With 51T rear sprocket for a smaller than stock drive ratio for ''more punch out of the hole'' or for track use to achieve higher than stock RPM. Big power and/or aggressive riding styles can break belts. A good chain can handle more torque; as Trask Performance knows from experience and the reason why this chain conversion kit is developed. The rear sprocket comes with a carrier which is compatible with the OEM 09-up style rubber vibration damper for an effortless install. Note: A chain is not included and must be ordered separately. High quality 530 O-ring chain is recommended. Use a 130 link chain with the 51T sprocket; a 140 link chain is recommended with the 54T rear sprocket. Note: See 701155 for the hassle-free Trask axle adjuster set for 20-up models with rear chain drive conversion. Note: Rear sprockets with a different tooth/ratio are interchangeable with the rear sprocket carrier. Cush Drive part numbers overview: - 51 tooth sprocket conversion kit (MCS 963442) gives standard drive ratio - 54 tooth sprocket conversion kit (MCS 963440) gives smaller ratio (more RPM) for more ''punch out of the hole'' and track use - 51 or 54 tooth replacement Cush Drive sprocket MCS 963443/963441 - 25 tooth replacement transmission sprocket MCS 964971