How to Sharpen Your Lawn Mower Blade

April 14, 2011



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Repair task: Sharpening Lawn Mower Blade

Machine type: Walk Behind Lawn Mower

Brand: Any Brand

Model Number: Any Model Number

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Your mower blade will get dull, worn and dented by hitting rocks and other flotsam and jetsam in your yard.

You can ignore this fact and keep mowing with a blade about as sharp as the handle of your butter knife. Or, you can quit pulling out your grass roots and start cutting the blades again.

I prefer to have my grass with clean cuts, not ragged ones. Sharp blades also place a lot less strain on your engine and save fuel as well.

Step-by-step instructions for sharpening you lawn mower blade:

  • Remove the spark plug wire and lock it off to the side so it can not reach the spark plug at all.
Plug wire removed

Plug wire removed

  • Using a piece of string, wire, a clamp or even some tape, hold the safety handle in the run position.
Handle Clamped

Handle Clamped

  • Lift the side of the engine that has the air filter. This keeps oil from plugging the filter.We want to check first if the blade or the engine shaft are bent. Use your eye to spot a point on the mower deck that lines up with the mower blade as it turns past. Bring the other end of the blade past the same spot and check if it’s on the same plane.
Eyeballing the blade

Eyeballing the blade

If it not the same, we need to confirm if the problem is the blade or the shaft.

It is pretty clear if the blade is bent because one end will droop visibly.

To check the shaft, first remove the spark plug. This makes it much easier to turn the blade. Turn the blade with your hand while watching the center bolt that holds it to the engine. If the bolt wanders away from the center, you have a bent engine crankshaft.

It is repairable, but it needs a special jig, some talent and some luck.

  • For now, let’s assume the shaft is straight and that the blade needs to be sharpened. Remove the center bolt from the mower blade. You may need to hold the blade from turning. There is a special tool for this,  or can use a block of wood. There may be two more bolts on either side of the center bolt. Remove them as well. The blade holder can have many different designs. The one pictured is very common. It has a square key bar inside that locks to the engine shaft. This key can be broken, so if it falls out when your remove the holder, it must be replaced.This holder is made of cast iron and has locking tabs on the side towards the blade. These can get knocked off also. This also requires replacement.The holder itself can get fractured into pieces. If it breaks, replace. Some are made of steel and have a pressed spot that locks to the shaft. This can get sheered off, too. If it does,  the whole thing should be replaced.I have welded these with success, but most people are not equipped to make this repair. Among the many other forms that blade holders can take are the plain flat ones that depend on friction to hold the blade and the star shaped ones that lock into a matching shape in the blade. There are also many others.You will also find that the hole in the blade comes in several sizes. The largest holes are on old Craftsman mowers with Tecumseh engines. These have a small step in the holder that connects to a notch in the end of the engine shaft, and they have threads on the outside of the shaft instead of inside it. They take a large nut, as you might guess.
Blade holder, cast iron

Blade holder, cast iron

Here is a generic holder or Blade Adapter:

Generic Holder of Blade Adapter

Generic Holder of Blade Adapter

If you need to replace the blade, note a few basic facts. Check the length.  Measure from cutting tip to cutting tip. If it is 19.5”, it is a 20” blade. Most commonly, you will need blade lengths either 20”, 21”, or 22” unless you have a very small mower. It will be either a standard blade, or a mulching blade. A mulcher blade has a longer or curved cutting edge.  It may have a larger wing and be called a high lift.  Notice the shape of the hole in the blade and get a holder that matches it.

This a sample of a mulching blade, 21” long with a star center hole:

Standard Mulching Blade

Mulching Blade - 21” long

  • Once you have the blade off, I suggest putting it into a vise and inspecting it.  Most likely, you will find it dull and dirty.
  • I use an angle grinder with a wire wheel to remove all the rust, scale and dried grass. The cleaning helps to ensure that the blade will balance well and look nice.
Wire wheel

Wire wheel

Be careful when using the wheel, especially as you near the edge.  It will want to dig in. The pattern that works best is to grind out over an edge.

  • Inspect the back side of the blade. You will find some mushrooming of the metal from the wear of the tip. I use a very light touch with the angle grinder to make the back edge flat.
Back Edge of the Blade

Back Edge of the Blade

  • Inspect the cutting edge of the blade.
Dull edge with nick

Dull edge with nick

Nicks like this one come from close encounters with rocks and metal. I do not try to work them out totally because it would remove too much of the blade.

You might notice on the pictures of the blade, that there are marks running parallel at an angle to the blade. These are the original sharpening marks and they tell you that your blade has not been sharpened since it was new.

  • Using a very light touch, sharpen the blade with an angle grinder.  I start sharpening  at the cutting edge and work length wise in stages until the edge is sharp and is in a straight line with the rest of the cutting surface.To avoid burning the metal, do not stay in one spot for more than a moment. Use the grinder in a way so that the sparks are flying toward the rest of the metal. This drives the heat into the metal and protects the thin edge from overheating.
Sharp Edge

Sharp Edge

The blade will get hot from the grinder. If you do not have an angle grinder, a wheel grinder can work instead. In a pinch, you can even use a good file.

  • Now to put it all back together.Put the bolt through the blade. Put the blade holder onto the engine shaft after putting a little spray oil onto the shaft.
Shaft ready for holder

Shaft ready for holder

This will reduce rust and make it easier to remove the blade next time.

Put the bolt through the blade holder and into the engine shaft. Notice that a few things need to be aligned. The bolt should have a lock washer and maybe a flat washer. The blade holder may have taps that need to fit into slots or holes in the blade.

This picture shows how the tabs can get out of position. Never tighten the blade if something like this is present.

Holder incorrect way

Holder incorrect way

Finger tighten the bold and check again that it is all aligned and nothing is pinched or held open.  Tighten firmly.

  • While the the mower is positioned like this, I like to put a block under the deck and oil the wheels.
Deck blocked

Deck blocked

Spray some oil inside and out while spinning the wheel. You will find that it pushes a lot easier.

  • Put the spark plug back on the mower and release the safety handle.

Happy mowing!

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