My Shimano Hollowtech II BB was feeling rough so I decided to replace it. I took off the LH crank then tapped the axle with a rubber mallet, I was rather shocked that the chainset and RH BB cup shot across the workshop as one piece. My first thought was the thread in the frame must be knackered but it’s fine, however on the BB the thread has completely worn away except where it was under the spacers and not in the frame. Any idea what could have caused this? BB had felt fine until the bearings got rough.

no idea but just be glad it was the BB threads not the frame. I had the more expensive version…

I can’t see how this would have happened while riding.
Did you fit the BB? Is it a steel frame?

It’s an On One aluminium frame so my first thought was the threads in it had gone! BB fitted by me with a Park BB tool… fitted lots so I think I know what I’m doing. New BB went in fine and I did lots of checking that it was securely in place.

fitted lots so I think I know what I’m doing.

I thought that when I mashed the threads installing mine

Done up too tight and popped under load while riding? Just a guess no aspersions intended.

I nearly put a 🙂 after ‘fitted lots so I think I know what I’m doing.’!
I didn’t find any remains of the thread in there so I don’t think it was a sudden failure.

More likely to have not been tight enough. HT2 type BBs need to be tighter than UN52 style BBs as they don’t self support like a UN type as they are 2 individual cups.

Over tightened and threads stripped ridding or not tight enough and its worked loose and just worn its threads off. Way to tell would be what kind of mess was left in the shell.

There was just muddy sludge in there! It does sound like it’s a tightening issue then, I was just surprise that the frame threads look fine and I hadn’t noticed any problems until I took the cranks off. Quite sensible making the BB out of a soft alloy so it wears out rather than the frame threads.

Just installed the worlds most expensive Deore HT2 BB. It was a tenner from CRC.

I’ve installed loads before as well…..

Tried to install it on my lads new Inbred – wouldn’t go in. Initial inspection suggested a lot of paint overspray on the threads in the BB shell. Scraped some off, gave it a bit more torque, still wouldn’t go in.

Took it to the tame LBS who are used to my ineptitude. I’d managed to cross thread it. And the BB shell. Which need the threads redoing. And a new BB at LBS prices.

Would have been nearly as cheap to buy him another **** frame!