I replaced the bearings on my xtr crankset last week, and I just realised that I re-fitted the bottom bracket cups but forgot to include that plastic sleeve that connects then within the frame’s bb shell.

What does this do?

I went for a pretty serious (and wet) ride earlier in the week, as well. I’m going to replace it in any case so I guess it’s more a case of curiosity as to what its purpose is.

To stop water getting in from the seat tube to the inner bearing seals

It’s just for show 😉

as mtbtomo said. It has fine ‘O’ rings either end that seal in the BB cups, preventing water getting in. Quite an important part of your BB 😉

Thanks all.