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I'm fixing up an old tandem, and the cup-and-cone bottom bracket at the front sounds very gritty, so I picked up my c-spanner and applied it to the lockring. With a bit of effort, left cup and lockring both rotated together. Not hugely surprising that they're stuck together - both bits are quite old and grimy. But then I noticed that the right cup was rotating as well - as if it were rigidly attached to the left. The entire set (two cups and lockring) won't easily rotate more that about 20 degrees,and I didn't want to push any harder. How can this happen - aren't cups meant to be separate? Will I damage it if I keep rotating it? How should I unscrew it?

Some more details: The tandem is a 1940s Claud Butler - spec here: http://www.nkilgariff.com/CBcats/Cat_40/CB15.jpg. The bracket is inside an eccentric casing. I can't shift this at all, so I don't know how it's all connected inside. BB make is Bayliss-Wiley.

Photos are here: http://www.flickr.com/photos/88429902@N04/

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Possibly there's some sort of shell/spacer inside that the two cups are bumping up against -- probably not supposed to, but the bearings wore down that far. –  Daniel R Hicks Oct 12 '12 at 1:50
(I'd flip it over and drizzle penetrating oil into the slot and around the cups and let it sit a couple of days.) –  Daniel R Hicks Oct 12 '12 at 1:56
You could also try penetrating oil in the holes on top of the bottom bracket that I see from another question are designed as greasing points. But if you then still don't get it apart, that might thin any grease left in it into uselessness. –  armb Oct 20 '12 at 15:01

3 Answers 3

This a request for clarification rather than an answer (sorry, I don't have enough rep to comment yet).

First thing to assess is the threading of the right cup.

I assume the BB is English threaded, because it's a CB product; if this is correct then when you unscrew the right threaded left cup, the left threaded right cup would rotate in the same direction, hence would unscrew itself. Can you see a gap developing between the frame and the right cup when you unscrew the left cup?

If my assumption were correct, the fact that they do not rotate more than 20 degrees might indicate that there's some sort of connection (intended or not) between the two. By unscrewing both sides simultaneously you are stretching the connection, hence the block at only 20 degrees.

Finally, sorry if the following comments sound offensive. Depending on your experience in bicycle repairing:

  1. are you sure you are not rotating the whole eccentric when you rotate the cups? I can see you have an eccentric in your pictures.
  2. you might be surprised on how much strength one needs to unscrew old BBs, so when you say "won't easily rotate", what do you mean exactly? Are you applying strength with the arm only or are you using your body weight?


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Unless the bracket is so corroded it's useless, both cups being stuck and the whole eccentric moving (as astabada suggests) is the only way I can see that the cups can be stiffly linked. I'd try doing the bolts that grip that back up first (assuming they are the ones on a saucer in your photo).

Have you tried undoing the fixed cup, or at least holding it it to stop it turning? You will probably need a better spanner/wrench than he one in your photo, something like http://www.dotbike.com/p/6301 or http://www.awcycles.co.uk/park-tool-36mm-box-end-fixed-cup-wrench-bottom-bracket-pin-spanner.html

Alternatively, sometimes you can grip it in a bench vise - might be tricky with a tandem, but if you can get the eccentric out of the frame it might help get the bracket out of the eccentric.

If you can get one cup out, see http://sheldonbrown.com/tooltips/bbcups.html for a tool to get the other one unstuck - while Sheldon talks about using it on a stuck fixed cup once you've got the adjustable one out, I've had one bike where I could get the fixed cup out but not the adjustable, it works both ways. Just remember that the threads are reversed.

(Since the eccentric looks like aluminium alloy, gentle heat might be more effective at unsticking the cups than it would be on steel in steel. But unless you are prepared to strip paint off the frame, it probably won't help.)

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up vote 0 down vote accepted

So I finally got the eccentric out of the frame today (it turned out that I really did just need to mallet it harder). The reason that both cups were turning was that they both screw into a cylindrical sleeve, and this sits in the eccentric casing. The sleeve doesn't seem to have anything preventing it rotating (apart from rust and grime) so when I tried to turn one cup the whole sleeve rotated. I haven't yet managed to get the cups out of the sleeve, so I don't know what's in there. I've put pictures of the eccentric here: http://www.flickr.com/photos/88429902@N04/sets/72157631852885547/, just in case anyone's interested!

Thanks to everyone else that replied - lots of helpful people here!

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So effectively the eccentric has a full-width standard bottom bracket shell inside it? Interesting, I'd not heard of one like that before. –  armb Jan 31 '13 at 14:29

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