Bonamici Chain Adjuster for Aprilia RS 660 in Black anodized T-6061 aluminum alloy. If your chain is tight it will be rough and cause excess wear on sprockets, bearings, and even the counter-shaft seal and it can also cause premature stretching and weakening of the chain. On the contrary, if your chain is loose it could hit the frame, cause a jerky throttle response, or worse even fall off! Bonamici Chain Adjusters are easy to install and adjust by turning the rotating knob by hand and clicking into place, the advantage to the Bonamici Chain Adjuster is that it allows the rider to make quick and accurate adjustment of the rear wheel on both sides so you're chain is straight, rear wheel even and at the proper tension. Sold in Sets (Left + Right). *During the first use, please loosen underside tension screw with allen key to free rotating knobs from their factory locked position for adjustment by hand (do not use a wrench or pliers to adjust which could cause damage).