I have a question, My Bianchi Giro rides a 12/26 Cassette. I have a spare wheel and have been looking for another 12/26 cassette so all i would have to do is throw the spare on should I need it. Will the shimano 12/25 work?
I guess what I am asking is the 12/25 compatible with the 12/26 (chain wise)?
Thanks ahead of time for any info given...............

As long as your current drivetrain in shimano, and the cassettes have the same # of cogs, there should be no problem....unless your chain is really tight right now in the big ring/big cog combo...which you shouldn't be ridding in anyway. Enjoy.

if they are both the same manufacturer of cassette (both shimano for example) AND they have the same amount of gears (7 or 8 or 9's) then yes, they are compatible. a 9 speed chain is thinner than a 7 or 8 speed chain.
a 12/25 and 12/26 are very close anyway.
it sounds as if they are the same number of gears (8?) and therefore compatible.

so the chain that drives my 9 speed 12/26 cassette "WOULD" work with a 9 speed 12/25....
Guess thats what I will have to buy since I cant find the 26 on ebay .
Thanks

You sould be able to find a SRAM R9 12-26 with ease...

TaDaaaah, see someone said sram r9....
That made all the differance in the world, I had been typing in sram 12/26 in ebays search and came up with nothing, I type in sram r9 and bingo there it is, a 12/26....
Thanks K6

The Sram is compatible with a Shimano drivetrain, but it sounds like you may have a Campy drivetrain. That would not work.

Generally though it's recommended that you change your chain whenever you change your cassette too.

Shifting with a SRAM chain and SRAM cassette is fantastic!

12/25- Chirstmas Day
12/26- St. Stephan's Day