My stock UN26 failed after 1600 miles, but it didn't fail by loosening up. I probably wouldn't have known mine was shot if I hadn't needed to change the BB length, and when I had the cranks off I noticed how poorly it turned, and that it had very sticky spots. I replaced it with an IRD BB and have been very happy, but have not passed 1000 miles with it yet. Most of my miles have been on an external bearing crank, with no bearing issues after 4k miles