Looking to replace an old Oval R900 Carbon Seatpost from 2008.

The mechanic used callipers to measure the exposed part of the seat post (just above the seatclamp) and it measured
27.2mm...so he said buy 27.2mm

However I measured the outer diameter of the seat tube and it is 32mm (31.8mm inner diameter I guess)

I found the 2008 invoice for the seatpost and it says "31.6mm/280mm" on it.

So Im confused...I have no appetite to return stuff as I Iive outside the UK and mainly use CRC and Wiggle.

Can someone help?

Measuring the seat tube will tell you absolutely nothing about the actual post. A 32.0mm seat tube could NOT have a 31.8mm inner diameter, the will thickness would only be .1mm. Not possible. Nor would a 31.6mm post work in a 31.8mm tube. Why don't you measure it yourself? Or remove it and read the size off the post?

I have measured the post diameter with digital calipers just above the seat clamp and it read 27.2mm. Im not a DIY bike savvy
person so not qualified to remove the post and put it back again.

I find it interesting when you say that a 31.6mm post would never fit inside a 31.8mm seat tube.
Im pretty sure the tube is 31.8 as I just bought a front derailleur for those specs and it fit perfectly.

So a 27.2 mm post would fit in a 31.8mm tube?

The only mystery now is why my invoice for the existing oval seat post is 31.6mm.

Unlike tire sizes, which are as much a name as an actual size, most bike components are sized by their actual measurements

Your Derailleur is a 31.8, because that is the outside diameter of the seat tube, and the FD goes on the outside of the seat tube.

The seat post measured out at 27.2, because that's the inside diameter of the seat tube, ant the post has to fit inside of it.

Your invoice is wrong.

I have to consider that as a strong possibility.
I needed to hear that from someone.

Don't want to open a new thread but does anyone have any opinions on the Ergon CF Allroad Pro Carbon Setback
or its Canyon equivalent: the Canyon VCLS 2.0 variants?

31.8mm is the OUTSIDE diameter of the seat tube...your seat post goes...wait for it...INSIDE the seat tube. Get it now?

the invoice might be referring to a 31.6mm seat post clamp - rather than the seatpost itself. If the post was measured at 27.2mm, then that's it

Ive had this Oval carbon seat post on the bike for 15 years and never had a problem.

During a servicing + fitting of a new saddle, mechanic says the seat is now tilting up and down even though
the nuts on the clamp are tightened to the max. He swears he greased the nuts and the threads aren't stripped or anything.

Been reading up on this situation where saddles tilt up and down and it seems to be happening to a lot of people...and its
always very old gear. So Im thinking it's some sort of wear and tear.

Anyway, it gave me an excuse to order a S13 from Canyon. Ive read good things s hopefully this will sort out my seat post issues.