On a Shimano 105 9 speed the gears are held 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.
Last edited by TLCFORBIKES; 12-30-10 at 02:40 PM.