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One of the best ways to
encourage a greener, fuller
and healthier lawn is to
sharpen your lawn mower blade.
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. Sharp
blades also let you complete your
lawn-cutting chore faster and with
less stress on the mower.
Sharpening is a simple task, even for a novice. It’ll take a few sharpenings to master the technique. After that, the chore will take less than 10 minutes. Plan to do it twice every mowing season. We show here the steps that will work for just about any walkbehind mower. Riding mowers require different blade removal techniques, which we won’t show here.
Play it safe when
removing the blade
We recommend always removing
the spark plug wire before you touch
the blade (Photo 1). The blade and
shaft are directly connected to the
motor, and in some cases turning
the blade by hand could cause the
motor to fire.
Then look for the carburetor and 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 (Photo 2), 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. Once the blade is off, set the mower back onto all four wheels until you’re ready to reinstall your blade.
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 (Photo 3) is simple, quick and safe. Don’t use your foot! A good tool to keep handy to loosen the bolt is a 10-in. breaker bar with a socket to match the bolt. It’ll give you plenty of leverage to loosen extremely tight bolts, and you can keep your knuckles well away from the blade when bearing down. Use a squirt of penetrating oil on really rusted, stuck bolts. Wait 10 minutes to give it time to work.
1. 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.
2. 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.
3. 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. Remove the bolt and blade.
TIP
Keep a second blade on hand.
The store will probably be
closed when you need it!
Mark your new blade
Mark your blade with spray paint before
you remove it so you know which way
to reinstall it. 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!
Sharpen it with a file
Once you remove the blade, examine
it to determine whether to sharpen it
or replace it. We recommend
that you sharpen it with a
hand file (Photo 4). 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 in. long.
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.
Always sharpen from the top side of the cutting edge; this will give you the longest-lasting edge on the blade. 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.
4. 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.
Sharpening mulching blades is sometimes more difficult. (See “Buying a New Blade,” below.) Mulching blades may have longer or curved cutting edges, and you may need several types of files to sharpen them. In some cases, you may have to resort to a 4-1/2-in. angle grinder. If your blade is too difficult to sharpen, take it to a hardware store or a blade sharpening service. You can have it sharpened for about $6.
No excuses!
To get in the habit of keeping
your blade sharp, dedicate a
set of tools for sharpening
only. Hang them nearby so
they’re ready to go. And keep
a second, sharp blade handy
too. You can slip it on and
sharpen the dull one later.
Balance it before reinstalling
Before you reinstall the blade, be sure
to balance it. An unbalanced blade
will cause vibration and possibly ruin
the blade shaft or bearings. To check
the balance, simply drive a nail into a
stud and set the blade onto it like an
airplane propeller (Photo 5). If one
side falls, it’s heavier, and you have to
file more metal off it. Keep filing
until the blade stays level.
Reinstall the blade and hand-tighten the bolt. Insert the 2x4 in the reverse direction so you can bear down on the breaker bar to tighten the bolt. It’s difficult to overtighten the bolt. Mower sharpening pros say that the second most common mistake they see is undertightening the bolt. A loose blade throws off the engine timing and sometimes makes the mower hard to start.
5. Hang the blade on a nail to check the
balance. If one side dips, file a bit more off
that side until the blade remains horizontal.
6. Reinstall the blade and screw in the bolt.
Then wedge the 2x4 back in and tighten the
bolt firmly with your socket and breaker bar.
Buying a new blade
Always replace your blade with an exact replacement blade,
or the blade recommended in your owner’s manual. Resist the
temptation to convert your regular straight-blade mower to a
fancier mulching mower by simply changing the blade. Your
mower probably won’t work any differently than before, and
it may not work as well. The mower deck on a straightblade
mower is shallow and has a side discharge
to eject the grass clippings quickly. A
mulching mower has a deeper deck
without a side discharge; the
grass is chopped three
or four times before it
drops to the ground.
The mower design is as
important as the blade.
Do you need a new blade?
Examine your blade when you
remove it and look for the problems
shown. If you’re unsure of the
condition of the blade, take it to a
hardware store or home center and
compare it with a new one.
Thin trailing
edge
The trailing edge, or fin, is the edge
opposite the cutting edge. This fin is
often slanted upward, which creates
an updraft to lift the grass and grass
clippings. Dust and sand will wear
this fin down. When it’s thin, replace
the blade.
Bent
Set your old blade on your workbench
and check for bends. If you’re
unsure, compare it with a new blade.
Dents in cutting edge
Replace blades that have deep dents
that you can’t file out and erosion
from wear and sharpening. Also
replace any blade that has cracked.
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