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Replacing rear cog
Hi all,
I have a shimano ultegra 6600 12-25 cassette (and deralieur).
I would like to add a lower gear - a 28 if possible.
I know shimano make a 6700 11-28 ultegra 6700 but AFAIK this isnt supposed to work with the 6600 setup.
Has anyone tested this?

Also, is it a viable option to just buy a new cog or 2 (say a 24 and a 28 and replace the 23 and 25)? Any 'gotchas' with this idea (e.g. deralier wont work with 28T, the jump is too big, or are the cogs locked together etc)

Many thanks,

You cannot change individual cogs, You need to replace the entire cassette. I think your set up can handle extra three teeth without problems. Just may need to tune the rear deralieur. Cassettes are not expensive.

Your photo shows a MB but 25T is short for MB so you must be doing a RB. Some info on bike is always helpful. On a MB you can put a smaller granny gear.
Never Give Up!!!
hi, thanks for quick reply,
sorry - thats my old bike in the photo-
my new one is a Cervelo S1 - Road bike.
Yes, 10-4 to that. Its not a major swap check out how to in repair guide and the link I send. The chain may get short but I doubt it.
Never Give Up!!!
Check your chain length with the new cassette installed. may need a link or two? Just set up the derailleur as if it was new. Check the limit screws to make sure there wasn't any spacing differences after the cassette swap. Should be all good to go.

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