You want to mess with the Cogs in the back and get it right the first time.

The Campy cassettes have A, B, C cassette cogs that are used in a sequence to maximize shifting. If a 2 tooth jump in teeth count is made as opposed to a single tooth, the cogs will be cut differently. The shift teeth and shift ramps on the cog are arranged in a helix type pattern. Placing an out of sequence cog in a cassette will cause a loss of shifting performance out of and into the out of sequence cog.

Check out the guide to all cog combos.

8 speed Table for all eight speed cogs including titanium 8 speeds

Cassette Cogs
11-21 11A-1P, 12A, 13A, 14A, 15A, 16A, 17A, 19A, 21A
11-23 11B-1P, 12B, 13B, 14B, 15B, 17B, 19A, 21A, 23A
12-21 12A-1P, 13A, 14A, 15A, 16A, 17A, 18C, 19C, 21C
12-21 12A-1P, 13A, 14A, 15A, 16A, 17A, 19C, 21C
12-23 12B-1P, 13B, 14B, 15B, 17B, 19A, 21A, 23A
12-25 12C-1P, 13C, 15C, 17B, 19A, 21A, 23A, 25A
13-21 13A-1P, 14A, 15A, 16A, 17A, 18C, 19C, 21C
13-23 13A-1P, 14A, 15A, 16A, 17A, 19A, 21A, 23A
13-26 13B-1P, 14B, 15B, 17B, 19A, 21A, 23A, 26A
13-28 13B-1P, 14B, 16B, 18B, 20B, 22B, 25B, 28B
14-26 14A-1P, 15A, 16A, 17A, 19A, 21A, 23A, 26A
16-23 16A-1P, 17A, 18C, 19C, 20C, 21C, 22C, 23C

9 Speed Table

Cassette Cogs
11-21 11A-1P, 12A, 13A, 14A, 15A, 16A, 17A, 19A, 21A
11-21 11B-1P, 12B, 13B, 14B, 15B, 17B, 19A, 21A, 23A
12-21 12A-1P, 13A, 14A, 15A, 16A, 17A, 18C, 19C, 21C
12-23 12A-1P, 13A, 14A, 15A, 16A, 17A, 19A. 21A, 23A
13-23 13A-1P, 14A, 15A, 16A, 17A, 18C, 19C, 21C, 23C
13-26 13A-1P, 14A, 15A, 16A, 17A, 19A, 21A, 23A, 26A

There are two sizes of lock rings for the cassettes, The bigger lock ring is used on cassettes that start with 12, 13,14, and 16 teeth. The smaller lock ring is used on cassettes that begin with 11 teeth. Keep this in mind if you are trying to add and 11 to your cassette. You will need the smaller lock ring also.

The cog spacers are 3.1mm for all eight speed systems. If you want to use the 8 speed Ti cassettes on a 9 speed hub, a 1.2mm spacer is used to make up the extra length in the 9 speed freehub. The cassette spacers for the 9 speed are thinner at 2.8mm.

It is important to replace the correct cog in the sequence with the correct lettered cog. The letters are specific to shift ramp and tooth profiles to allow easier shifting. It is possible to use a cog out of sequence, but the shifting performance will suffer.