Bottom bracket failure

[Jay Gee]

Dismantling my old mountain bike today, well used in the past but neglected over the last year. Cranks removed, I was surprised to find the drive side bearing cup (Hollowtech II type) was very loose and more or less fell out of the frame. The threads, both of the bearing cup and in the frame, were flattened off, enough to give about a millimetre of free play. (The wear was across the whole width of the thread, so it wasn't as if the cup had unscrewed and the last few threads stripped off)

I was going to give away or scrap the frame anyway, so the damage doesn't matter to me - but thinking of my other bikes I'd like to know how the bearing cup came loose without me knowing it and what might be done to prevent a similar failure.

The bottom bracket was a pricey Hope one, the bearings themselves still seem to be running smoothly despite my neglect.

[Brucey]

HTII bottom brackets (fairly obviously) have the bearings beyond the BB shell. This means that cups don't precess (i.e. self tighten) as well as they do with an old-style BB, should one cup come loose. The result is all too often knackered cups and/or a knackered BB. Since this is always accompanied by some movement (rather than none), it is indeed a bit surprising that you didn't notice that there was something wrong.

cheers