To get the best results from your lawnmower, sharpen the blade regularly. Under normal conditions, you'll likely need to do this every four to six weeks. If the blade bends or is damaged, replace it. Purchase a blade either from the mower manufacturer, or a universal blade suited for your machine. An incorrect blade will likely damage your mower, and could cause injury to you. Once the old blade has been removed, install a new blade by reversing the steps of the removal process.
Disconnect the sparkplug wire from the spark plug. Grasp the rubber plug cap. Pull and twist the cap gently away from the sparkplug. Turn the mower over on its side so that the carburetor is farthest from the ground.
Hold the new blade with two hands and place it on the crankshaft adapter so that the raised lugs line up with the proper holes in the center of the blade. The blade should only fit one way. The non-sharpened edge of the blade should face toward the mower body.
Locate the bolt, lock-nut and washer you set aside after removing the old blade. Slide the lock-nut onto the bolt. Slide the washer onto the bolt behind the lock nut. Insert the bolt, nut and washer into the opening on the blade adapter and crankshaft. Turn the bolt clockwise with your hand to tighten it.
Turn the blade to a vertical position. Place the block of wood on the lower side of the housing to the left of the blade. Place your foot (with shoes on) to the left side of the wood to keep the blade from moving.
Place the end of a proper-size box wrench onto the bolt and tighten further by turning clockwise. Turn the bolt until it stops tightly. Don't overtighten the bolt.
Set the mower back on its wheels. Reconnect the sparkplug wire to the sparkplug. Grasp the plug cap and press it gently down onto the sparkplug, twisting slightly, until the cap fits snugly.