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Potentially need a 31.9 QR seatpost clamp. Any recommendations? If not, will a 32 be within tolerance?
32 will be fine, as will 31.8
31.9 is a bit unusual 31.8 will be easier to get and as said you should be able to get it to fit .
At some point I’ll take the clamp off and measure it. I can get a Salsa fliplock in either size; is it better to size up or down?
It's +-0.3mm so not much, given the choice I would size down.
Contrarily, I would size up and wedge a bit of aluminium can in there if it felt loose. But Ive used 32.0mm and 31.8mm clamps on the same frame and not noticed a different, so I suspect it doesn't matter in the slightest 😀