a cylindrical cassette body contains the movable balls and ball - biasing springs of a ball - type clutch . the balls ride in grooves in an element disposed within the body . exteriorly , the cog body has longitudinal keyways to receive internal teeth of a set of cogs all of which have the same internal diameter . the exterior of the body is also threaded and is selectively employed in combination with a free - wheeling cog adapter having two different outer diameters , such adapter receiving a set of cogs having two different inner diameters .