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I currently have a Shimano Dura Ace 10 speed rear derailleur with a 12-25 cassette. For some hilly rides I'd like to change to a 12-30 cassette.

The maximum the rear derailleur can handle is 28, so I'm thinking of getting a Shimano 105 RD. I'd like to be able to switch between these two set-ups but fear that breaking the chain each time is not a good thing to do.

Is it possible to change the RD without breaking the chain by taking apart the pulley wheels?

Also is it possible to fit a 12-30 cassette on a Shimano Dura Ace 10 speed short cage RD?

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    Put on the 12-30 cassette and try (carefully). It might work if you DON'T CROSS the gears but most likely the chain is too short. Also, you can fit a derailleur by taking apart the cage. – Carel May 28 '15 at 7:55
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    Did you consider adding reusable missing link to your chain? This way it's possible to remove chain without compromising the integrity. – Klaster_1 May 28 '15 at 8:38
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    In a shop you can disassemble the derailer to free the chain, but you need to be in a place with a clean floor so you can find the parts you drop. – Daniel R Hicks May 28 '15 at 11:00
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    @Blam - Good point. I think in this case, the idea is you have a chain long enough for both cassettes, remove the wheel, open the rear derailleur to swap in one which can take a bigger cassette without removing the chain from the bike. But the whole exercise is somewhat inane given that you can just leave the bigger capacity derailleur on and deal with a few extra links when using the smaller cassette. – Batman May 28 '15 at 23:00
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    If you fit a chain and derailleur that will handle the 12-30 then you should only need to swap the cassette between rides because 12-25 is a subset of 12-30. – Chris H Jun 2 '15 at 7:52
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You're likely to destroy the derailleur by doing that too many times.

KMC and Wipperman both have 10 speed quick links which are a better thing to do. Note that you'll need a chain big enough for the larger cassette (or, with a quick link have 2 chains).

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    This not true, unless you are incompetent with your tools. There is no issue with disassembling the dérailleur to remove it. – zenbike May 28 '15 at 18:19
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    @zenbike - derailleurs have Loctite on the screws that hold the cage together. Unless your are refreshing this eventually the screws will be able to work themselves loose. – DWGKNZ May 28 '15 at 19:52
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    @DWGKNZ: Yes. You need to do the job correctly. That doesn't make it unsafe to do the job. – zenbike May 29 '15 at 21:04
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It is possible to remove your rear dérailleur without damaging it. Care should be taken (as with any job) not to damage the machine screws that hold the dérailleur cage together, or to lose the pulleys or screws.

The pulleys do come in multiple pieces, so real care should be taken.

As for fitting a 12-30 cassette on a short cage dura ace 10 speed dérailleur, Shimano says 28t is the maximum. You may be able to fit a 30t if you avoid cross-chaining, but it's better not go take that risk, as the failure consequences can be quite high.

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