The bar tensioner plate lets you adjust the chain tension of the Good Luck chainsaw: it moves the bar along the guides as the screw is turned.
Replace it if the thread is stripped, the gear teeth are broken, the plastic is cracked, or the chain does not hold its tension. A loose chain wears the bar and is dangerous, so it is better to replace a defective mechanism right away.
Good preventive care: regularly clean the saw of sawdust and resin, use fresh fuel mixture and chain oil suited to the season.
