Tested extensively by the Dutch National BMX Team, Damien Godet, Kory Cook, and numerous other elite racers, Box Two™ Freewheels combine durability with lightning-fast engagement. They feature nine, super durable pawls and are available in one of three choices -- 16, 17 or 18 teeth. To install, use the Park Tools™ FR-6 or other installation tool designed for single speed freewheels with four notches that are approximately 32mm across measured inside to inside and 40mm outside to outside.