Plainsman

Are 10 speed Ultegra and Dura-Ace cogs compatible? Can a 12-23 be turned into a 12-27 easily? I know the diff between a 12-25 and a 12-27 is just the rear cluster (21-23-25 vs. 21-24-27) I think the jump from 12-23 to either also affects the last cog before the rear cluster. In any case, are the two groups compatible, and is it possible to pick up the rear cluster without getting a whole new cassette?

Garthr

I was following you until the last sentence. . . if I understand you correctly, you're asking if you can just buy the clustered part of a cassette? Not in Shimano's wildest dreams! Or yours. Shimano wants you to pony up for a whole new cassette. I don't even think you can dismantle DA cassettes any more for customizing. https://sheldonbrown.com/harris/k7.html#10

You can however customize 'till your minds content with Miche cassettes.

Plainsman

No, you are right, that would be TOO convenient, haha! Basically, I have an Ultegra 12-25 that is in decent shape, but with some rust on everything up until the 19 and teh rear cluster. What I'm wondering is if I can take this 12-23 DA I have coming in, remove the rear cluster and last floating cog (18 I think), and replace it with the 19 and 21-23-25 from the Ultegra. That would give me almost new top end gears, and then the less used low end cogs from the Ultegra that are still in okay shape. Would that work?

DMF 

Hey, rust is easy. You don't need to worry about replacing until your sprockets have too much wear. First take the rust off the contact surfaces (rust remover/brass wire brush .. or whatever), then try riding it. You might not need a new one.

IK_biker

This will definitely work. Same cog thickness, same spacer thickness, etc.

Plainsman

Thanks!