The definite signs of wear on the aluminum sprocket: the teeth are pointed and the “shoulder” of the sprocket is non-existent. These sharks’ teeth indicate this sprocket is worn out and has the potential to cause a failure. Another visual sign is “hooking” where the teeth start to have a hook on one side.
Most manufacturers recommend you replace the sprockets every time you replace the chain. Chains and sprockets wear at different rates. It is possible to wear out a chain and not wear the sprockets sufficiently to replace. How do you tell? Look at the sprockets “teeth”. In this illustration you notice the alloy sprocket on the left has sharp pointy teeth (left) compared to the new sprocket on the right. Worn sprocket teeth can fail, causing the chain to jump with nothing good following that sequence. The drive sprocket can wear in a similar manner and requires inspection periodically.
As mentioned previously, steel sprockets will (generally) give the longest service life when compared to alloy sprockets. When inspecting sprockets for wear pay particular attention to the shape of the teeth to look for wear:
Examine the picture comparing a new steel sprocket with an aluminum (alloy) sprocket with about 10,000 miles.
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Ducati - D.I.Y - simplified
The next picture shows a steel sprocket with about 30,000 miles to a new steel sprocket. The wear isn't as dramatic as the previous picture, but close inspection reveals the sprocket is worn and there is visible cross-hatched wear across the teeth. Circled is worn area on the tooth of this sprocket. The sprocket should be replaced.