Do I need a new Chain?

I went out for a ride tonight and my chain, which occasionally (just when I don't want it to!) slips in 4/5 cassette sprockets, now is slipping in all of them.

It and the cassette are 9 speed Shimano.
I replaced it under 5 000kms (the usual distance I replace chains - if I remember) ago.

Could it need replacing?

I've probably had 3 chains on the cassette so that'll be getting worn but the bike shop guy (and me) said it looked ok.
I could understand if the cassette was worn as surely the chain would only slip on certain rings.

Measure the "stretch" of the chain.
A cassette typically still looks OK when it isn't. Many cassettes don't last 3 chains worth of wear.

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Artemidorus is correct about the stretch.

BTW, my personal experience is that it's extremely difficult to make a sprocket become old... I'm currently running a chorus (10) of 2005, it has 50000 km, it's still undistinguishable from a brand new one......and I'm not a cyclotourist (I do about 60 races / year). I change the chain whenever the "caliber" measurement fail... that is, with Record chains (I always use Record chains) nearly every 6000 km; with those Chorus sprockets I would have ridden on 7-8 chains... and they're still new, so don't think about how many chains but look at the teeths

The ability of a sprocket to last depends on the wear on the chains that are used with it. If you let a Shimano chain stretch beyond 1%, the cassette is reliably toast. Alternatively, if you replace at 0.75% stretch, they last a lot longer. I'm in the process of finding out how much longer this is.

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By all means measure the chain stretch as others have said. Also, inspect the rear derailler for freedom of movement, it may be a good idea to put a drop of light oil on the pivot points anyway. As a troubleshooting techneque try the following:

Remove the cassette cogs and put them back on with the frame side now facing towards the spokes. This changes the bearing surface of the chain on the cassette tooth. If your problem goes away or is much improved the cassette is worn and needs to be replaced. THIS IS A TROBLESHOOTING TECHENQUE NOT A FIX.

Campy riders would ruin a good cassette doing this.

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I would check the rear derailier and make sure the top pulley still "floats" side to side a little. I'd also of course measure the chain. Boy I love my campy stuff

Thanks guys

I might just try that turn the sprockets around trick (non would call it a fix!) & lubricate the derailleur - although its likely to be fine now.