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I am building a bike with a Rivendell custom frameset. The seat tube size is 29.7mm. I am mounting a Dura-Ace FD-R9150 front derailleur that is braze-on.

I'm looking for an adapter clamp but the smallest says it is 28.6mm. Can I use a shim with a 28.6mm clamp or is there a clamp that will fit that size of seat tube?

Any suggestions would be very helpful.

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  • Band-on to Braze-on adapters aren't expensive - you might want to get one and see how it goes. Having one in your hand can make all the difference.
    – Criggie
    May 7, 2023 at 0:54

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That is a very weird situation. Sometimes thick finish can inflate the tube OD, but not typically by that much. It's curious what the seat tube used is. Seat tubes are typically built to be seat tubes, and their diameters shouldn't be that far off on a bike without a braze-on mount.

I would not try to force on a 28.6 adapter. The difference in diameters is too great. The good option is likely get a 31.8 adapter and shim it down. Ideally get the Shimano one so the back support screw contact will "just work." Alternatively, you could get a chunky 28.6 one and machine it out. Some FD adapters are dainty and I wouldn't go assuming they all have the material to give.

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