Bottom bracket condition
I'm using Shimano's XT Hollowtech II cranks from 2004/5 on my mountain bike which does part time work as commuter, XC rig, as well as my tourer.
I changed out my worn-out the original XT external BB with an Aerozine bottom bracket (Hollowtech II style) using SKF bearings... and it was sweet for a couple of years. OK, before I get remarks that I'm cheap... I'd like to share some findings even for a fairly tech-head like myself. My aluminum bike frame came with the BB threads already tapped, and I am not aware of tapping a BB shell - until a few years later. My original XT BB died on me - the non-driveside unit was totally dry, the remnants of the greased was dry and caked up.. hence the change to the Aerozine/SKF unit - I believe I changed it in 2006/7.
However early this year after my two short tours, I started to note that the bearings were getting out of condition... to the extent that I thought then, that I cracked my BB shell. Scary thought, since I use my bike for everything, but I didn't want to part with it early. Two months later of observation - increased drag, that grinding vibration from the downtube when I pedalled away.... was enough. Somehow I remembered that Rotor made these SABB (self aligning bottom bracket) and I just got it fitted into my bike. Haven't done anything adventurous with the bike yet - but it was nearly hell-to-heaven in a 15 minute operation at the bikeshop, fitting the SABB on.
I'm sharing this experience I just had... with two external-type bottom bracket units that died with identical signs - from what I guessed was improper factory-facing/tapping that killed the non-driveside bearings much earlier. I also had this "rust ring" where the plastic spacer from the bearings kind of ate into the key shaft that joins both crankarms together. The Q-factor on the bike might have been adjusted slightly with the change, but so far I haven't noticed anything major... yet.