Shimano Dura-Ace CS-9000 Ultegra 6800 11-Speed 12t 1st position Cassette Cog

It works for my application;

Used (along with 13t 2nd position) to replace the silly OE 11/12t combo on both Ultegra 6800 and SRAM 22.
Dura Ace and 6800 are identical part btw.;

This is a very particular product for a very specific purpose. The way modern Shimano cassettes are built the largest three cogs are always attached together the middle 5-6 cogs are individual gears and the smallest two cogs are built in a special way. The smallest cog is made so the cassette lockring threads into it and the second to the smallest cog is made to mediate between the odd small cog and the normal third cog.

This cog is made to fill that weird first cog position. But you'll also notice that it has 12 teeth while most modern cassettes have an 11 tooth small cog which dictates a 13 tooth second cog. Your normal cogs will start at 14t on the third smallest position.

I bought this cog along with  this  cog and another regular 15t cog in order to turn my 11-32t cassette into a more usable 12-32t cassette. I don't find my 11t cog very useful especially when I'm not racing and having more closely spaced gears in the middle of the cassette where I actually use them is much more useful. In this case I now have single tooth jumps from 12-16t and my biggest jump is from 16-18t instead of from 14-16t. Everything runs and shifts as smoothly as it ever has.

I also later used the new 12t and 13t cogs (and another 16t cog) to modify my 11-28t cassette into a 12-28 and that also worked very well.

This part is somewhat overpriced at $7 especially when you can get some complete 105 11spd cassettes for as low as $30. But if you just want to do a conversion like I did it's what you need.;