Installing a blade on your lawn mower should be part of a routine maintenance to keep the machine operating at peak performance. This does not mean using a new blade each time. Most lawn mower blades can be professionally sharpened in a matter of a few minutes and then reinstalled on the mower. Keeping a sharp blade on the mower not only helps the engine, but will make for a cleaner cut.
Put on your gloves. Even old blades can have extremely sharp edges.
Remove the rubber sparkplug wire from the sparkplug. The sparkplug is typically located on the rear of the engine and faces the push handles of the mower.
Pull the fuel fill cap from the gasoline tank. Tip the lawn mower over to drain the gasoline into the gasoline container. Replace the fill cap.
Lay the lawn mower on its side. Use the push handles to balance the machine. Insert the 10-inch 2-by-4 between the edge of the blade and the side of the mower. This will hold the blade in place while you remove it.
Fit the correct-size socket over the main nut that holds the blade to the engine shaft. Generally, the nut's size will range from ¾ inch up to 1 inch. Consult your lawn mower's manufacturer's instructions for the correct socket size.
Place the ratchet in the socket. Turn the ratchet counterclockwise. It may take some effort to break the nut loose. The 2-by-4 may fall from the blade. Place it back into position. Loosen the nut and remove the old blade.
Place the new blade onto the engine shaft. Take care to position the blade, the correct side upward. The sharp edge of the blade must be facing properly so it will cut when the engine revolves in a clockwise direction.
Start the nut onto the threads of the engine shaft with your hands. Be sure you have installed any washers that were removed when you took off the old blade. Tighten the nut clockwise. Insert the 2-by-4 between the blade's edge and the side of the mower. Finish tightening, using the socket and ratchet.
Lay the mower right side up. Attach the rubber sparkplug wire to the sparkplug. Fill the gasoline tank with fresh fuel. Allow the lawn mower to sit for 15 minutes so the oil can fully drain back down into the crankcase. Test the new blade by starting the mower and cutting some grass.