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Just faced the BB shell on one of my bikes. It is a carbon frame with an aluminium threaded BB shell (so old it is coming back into fashion).
When putting it back together it seemed that the central BB sleeve served no purpose so I didn't install it. A good easy way to save a couple of grams or am I missing something?
The sleeve uses o-rings to provide the interference fit so I guess won't provide concentric centering, and with the BB shell being an un-perforated tube there doesn't seem to be anywhere for internal contamination to come from.
Its to help keep crap out of getting to the back of the bearings and keep crank axle clean as well.
you might find to be replacing you BB sooner than normal depending on type of conditions you ride in.
No for you in theory. Most folk have perforated bb shells so crap and water usually makes it's way down there.
I take the sleeve off (shimano BBs) and then drill a drainage hole in the BB shell for even moar grammyshavin'
Just checked the weight saving, 18.7g. Not massive but better than nothing.
Drilling a drainage hole in the bottom would only be pointful if I drilled all the way through into the seat tube to let in whatever liquid that seeps in there😉