This in my second Stihl and they have served me well until recently. The clutch flywheel/sprocket that drives the chain is ceased on the crank shaft. There is a caged needle bearing that it turns on and the crank shaft that more than likely why it's ceased. The You Tube gods must have read my mind as a video covering this assembly showed up a week latter. The presenters assembly wasn't ceased but the flywheel/sprocket had worn oval necessitating replacement. He said in the video it was a much over looked part to grease and there is a chance the crankshaft is damaged as well. An expensive repair if the crank is damaged.