Bicycle Mechanics - Swapping out cogs on Ultegra cassettes

I just got a new bike with a 10-speed Ultegra 6700 12x25 cassette. I want to change it to an 11x25, which has the exact same gears as the 12x except that it has an 11 and does not have a 16. In other words, 11-12-13-14-15-17-19-21-23-25 versus 12-13-14-15-16-17-19-21-23-25.

My question is, does anyone know if the 16-tooth cog on the 12x25 can be removed and an 11-tooth cog added to the end? I ask this because I know the 21-23-25 is one piece. They cannot be broken up. And on my old bike, the 17-19 was also one piece.

Sorry, I don't have the proper tool to remove the cassette lockring (I know I should get one) and can't verify for sure. It certainly does look like all the cogs are separate except for the 21-23-25.

Thanks!

cthenn-

Loose cogs up to the last 3. Order an 11T (and an 11T compatible lock ring...) and you're good to go.

Actually, now that I think about it...probably not. The 12-tooth on the end has that extra flange bit to keep the spacing right, so I can't just throw an 11 on the end and keep the 12. Also I'll bet the ramping pattern will be screwed up if I take out the 16. Am I right?

I'll just try to get my shop to do the swap. The bike's brand new.

You need a second position 12 tooth cog to do what you want to do in addition to the new 11 tooth cog and matching lockring. The first position 12 tooth cog you have currently will not work.

You need a second position 12 tooth cog to do what you want to do in addition to the new 11 tooth cog and matching lockring. The first position 12 tooth cog you have currently will not work.

And instead of a few bucks for an 11, now it's 3 pieces and probably some shifting issues. This deal is getting worse by the minute!

The 12T cog is both a top position and a second position on the 11-25 I have here. If your 12T cog has a small recess inboard of the serrations, it's designed to take an 11T on top.

The 12T cog is both a top position and a second position on the 11-25 I have here. If your 12T cog has a small recess inboard of the serrations, it's designed to take an 11T on top.


What about the lockring?

You'll still need an 11T compatible lockring.

The 12T cog is both a top position and a second position on the 11-25 I have here. If your 12T cog has a small recess inboard of the serrations, it's designed to take an 11T on top.

You're right. I looked at a pic of an 11T cog and forgot that the 12T cogs aren't capped on the end like the 11T's.

In case you were curious, you can remove/add individual cassette rings without much difficulty.
You will still need the locking ring, normally an 11 or 12 tooth.
Use your cassette removal tool to remove the cassette from the hub.
It can be tightly held, but show restraint.
The smallest rings are normally loose, separated by a plastic spacer.
The large rings are riveted together, either hacksaw the rivet or grind off the head. Save the spacers!
Pick the rings you want to use.
Simply slide them on largest to smallest, each separated by a spacer.
It will shift just fine.