The clutch on a Stihl chainsaw works through a process called centrifugal force. When the clutch reaches a certain speed the outward force expands the clutch shoes, which engage the clutch drum and sprocket, spinning your chain. Clutch removal on a Stihl chainsaw will need to be done if you notice any grinding noises or if you notice you are wearing out chains more quickly than usual.
Clutch removal requires a few specialty tools: a rubber piston stop tool and a circular piece, called the clutch removal tool, that fits over the clutch and allows you to rotate the clutch hub without damaging the clutch. While some places may recommend blocking the piston with a piece of your starter rope, if it isn't inserted when the piston is on its upstroke you can push the rope into the exhaust port of the muffler. If the rope gets stuck in the exhaust port you can damage the cylinder, piston and exhaust port. It is far cheaper to order the specialty stop tool from Stihl. You will also need a wrench, socket wrench, screwdriver and multipurpose engine grease.
After you've blocked the piston, you will be able to turn the clutch without also rotating the crankshaft. On Stihl saws, the clutch drum and sprocket sit above the clutch assembly so you will need to remove these before you can remove the clutch. The clutch removal tool fits precisely around the outer ring of the clutch and will allow you to pull it off gently, as it keeps the clutch from spinning while you remove it. Stihl clutches use a reverse thread on the nut, so to remove the clutch you must rotate the clutch tool clockwise, opposite from most screws. Do not use any hammers or punches to try and pound the clutch free from the crankshaft, as you can damage the crankshaft and engine.
With the clutch assembly free from the crankshaft, you can separate the clutch springs and clutch shoes from their holding slots. Stihl manufactures a specialty metal hook for the springs, but if you're careful you can use a screwdriver to wedge the springs off their holding hooks. Make sure you wear safety glasses as these springs can fly off unexpectedly and cause injury. The clutch shoes can be pried off their retaining slots with the wrench.
Installation of the clutch should always be in reverse order as removal and disassembly. The clutch shoes need to fit into their holding slots. The clutch springs need to be bent into place. The clutch assembly will need to be anchored to the crankshaft using the clutch removal tool and screwing the bolt counterclockwise. The needle bearing will also need a coating of multipurpose engine grease to ensure too much friction doesn't overheat the clutch parts. The clutch drum, sprocket and washer will fit on top of the clutch assembly. Remember to remove the piston stop tool carefully before restarting the engine.