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Welcome to the Blade Sharpening section of Maintenance and Repair. If you don't want your mower to do what the one above is doing the blade will need to be balanced at the time you sharpen it.

I shouldn't even justify the mention of devices meant to sharpen your blade while on the mower, but I can't resist a chance to denounce them with a sound flogging. They're JUNK! It would be difficult to put a good edge on a blade with them and impossible to balance a blade. You may be able to use a drill motor attachment to sharpen your blade but remove it from the lawnmower first.

Tools you'll need.
1. Spark plug wrench. 5/8", 3/4", or 13/16" depending on the plug you have. Preferably a deep socket wrench.
2. Blade bolt wrench. 9/16" for most mowers, 5/8" for most Toro's. Some mowers require a 1/2" in addition. Honda mowers will require a 14mm socket. It's really very preferable to have a 6 point socket on a short breaker bar for the main blade bolt(s).
3. Some sort of grinder or if you're a glutton for punishment, a big file.
4. Heavy gloves.
5. Safety Glasses.
6. Screwdriver.
7. Ear protection.

First off, remove the spark plug. Anytime you're doing repairs under the deck remove the spark plug. An accidental engine firing could be the end of a set of fingers. The mower will need to be tipped in some direction but first remove the air filter to prevent any fuel dribble from the carb from fouling the filter. Remove the fuel cap and place a piece of plastic wrap over the fill hole and replace the cap. This will prevent fuel from leaking from the tank through the cap breather hole but don't forget to remove the plastic when done.
Preferably tip the mower with the spark plug end of engine pointing up. If that's not practical, never tip the mower where the plug hole is down and never turn the mower upside down. Don't tip it further than vertical or 90 degrees. A messy oiling of all things not meant to be oiled will occur if the engine is tipped wrong. If you have an impact gun it will make the blade R & R much easier but most folks aren't going to have this luxury. Use a six point socket and a breaker bar or at least a decent length ratchet to remove the blade bolt(s). Mowers without a blade clutch 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 and you won't like putting it back together. An extra set of hands can be useful to hold the blade. Make sure you and your helper are wearing gloves. If no one is around to help, a wedge of wood can be jammed between the blade tip and deck or better yet a large pair of vise grips can be clamped to the deck as a blade stop. Make sure the head of the bolt is clean so the socket can be engaged to the full head surface. When the bolt breaks loose, if you're not braced, you could slip so don't leave yourself in an awkward, vulnerble position. Before removing the blade from the mower look how it's mounted. Any backing plate or anti-scalp plate will need to go back on just as it comes off. Even if there's just a blade, some will go on up-side down and that would put the un-sharpened air lift to just dig at the grass. A week doesn't go by that we don't see one or two blades put on up-side down by well intentioned homeowners who are complaining how bad there mower cut the grass.

Now to the sharpening. Well, not quite. First clean the blade of grass build up. If left on it will throw off the balance of the blade.
A bench mounted grinder is the easiest way to sharpen the blade but you can mount the blade in a vise or clamp to a table and use a drill motor grinding attachment or a big file. In any case, remove material from the engine side of the blade only. You'll see an existing bevel and that is where you want to grind. Approximately at 45 degrees. DO NOT remove material from the grass side of the blade. The cutting edge has to be the lowest point and if you grind on the bottom side of the blade it will bevel up and cause the bottom of the blade to drag and beat the grass. This will render your sharpening job useless and put an un-needed load on the engine. When you have a good edge on the blade a light, flush deburring on the bottom is ok, just don't bevel up. Next take a screw driver or anything with a round shaft which fits easily in the center hole of the blade. Hold the screw driver horizontal and hang the blade from it also horizontal. If the blade remains horizontal it's balanced and ready to go. If it wants to swing down on one side, that side is heavy. Go back to the grinder and remove more material as you did when sharpening. Make four or five passes, debur the bottom and check again for balance. Continue until the blade remains horizontal.

Re-install the blade and be sure the blade is back in the correct position. Torque spec vary and most homeowners don't have a torque wrench, but to be sure, it has to be tightened a bunch. Don't slip off and get yourself hurt but don't be a wimp about it either, It has to be tight.

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