13 Tooth 11mm Bore Sprocket for 1/2"x1/8" and 1/2"x3/16" Bicycle Chain
Motor sprocket for MY1018Z and MY1016Z3 gear motors. 2-3/8" (57mm) OD, 11mm ID, 4mm wide x 2mm deep keyway. 9mm wide hub.
For 1/2"x1/8" (#410) and 1/2"x3/16" (#415) bicycle chain.
http://www.electricscooterparts.com/bicyclesprockets.html