower blade. Grass trailings, uncut grass, clogged mower and the list goes on…

So do you need to sharpen a new mower blade? No, there’s no need to sharpen a new lawn mower blade. Mower blades come ready sharpened.

In this post we’ll answer beginner questions about lawn mower blades. And also what you must not do when changing your lawn mower blade.

Do I need to sharpen lawn mower blades?

Yes, lawn mower blades need to be sharpened. A dull blade will pull and tear on your grass blades causing jagged tips. The dull blade stresses the grass and the damaged grass tips turn yellow. A sharp blade will cut cleanly which is healthier for the lawn, easier on you and your mower engine.

How often should mower blades be sharpened?

At the very least your mower blade needs to be sharpened at the start of every new season, and more often if your yard is stony or large. Setting your mower cutting deck to lowest level will cause the blade to impact objects more often and will obviously need more regular sharpening.

When should I replace my mower blade?

A mower blade should be replaced about every third to fourth season. But it really does depend on it’s condition, a mower blade that cuts a small mature yard will last longer. New yards usually have rocks, stones and debris that impacts the blade causing it to become dull and wear out quickly.

Mower blades

Blade wear

Check out the wear on this blade, this mower was having all kinds of cutting and bagging problems - New blade solved all.

How do i know if my mower blade needs to be sharpened?

A mower blade doesn’t need to be knife sharp, but the leading edge of the blade does need to be bevelled and the face free from gouges. Examine your blade by turning your mower over on its side but always with the carburettor side facing the sky (prevents gas spill).

Wearing a glove, run your finger along the bevelled blade. Are there large indentations or gouges? If the answer is yes, then your blade needs to be sharpened.

Can you sharpen a lawn mower blade without removing it?

Yes you can sharpen a mower blade without removing it, so long as it is not badly damaged or worn. Best practice is to remove the blade from the mower, sharpen and then balance it. Sharpening the blade on the mower runs the risk of unbalancing the blade which will cause the whole mower to vibrate. 

Mower blade sharpening

tools

This method is only suitable for relatively new blades. You'll need a big flat metal file, move the file at 45 degrees with the bevel and to finish run the file across the rear to clean the burrs. If your blade is heavily damaged or badly worn, this method isn't suitable.

Should i sharpen or replace my lawn mower blade?

If your mower blade is more than four years old and seen average use, then go ahead and replace the blade. Good quality blades aren’t expensive, but note you will need a torque wrench to tighten the blade bolt to specifications.

Tightening a mower blade without using a torque wrench runs the risk of the blade coming loose or destroying the engine. I wrote a post about my torque wrench here “Best torque wrench for mower blades”

Mower blades

Sharpen or replace

Unless your blade is in good condition, I'd spring for a new one. They're not expensive, but you'll need to buy or borrow a torque wrench.

How tight should mower blades be?

It’s important to tighten your blade to the correct specification. A mower blade may not be as tight as you might expect.

A blade fits on a blade boss (metal block) which is then fitted to the end of the engines crankshaft. A blade is designed to slip on the blade boss if the mower blade hits a solid object. 

Over-tightening the blade will prevent the blade slipping on the boss, and will instead bend the crankshaft and destroy the engine.

Can you sharpen a Mulching blade?

Yes a mulching blade is sharpened in the same way as a collecting blade, best results are achieved by removing it before sharpening and balancing.

Which way do mower blades tighten?

A single blade lawn mower blade bolt will be a right hand thread. Viewing the mower on its side (carburettor side up). To tighten the bolt – turn clockwise. To loosen the bolt – turn anticlockwise. I wrote a post on a similar subject “How to remove stuck blade bolt”

Which side of the lawn mower blade is up?

All mower blades will have a lifted trailing edge. The lifted edge points upwards, to the underside of the mower body.

Do lawn mower blades need to be timed?

A single bladed mower will not need to be timed. A twin blade walk behind mower like a Toro or some lawn tractors will need blade timing. The purpose of the timed blades is so the blades can overlap.

Overlapping blades helps prevent uncut grass between the blades. If your blades can touch each other, then yes it’s likely you have a timed deck and the blades will need to be timed.

I wrote a post about timing your mower blades here ” Replacing timed mower belt”.

What causes a lawn mower to cut uneven?

There may be several reasons why your mower is cutting unevenly, here’s some of the more common reasons:

  • Deck wheels are set to different heights (Walk behind mower)
  • A bent/worn blade
  • Uneven tire pressures (tractor mower)
  • Deck needs to be levelled (tractor mower)

Should the front of a push mower be lower than the back?

A lawn mower deck should be level, front to back and side to side.All mowers will have a height adjuster, some will have a single lever and others will have a height adjuster at each wheel. Level the deck by adjusting the wheel heights.

Mower spark plug testing tools

Level

Adjust all wheel heights until the deck is level.

What does a high lift mower blade do?

A High lift blade has a trailing edge that lifts sharply upwards. This causes a powerful vacuum effect on the grass blades, lifting them upright before cutting them. The high lift blade is specially designed to collect grass and it does so really well.

An engine will need to be sufficiently powerful to drive a high lift blade, not all mower engines are. Check with your dealer before fitting a high lift blade.

Auto Technician and Writer at | Website

John Cunningham is an Automotive Technician and writer on Lawnmowerfixed.com. I've been a mechanic for over twenty years, I use my knowledge and experience to write "How to" articles that help fellow gear-heads with all aspects of mechanical repairs, from lawn mowers to classic cars.