This is a set of sprockets that can be used to hook up a rudder to a servo. Inexpensive ball chain (#6 chain or 1/8" balls) from a lamp pull is used to connect the two pulleys. One sprocket has a 1/8" hole and has provisions for a set screw. The other sprocket is recessed to allow it to fit over a servo horn. Two holes are provided to screw the sprocket to the servo horn.I recommend at least 40% infill.