Sharpening and Balancing a Lawnmower Blade
A sharp mower blade is critical to a good-looking lawn. Mow with a dull blade and you'll tear the end of the tiny blades of grass, creating a ragged cut that quickly turns brown, giving your lawn a dry, beige cast even if it's actually quite moist. Clean cuts with a sharp mower, on the other hand, promote good lawn health by preventing disease from getting a foothold in a sloppy, uneven cut.
Devices meant to sharpen your blade while on the mower are not the greatest, it is difficult to put a good edge on a blade with them and impossible to balance a blade also. You may be able to use a drill motor attachment or grinder to sharpen your blade while on the mower but it is best to remove it first.
The tools you will need:
Spark plug wrench. for the size of your spark plug.
Socket set and ratchet.
Some sort of grinder (angle or bench) or a file for sharpening
Safety glasses.
Gloves while working and sharpening.
Some type of vise if you are going to use an angle grinder or file.
C-clamp or vise grip to hold the blade from turning while undoing the bolts.
WD-40 or similar product to help with undoing the blade bolts.
Scraper or similar item to clean material off of and under the mower deck.
Whenever you do repairs under the deck of a lawn mower remove the spark plug or disconnect the spark plug boot and make it is kept away from the plug, an accidental engine start could cause serious injury. The mower will need to be tipped, so remove the air filter to prevent any fuel leaking from the carb from fouling the filter. Remove the fuel cap and place some kind of plastic wrap over the fill hole and replace the cap to seal it and stop fuel from leaking from the tank through the air breather hole and remember to remove the plastic when you are done.
Tip the mower with the spark plug pointing upward. Don't tip the mower where the plug hole is pointing down and never turn the mower upside down. Don't tip it more than 90 degrees. Oil can get into the cylinder and this will cause a hydraulic action, if the cylinder does get a lot of oil into it, you will not be able to pull the starter rope. Oil does not compress! If you do get oil into the cylinder and you can't pull the rope remove the plug and pull the rope a few time and this will blow the oil out of the cylinder out the plug hole, then reinstall the plug. When you first start up the engine you might have a lot of blue smoke coming from the exhaust but this will burn off and it will run normally.
Use the correct size socket size ratchet to remove the blade bolts. Most mowers will have a center bolt and sometimes two outer bolts to secure to the blade adapter. Mowers with a blade clutch will just have two outer bolts. There will be a center bolt on blade/clutch units but leave the center bolt alone, it's for the clutch. Use the vise grips or C-clamp to clamp to the deck as a blade stop. Clean the head of the bolt so the socket can make good contact with the head surface to prevent stripping the bolt. Spray the bolt area with the WD-40 to make it easier to remove. When the bolt breaks loose, and you are not braced properly, you could slip and injure yourself, so use caution during this time. Before removing the blade from the mower study how it is attached. Any backing plate or anti-scalp plate will need to go back on again the same way.
Once you have the blade off, scrape any debris that has built up on the blade with the scraper. This debris will throw off the balancing of the blade later on. This is also a good time to scrape off any debris that is on the underneath side of the mower deck. This will help prevent the deck from rusting, as this debris which is usually dead grass, will hold moisture against the metal of the mower deck which will promote the rust. This A bench mounted grinder is the best way to sharpen the blade but if do not have one you can mount the blade in a vise or clamp it to a table and use a drill motor grinding attachment, angle grinder a file. Grind the engine side of the blade only. You will see an existing bevel and that is where you want to grind. Approximately at a 45 degree angle. Do not remove material from the grass side of the blade. After you get a sharp edge on the blade, a light, de-burring on the bottom of the blade should be done, just don't bevel it. Next, take anything with a round shaft, like a screwdriver, and hold it in a horizontal position and then place the blade across the top the screw driver shaft and try to balance the blade on it. Look through the center bolt hole and if the shaft of the screw driver is in the center of the hole, it is balanced. If the blade is not in the center of the blot hole it is not balanced. The end of the blade that the screw driver shaft is closest to is heavier, or if the blade wants to swing down on one side that side is heavier. Go back to the grinder and remove more material from that side of the blade, de-bur the bottom and check again for balance. Continue until the blades center bolt hole is centered on the screw driver shaft, and the blade remains horizontal.
Re-install the blade and be sure the blade is on in the correct position. The bolts have to be tightened a lot. And use caution, do not slip and get injured when tightening the bolts.
Ideally, a mower blade should be sharpened once every month or two. It's easy to do and even all-thumbs types can do it quickly!
Also see Complete Engine Rebuild, Carburetor Rebuild, Loose Valve Seats, Starter Rope Repair, Carburetor Troubleshooting Chart or My Engine Won't Start for more troubleshooting.
Visit our Help and Tips Forum to ask any troubleshooting questions.
Visit our new Repairspot FAQs Page.
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