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2,643 Posts
can't see what you have now

You may have to add a link if you're running with the chain pretty tight.

I don't know how much chain drop your cage can handle, but my guess is the cage will be fine. At worst you might have to stay off the small ring and the 2nd smallest cog, a gear ratio that will be duplicated somewhere on the big ring.

It's also possible that the upper pulley will hit the large cog if the derailleur itself won't accomdate a 27.
 

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No Crybabies
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11,692 Posts
ok

Bertrand said:
I'm changing from an 11-23 to 12-27 (Ultegra 10sp).

1. Will I need to change chain length?
2. Will I need to change the rear derailleur cage?
It will probably work with no changes whatsoever. If your chain was as long as possible in the prior configuration, such that in the smallxsmall combo it was ok, then it was as long as you can get away with, anyway.

The derailleur will be fine. Might need minor tweaks.

If you have used the old chain and cassette for a long time, it might well be worn and have some chain stretch. It would be a shame to use the old chain and then immediately start destroying your new cassette. I'd replace the chain, anyway, unless you have low miles on the prior chain/cassette.
 

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Spicy Dumpling
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9,723 Posts
Will probably be ok depending on your current length. I swap between those two cassette combos depending on the ride I'm doing. I dont' need to change the chain length.
 

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What is your current set up? Have you already purchased the 12-27?
Unsolicited, for me I found the gear jumps between on a 12-27 to be uncomforable. I rarely was able to find "the right gear" when riding in groups. When you are on your own, you can set your own pace and this is much less of an issue. A more expensive option, but I believe a better one is switch to a compact. That will give you nice tight gearing, and the extra hill climbing ablity.
FWIW.

As to the chain, try out your current one and see if it works. If it's too tight, get a new one.
 
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