ridelikeaturtle

In case anyone searches for this information in the future, I thought it might be useful to relate my experience, relating to the seatpost size of a Bianchi Sprint 76.

The previous owner had inserted a 25.8mm seatpost and crushed down the seat tube binder bolt. To address this temporarily, I made a small shim out of an aluminium coke can (beer can, actually), which worked; but it wasn't really a great fix, as the binder bolt and seat tube was still overclamped.

After a lot of searching and a bit of measuring, it seemed that a 26.2mm seat post would be the correct fit. That's what I measured the ID of the seat tube to be.

Well, my friends, it is not - not for this bike, anyway.

The 26.2mm seat post is exactly the size of the ID of the seat tube, which in this case means the seat post will not enter the seat tube. Of course, it could probably be bashed a bit and forced in there, screwing up both in the process... but since I don't like destroying stuff, I figured for the 20 quid it'd cost for another seat post, I got a 26.0mm . Having arrived, that one slides in perfectly and tightens down nicely without having to force anything, and the seat tube binder is not overclamped and all is good.

Lesson learned: on an old steel frame where you've no idea what the manufa's seat post size was supposed to be, and you're measuring using cheap calipers and a tape measure, get a seat post thats 0.2mm LESS than the ID of the seat tube.

p.s.: throw those cheap crappy digital calipers in the bin, they're useless. Get a micrometer, or a decent set of dial calipers.

T-Mar

Standard practice is to install a seat post that is 0.2mm smaller than the nominal inner diameter of the seat tube. You don't use a post that's the same size as the inner diameter of the seat tube. You always want some clearance and a 0.2mm smaller post provides 0.1mm clearance.

ridelikeaturtle

"Experience is something you get just after you needed it."


Lucky for me this wasn't a terribly expensive lesson. But yeah, it seems obvious in hindsight, I should've copped on to that when I measured it the first time!