If your Gravely mower is no longer cutting properly, leaving strips of untouched grass as you go over your lawn, a major vibration appears from beneath the mower deck or the blade has been damaged by hitting a foreign object like a rock, brick or a tree stump, it's time to replace the blade. Mower blades can be found at virtually any hardware or lawn-and-garden shop, and many generic brands will fit. The easiest and surest way to get the right blade is to take the old one in to match the type that you have.
Turn the lawn mower over and onto its side, so that the blade will be easily accessible.
Locate the center attachment bolt that holds the mower blade in place. The blade will be bolted to the center of the engine driveshaft, directly underneath the actual mowing deck.
Wedge your piece of wood between the blade and the deck to make sure the blade does not rotate when you begin to unscrew the bolt. You may need to use a bit of muscle to do this, so wedge it firmly and tightly between deck and blade.
Press your wrench over the center bolt until it is secure in place over the bolt head. Always use the closed or box end of the wrench. The most common bolt sizes are 9/16 and 5/8, but sizes may vary from model to model.
Start turning this bolt in a counterclockwise rotation, and plan on using some elbow grease to get it started.
Loosen the bolt fully until it can be removed with your fingers. If the blade is stuck on the spindle, tap it lightly with your wrench to make it fall from the mounting. A mounting lock washer will also come off when the blade does, so be sure to grab it.
Replace the new blade in the same way it came off. Tighten the new blade firmly. A good way to tell when it is tightened is by watching the lock washer compress fully as you tighten the center nut.