The bar tensioner assembly with the plastic part lets you adjust the chain tension of the STIHL 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.
When replacing, clean the seat of dirt, oil and sawdust and inspect the neighboring parts: a worn unit often drags the others with it.
