Normal? Depends on how much you ride in a year. A good quality O-ring chain should last 15,000+ miles if well cared for. A set of steel sprockets should last two chains if the chain is changed before it is so stretched out it begins to really eat the sprockets. An aluminum sprocket (rear, that is) will likely only last the same as one chain. It is best to change both sprockets and chain at the same time for the optimal wear/life of the chain, but it is not absolutely necessary. You can run the stock 630 or 530 or 520 and all will be fine. If long life is important, the stock size is best, then the 530, then the 520, but it shouldn't be a major difference between them. If performance is most important, use the 520.
Mark

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