Regularly clean your rotary lawnmower blades to extend the life of your mower. Inspect your blades so you are aware of any damaged blades that may be causing your mower to not work as well as it used to. Always follow safety precautions when you are dealing with lawnmower blades.
Disconnect the spark plug cable to prevent any chance of the engine accidentally starting. Remove the battery completely from a mower with an electric starting system to further ensure that it does not accidentally start up. Drain the gas from a gasoline powered mower.
Remove the spark plug. Turn the engine with your hand until the piston is all the way down inside the cylinder. Push a piece of cloth into the cylinder to protect the piston from being damaged.
Turn the mower over onto its side. Make sure that the oil fill hole is elevated above the crankcase to keep any oil from draining out of the mower.
Insert a block of wood between the blade and the deck of the mower. Make sure that the wood is lodged so that the blade cannot be turned all the way around. Put a pair of heavy work gloves on and turn the blade counterclockwise against the wood block.
Hold the blade firmly against the block while loosening the retaining nut. A wrench may be necessary to remove a firmly secured nut. Some models may also require the removal of more than just one retaining nut, so inspect to see if this is the case.
Remove the blade from the shaft. According to "Garden and Yard Power Tools," there may be a half-moon key installed on the mower to protect the engine from damage. Look for this key and inspect for damage if you find it. If the half-moon key is damaged, replace it with a new one before remounting a new or sharpened blade.