To maximize the life of your lawnmower, you must remove and sharpen its blades after every 8 hours of use. When you sharpen your blade, examine it to see if it needs to be replaced. A bent blade or one with nicks that are too deep to be filed away must be replaced immediately. In fact, it is a good idea to keep an extra mower blade around. That way you can change the blades before your next mow and leave sharpening for when you have the time.
Empty all of the gasoline out of your lawnmower's tank. To do this, sit the lawnmower outside and allow it to run until all of the gasoline is used up.
Pull out the spark plug with your fingers. If you are unsure of where it is located, consult your owner's manual.
Lay the mower (gently) on its side so that the spark plug is facing up.
Put on your work gloves. Then immobilize the lawnmower blade by holding it firmly with your hand.
Remove the blade from the crankshaft. For most lawnmowers, this can be done by removing one or more bolts that thread through the blade. If the bolts are difficult to remove, apply a few drops of penetrating oil to the bolts first. Let it sit for 15 minutes. Then use a long handle wrench to remove the bolts.
Pull the lawnmower blade off of the engine crankshaft with your hand.
Apply an ample amount of engine grease to the crankshaft.
Slide the new lawnmower blade onto the engine crankshaft. Then replace the bolts. Be sure to tighten them as tight as they will go. But first, check your lawnmower's owner's manual. It may specify exactly how much torque is needed to secure the bolt. If it does, use a torque wrench to gauge exactly how much torque you are applying.