Originally Posted by
TLCFORBIKES
On a Shimano 105 9 speed the gears are heald together by 3 removable pins. This makes for a very easy cassette to make custom. On your Ultegra the last 5 cogs are joined together by 2 seperate carriers 21 - 24 - 27 together and the 17 - 19 together. Go to a 105 cassette and ask your LBS to keep a eye out for a retired cassette that will have the extra cogs that you want. Or go to Craigs List/ Ebay/want ads and get the desired cogs.
This is correct. The 105 9-speed cassettes have all full-plate cogs held with removable bolts and can be used individually. Ultegra 9-speed cassette have the #5 and #6 cogs on a spider and the three largest cogs on a single spider and they can't be used individually.