SeattleJG

Hello everyone, yes, I know this is a very old thread. I have a 1991 Trek 2300 Pro (the one with carbon straight tubes and aluminum lugs/fork/triangle). My bike has a measured 126.1mm distance between the rear dropouts with no rear wheel mounted. I had the original Shimano 600 7-speed downtube shifter setup. The bike was great, but I've been hoping for an update to brifters.

That led to a rabbit hole of planning, reading, talking to mechanics and buying parts. I found a great deal on a good condition full Shimano Dura Ace 7800 series 10-speed groupset. From this thread and many others and Sheldon Brown's archived website, I assumed I was going to start with a "9 out of 10" setup since I would need to measure my original hub body length and start planning for new washers and maybe a narrower cone nut on the drive side etc. to fit my 126mm OLD. I bought a brand new 10-speed cassette (Ultegra 6700 11-25) and confirmed that I could just take off the 11 tooth cog.

Believe it or not, after examining the Shimano literature, the part number for the 12 tooth cog that is the second smallest cog on the cassette I bought is the exact same as the smallest cog of a 12-X Ultegra 6700 cassette, knurled to mate with the lock ring and everything! The only difference is the lock ring is smaller for an 11 tooth cog, so I just bought a 12 tooth lock ring and thought I was ready to go for a 12-25 9 sprocket cassette with 10-speed spacing. When I started the transition to my new groupset I took off the rear wheel and took off the original 7 speed cassette. I also took off the clear plastic protector ring between the largest cog and my rear spokes.

So I decided to try the new cassette without the metal spacer washer that comes with it that one is supposed to use for a 8/9/10s hub body. It fit great but was *too narrow* with my intended "9 out of 10" setup (i.e. there was space between the smallest cog and the lock ring!). So I just slipped the 11 tooth cog on, switched lock rings back to the 11 tooth lock ring, and voila! Everything fit! I'm not sure how or why, but I didn't even touch my axle/spacers/locknuts/wheel dishing etc. Examining it more, it appears that the largest cog is suspended toward the middle of the hub (the largest 3 cogs are all on a central tree, I forget what it's called, on this cassette), and this is probably is what allows the extra gear space.

I've set up the bike completely and was stunned that I didn't have any derailleur or chain interference problems with the rear triangle or rear spokes. I've done multiple 10-20 mile rides to get things dialed in and I'm super happy. Anyhoo, I just wanted to share what I was expecting vs. what my reality is currently, i.e. with the Dura Ace 7800 groupset (shouldn't really make any difference vs. any other 10-speed group?) and the CS-6700 Ultegra 11-25 10-speed cassette, I have a working 10 speed setup on my original 7 speed Shimano 600 hub/hub body WITHOUT MAKING ANY CHANGES TO THE 126mm OLD or WHEEL DISH!