Originally Posted by kahn

I did exactly that with my new Madone. I ordered from harris cyclery. Sheldon Brown can be found hanging out on the Mechanics forum. I ordered a TA 26 tooth and the local shop replaced it for me.

I did have a bit of a problem with the chain dropping off and 'chain guard' type fangs would not fit on my larger diameter tubed Trek.

And I'm still thinking of replacing the cassette with one that goes even lower than the 27 my ultegra now sports. My knees and age like easier pedaling even if I have to balance more to keep the bike upright at very slow speeds.