I've cleaned and replaced sealed unit bottom brackets before, but have never come across this... On removing the bb unit from the chain side, I noticed the threading doesn't seem to engage tightly while being withdrawn - there is back and forward 'play' - that is, the bracket will move in and out of the socket after only a few turns of the removing tool while there's loads of thread on the bb unit still technically to be unscrewed. And only on the final few turns of the bb tool when refitting it does it seem to grip the bb socket. Now, I haven't had any problem with the bb or the cranks - no noticable excess play in them while riding - but I'm wondering if I should get the LBS to examine it. Could it be a case of the bb socket threads needing re-cutting? (Any idea of cost?). Or is it more likely to be the threads on the bb cartridge that are at fault? I suppose it's not likely that a previous owner has installed a bb cartridge of slightly the wrong diameter? The frame is 531 tubing by the way. Any ideas what I'm looking at here?