Oh I know it'll be a make to fit kind of scenario, I was just wondering if there's any kind of convention. Now that im thinking about it though, I guess tubes have a nominal ID, and if that size post doesn't fit, the framebuilder reams the tube to fit the nominally sized post.
I have digital calipers- $8 garage sale with foam case branded with some cartridge reloading company on it..
I'm thinking standard tolerance is ~.15mm. My "25.4" Thomson post is actually 25.25 but the stock 25.4 shim is an honest to goodness inch. That's about .004". So tighter than plumbing fittings.
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Shimano : Click :: Campy : Snap :: SRAM : Bang