This is an anodized aluminum tensioner made by Righetti Ridolfi. It mounts to the bearing cassette and the plastic bolt presses against the mount to keep you motor in place. It is adjustable so that you are able to change the slack on your chain when changing sprockets, etc. If your kart did not come with one than it is highly recommended as it can stop your motor from sliding on the frame rails during a race which can cause a thrown chain.
Mounting Holes Center to Center: Approx. 93mm apart (3 5/8")
Bolt Length including red stop: 85mm (3 3/8")