Inspecting and Removing Old Blades
Lift the mower deck to expose the blade. It's important to look closely at the carburetor and the oil compartment to make sure you're not tilting the mower in such a way as to spill oil all over the engine, the grass, and yourself. Generally, the safest way to do this is to tilt the mower back, toward the handle, and prop it with some kind of weight, or the help of a partner. This won't necessarily be true for all mowers, however, so use your judgment and consult the owner's manual, if necessary. It's also best to do this when there's no gas in the mower. Wait until you use it all up to change the blade, or you might consider draining out the gas with syphon hose. Typically, self-syphon pumps are sold at any hardware or automotive parts store. This prevents gas spillage onto the mower body.
Unplug the spark plug. It's best to stay on the safe side and prevent a short-out or electrical flare-up if any oil or gas should come in contact with the spark plug. It shouldn't be a problem if you hold the mower properly, but it's still best to be on the safe side.
Remove the blade mounting bolt. Use a socket wrench of the appropriate size and unbolt the mounting, while using your other hand to keep the blade from turning. Be careful not to lose any washers or mounting hardware that hold the blade into place, which can be reused. Pay attention to the position of the blade as you're removing it. You'll mount the new one in the same orientation, usually with the sharp edge of the blade going counter-clockwise with the turning of the assembly. Again, this may not be true on all mowers, so pay attention to the way this blade is installed and install the new blade accordingly.
Installing New Blades
Purchase replacement blades. Replacement blade kits for push mowers are generally only a few dollars at most home improvement or hardware stores, featuring new pre-weighted and sharpened blades, and usually replacement nuts. If your blades are particularly worn out, it's a good idea to invest in a new set. Some mowers feature a bottom cap on which two shorter separate blades are attached, while some newer push mowers feature a single longer blade, that looks kind of like a ruler. Tilt the mower back to inspect the blade, or talk to someone at the hardware store about the type of blade appropriate for your brand of mower. You can also check in the owner's manual, if you have it. Alternatively, you can salvage the old blades and have them sharpened if they seem to be in decent condition. If the blades are worn out, with chips or chunks out of the metal, it's probably wise to get a new set.
Mount the new blade in the appropriate direction. Line up the blades as they were lined up before and reinstall the washers and nuts, or use new washers and nuts of the appropriate size. If you have the owner's manual, there should be torque specs for tightening the nut. If not, just make sure not to over-tighten and warp the blade, which can cause vibrations in the mower. Most blades are either specific or universal fit. Be sure you have the same length as the old blade before mounting the new one, and make sure the clearance from the lawn mower deck is the same. Tighten the new blade onto the bolts carefully, since it'll likely be much sharper than the old one. It's a good idea to wear thick mechanic gloves when doing the job, to keep your hands safe. It's also advisable to use a small piece of wood to stop up the blade from turning while you reinstall it. You can jam a small piece of wood between the blade and the mower deck to keep things from turning.
Check the blade for play. Be sure the blade is mounted correctly and has no wobble when you move it up and down, firmly. Remove any jacks or props used to hold mower in place and wait about 30-60 minutes for oil to return to motor to prevent issues or motor damage. Check the oil before use to ensure it is within the proper limits.
Refill the gas tank and do a pre-check before mowing. Check the air filter to be sure that oil did not saturate the foam filter, if necessary, and reattach the spark plug wire. After a quick inspection, you should be able to start up your mower and start cutting that grass much more efficiently with your new blades.
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