skid777

Had a cog just break into two pieces and fall off from the middle of the cassette last night while spinning easily on a ride. It was the 14t in an 11-21 10 speed Campy cassette. I believe it is Record as I think that is the only one that comes in 11-21. At the very least it's chorus. Either way pretty steep to replace. Not sure how many miles are on it but I don't believe it was that old and it still shifts very smoothly. Do I need to replace the whole cassette or will just replacing the cog be OK? Just wondering if the fact that one broke is a fluke and the rest of the cogs could be fine or if this indicates the whole thing is probably shot.

Homebrew01

Probably a fluke. Buy a new cassette, and save the current one. When the new cassette wears out, take the 14t cog which is probably not that worn, and use it with the saved cassette.

Drew Eckhardt 

Just the cog should be fine, although you probably have an aftermarket hub like Zipp or G3 PowerTap which allows incorrect mounting and need to be sure the stepped splines on the cassette and hub match.

From Lenard Zinn's VeloNews column:

skid777

Thanks - someone else told me about that last night as well and I found that article by Zinn. This was on a Mavic freehub which is one of those noted in the article that can be mounted incorrectly. I had no idea. Been riding Campy on all my road bikes for over 10 years on various hubs and I always thought they would only go on one way. Going to look at those pics and check the rest of the cassette tonight. Although no way I'll ever know on the one that broke as it is in two pieces on the road somewhere....