40A21 sprockets are an ANSI standard size that has 21 teeth and works with #40 roller chain. These sprockets are known as a “Type A”, which is a hub less roller chain sprocket. Our 40A21 plate sprockets are a durable, high strength sprocket that is manufactured from 1045CD carbon steel. After manufacturing, the teeth are hardened to help minimize wear, then a black oxide finish is applied for corrosion resistance. Bored for 2-1/2 diameter tube.