Typically the outer edge of the gear thins out. I have seen some you could shave with. Good filtration will help keep the nasties out of the oil.I had an interesting deal happen one time when a customer neglected to replace the block bushing while using one and his motor burned one up in record time. The teeth on the timing chain were showing bronze being transferred to the teeth to the point it "looked" like the billet steel gear was wearing out according to him....