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I have a one-year-old REI CTY 1.3. Aluminum frame, belt drive, with an eccentric BB shell for tensioning the belt. The bottom bracket itself is a standard Shimano BB-RS500, but I cannot identify the eccentric shell. At first glance it looks like a Bushnell unit, but it seems to be a solid machined aluminum cylinder (on the Bushnell the crescent shaped piece with the holes is separate).

I cannot get the shell to rotate in the frame at all.

There is a single bolt (socket-head cap screw in a counter-sunk hole) that runs the full length of the shell. I assume that bolt tightens some sort of internal wedge against the frame.

I've tried removing the bolt completely, as well as unscrewing it part way and tapping the bolt head with a hammer. I've tried to move the shell with a pin spanner, and also dropping a long allen key in one of the holes and using the crank arm to get more leverage. Nothing budges.

I assume the wedge mechanism is not un-wedging. Since I don't know what's going on in there I don't know what to try next without potentially damaging something... Any ideas?

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Yours looks like the one described in this blog post: https://handsonbike.blogspot.com/2013/11/avanti-inc-3-eccentric-bb-for-belt.html?m=1. The writer was able to free it up by putting the bolt partway in and hammering it, so I would try that. It’s the same principle as old quill style handlebar stems.

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