Loosen the three screws securing the cylinder cover to the Craftsman chainsaw with a Phillips-head screwdriver. Lift the cover away from the chainsaw.
Pull the spark plug wire boot off the spark plug. It might be necessary to twist the boot left to right to break the seal.
Remove the spark plug from the cylinder with a socket wrench and ¾ inch spark plug socket. Gap a new Champion CJ-7Y spark plug to .025 with a gap disc. The gap disc is available at auto parts stores. It resembles a large coin with either a graduated edge or graduating wire sizes. The hook on the gap disc bends the ground electrode of the spark plug up or down as necessary to obtain the correct gap.
Thread the new spark plug into the cylinder until it is hand tight. Turn the spark plug clockwise with the socket wrench and spark plug socket until the plug is tight. Do not overtighten the spark plug.
Push the spark plug wire boot over the spark plug. The spring inside the boot will snap over the top of the plug. Position the cylinder cover back over the chainsaw, and secure with the retaining screws.