lcrider

I just installed a brand new Ultegra 12-23 ten speed cassette, replacing my 11-28 that came with the bike. Thinking that I needed to learn how to do this, I decided to get the tools and do it myself.

After installing the new cassette and remounting the wheel, the smallest gear runs really "rough". I took the wheel back off and noticed that there is some play in the smallest few cogs. Is that supposed to be there? Neither the old nor the new cassette came with a spacer ring outside the smallest cog, but it seems like the cogs should be tighter...

Al1943

All of the cogs need to be tight. Loose cogs can damage the freehub as well as causing adjustment problems and noise.
In general Shimano 10-speed cassettes need a 1 mm spacer at the back of the cassette. This spacer comes with all new Shimano cassette.
If you have a Mavic hub you will probably also need a 2 mm Mavic spacer in addition to the Shimano spacer.

nwbikeman

Is the spacer still stuck to the old cassette. Make sure the high gear cog is on square to the free hub, it sometimes moves while tightening.

reptilezs

1mm spacer in the instruction packet of the new cassette. check the trash

lcrider

Thanks everyone. I looked closer at the old cassette and noticed the inside spacer (inside the large cogs) was thicker than the one that came with the new cassette (I have Mavic Cosmic Carbone SL wheels). I used the old spacer in its place and things tightened right up and it works great. FYI, never found an outside spacer (outside the small cogs).

Al1943

See post #2, you may need both spacers. The cogs need to be good and tight.

DannoXYZ 

Make sure you get the lockring up to the proper torque...

Jed19

+1. I am assuming your wheelset has the Mavic FTS-L hubs, in which case you'll need the Mavic spacer, Shimano spacer, then cassette.