The nut secures the clutch cover and the guide bar on the studs of the STIHL 180 chainsaw body. It holds the bar in the correct position.
Replace it if the thread is stripped or the flats are rounded: such a nut will not let you tighten the bar properly, and the chain will start to derail. Choose by the thread of the studs on your saw.
Good preventive care: regularly clean the saw of sawdust and resin, use fresh fuel mixture and chain oil suited to the season.
