Do I need to change my cassette if I can't see any wear? I bought a new road bike with Campag Athena 11 speed in March last year. To the end of Jan it had covered 9000km.

In all that time I hadn't changed the chain because I don't have the Campag tool, and the KMC chain was not available. I finally got a new chain recently - unsurprisingly the old one was very stretched - so I bought a new cassette as well.

The thing is, the new chain on the old cassette runs very smoothly, and even dismantling them and comparing the cogs I can't see any change in shape or size! I don't know what Campag are making them out of, but it's fantastic stuff.

Everyone I've spoken to is convinced that after 9000km on one chain the cassette will be useless, but the prevailing advice is also to change the cassette when there are issues, and there just don't seem to be any.
That's an impressive amount of mileage on a single chain.  Especially on a narrow 11-speed one.  I'm guessing that you keep everything very well maintained and don't ride in much wet weather?

The two main problems you'll get from a worn cassette are:


Skipping chain (either between cogs or jumping on a single cog)
Premature chain wear (as the chain stretches to match the worn cassette's teeth)


If you're not seeing the first problem and visually the cassette looks fine, then you're probably ok to keep using it.  Best case, everything works fine and you've delayed the cost of using your new cassette.  Worst case, your new chain won't last as long and at that point you'll have to change both.

Since chains cost much less than cassettes, I'd suggest running with the old cassette if it's not skipping and shifts smoothly.