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My road bike needs a new seat clamp and the old one is marked 30.5 mm. This seems to be quite an odd size and now that the frame manufacturer is out of spares I'm scratching my head a bit.
I've found a 30.9 mm and a 30.0 mm. The latter is much nicer and I'd prefer it: is it likely that it can be teased open the extra 0.5 mm with a screwdriver? If not, how likely is it that the 30.9 mm will clamp down enough?
I'd bet either would be ok. The smaller one might take a bit of paint off the frame. The larger one might need doing up pretty tight to get to work, but you can always add a layer of tin foil for a shim?
Coke can can make a good shim.
If it’s too thick just cut it so it doesn’t go all the way round.
If using the smaller size, sand the paint off the frame in the area covered by the shim.
A cursory google says 30.5 collars are available though. .
I could only find a no-name eBay special?
Either will work fine. I've exchanged clamps marked 28.6 and 30.0 with no problems at all so long as the two sits of the gap don't touch. the ones with a swivelly fasteners are best for accommodating extremes.
30.6mm Salsa clamp - Done.
If you want one in black give me a shout.