Small drive sprocket 420 - 16: a drive sprocket for Alpha and Delta mopeds. The sprocket transmits rotation to or from the chain. The numbers in the name indicate the chain pitch and the number of teeth.
Replace the part if the teeth are worn ("hooked"), the chain is stretched, or there is runout. Replace the sprocket together with the chain. Designation 420-16: chain pitch 420, 16 teeth. When replacing it, inspect the neighboring parts of the assembly: they often wear out together.
