How'd I do this?!

[b_S]

Bike was clunking on the way to work. I assumed it was the old cassette, had a quick look when I arrived and...

That's the spacer you can see on the floor, it'd cracked and allowed the cup to screw in a fair way.


My only thought is overtightening may have cracked the spacer but that was done months ago, for the cup to wind itself in like this was surprising.

Unfortunately I didn't have any spare spacers on hand but a cassette spacer - with the lumps filled off - worked fine
Now I have a happy bike again

Yes it's a slightly pointless thread, but I am still taken back by what'd happened.

 

[Tomas]

Did you try and disassemble the bottom bracket? I heard when you attempt that crazy things happen - like an LSD trip apparantly.
(equally pointless reply)

 

[maarrggh]

Mate that's unreal! I've been working as a fitter for 18years and the only time I've seen something like that happen is when things have been tightened way too tight. If you do find out how it happened I'd love to know.

 

[floody]

Thats amazing. I am an, uh, substantial gentleman, with significant tendencies toward violently overtightening things, and I can say that even with enough force to twist a sidchrome breaker bar head I have not managed to split a BB spacer.

 

[Tazed]

Most likely a faulty spacer. They're the cheapest plastic they can get away with. Overtightening is pretty hard with the standard tools, but not impossible. Tightening probably just caused a micro fracture that eventually led to a complete failure.
And it's no surprise the cup screwed itself in - that's designed to happen. It's no coincidence the BB threads are cut so that each side screws its cup inwards with pedalling - hence the 'reverse' thread on the non-drive side, and the instructions laser etched into the BB cup. Stops the BB/cranks coming out completely in case of component failure, as you've just witnessed. Perfect example of a safety design doing its job.