The blade on a chain saw is actually made up of a guide bar and a cutting chain. The components of the "blades," or bar and chain, can get worn out, bent or broken, and should be replaced on a regular basis. Most guide bars also have a nose sprocket at the tip, which can also wear out, requiring bar replacement. Determining the correct size for your replacement bar and chain is crucial for the proper maintenance of your chain saw and you should never run a chainsaw with an incorrect bar and chain.
Release the chain brake by pulling back on the chain brake handle that runs across the top of the saw's body. Locate the clutch cover on the side of the saw body; the bar and chain attach to the saw body behind the cover.
Use the wrench to loosen the bolts on the clutch cover until they are finger tight. Loosen the tensioner screw until the bar backs up enough so the chain hangs slack below the bar. This tensioner screw will be located in the clutch cover or on the bar itself.
Remove the bolts the rest of the way by hand and take off the clutch cover, guide bar and chain.
Hold the bar up by the tip with one hand and with the other measure it with your tape measure. Typical bar lengths are 14 inches and 16 inches for general-use saws, 18 inches or more for professional-grade saws. As a rule, do not install a larger bar than came with your saw originally, but many saws can be equipped with shorter bars.
Determine your chain saw's correct chain by referring to your owner's manual. The packages of new chains often list the makes and models of compatible saws. Or consult a chain saw dealer who carries your chain saw's make, as they will be able to recommend the right chain.
Identify the correct length of the chain. This is the number of guide links, which run across the guide bar. All chainsaw bars are measured in two inch intervals, and the most common are 16 to 24 inch bars.
Identify the correct gauge of the chain. That is the thickness of the drive links, which pass over the guide bar's slot as well as the drive gear on the saw body.
Identify the correct pitch of the chain. That is the measurement between three consecutive rivets on the chain. The rivets are the links that hold each section of chain together.
Chain saw bars are measured in two-inch increments, ranging from about 10 inches (homeowner's models) to 72 inches (big-timber fellers). The most common are 16 inch to 24 inch blades. Chains are available for different purposes. For example, if you plan to use your saw to cut rough lumber, look for a "ripping" chain that is designed to cut with the grain of the timber.
It is dangerous to operate a chainsaw with the wrong bar and chain. It can damage the saw and can also lead to kickback, jamming, or other dangerous situations. Always wear proper eye, ear, and foot protection when operating a chain saw. If you are working with large trees, also consider wearing a hard-hat and safety chaps.