I purchased a used ultegra 6600 10 speed cassette. In the package I got this extra ring that I have no idea where it goes.



In this video he says that the ring goes on first but this ring doesn't fit on my hub. The gears fit fine. I put the cassette in and it seems fine without the ring. Any ideas?

https://www.youtube.com/watch?v=Xoq_xNYcYlA

I know one of my old Mavic wheelset came with a spacer from the factory. Is it splined or just a spacer?

It's not splined. Just a spacer.

You use the spacer when installing a 10-speed cassette on a Shimano 9/10 speed hub. Cassette How-To - Part 2 - Slowtwitch.com

If the ring doesn't fit, you might have a tall spline 10-speed only hub which doesn't use the spacer. What model is your hub?

Shimano is now shipping the 10 speed cassettes with the spacer needed to mount them to new, wider 11 speed freehubs. If you don't have an 11 speed freehub you don't need it.

The hub is a WH-6600 so yeah 10 speed only.

'Fixed.

Those spacers come with the 11 speed hubs. Compatibility always works backwards.

Well, that certainly seems logical. I could have sworn the spacer came with the cassette but there was a wheelset with an Ultegra hub that showed up at the same time.... The Shimano site only shows the 1mm spacer with the cassette, as Cale mentioned. Clearly I am JUST WRONG!! Apologies to all!