https://www.mrosupply.com/chains-sprockets/261304_40bs12-1_martin-sprocket/
Martin 40BS12 1 Bored-to-Size Sprocket #40 Chain 1/2" Pitch 12 Teeth 1" Bore Steel - MROSupply.com
Martin 40BS12 1 Bored-to-Size Sprocket. #40 Chain (1/2" pitch), 12 teeth, 1" finished bore with keyway and setscrews. Steel. Outside diameter: 2.166". For...