Originally Posted by
bikemig
That's what I thought as well. But here's the kicker. The bike came to be with the wrong seatpost, a 27.2. A 27.0 didn't want to slide down either but a 26.8 works like a charm. Now I've seen one bike that I am fairly confident was made of columbus SP but it had a 26.8 seatpost. Possible?
Oh, definitely. It's not uncommon for tubes to distort to non-round during the brazing process. Then there's scale and cinch slot burrs, which can result in an apparently undersize tube. Most reputable builders will ream/hone a seat tube after manufacture to bring it back to OEM spec, but a lot of the Italian builders had a poor reputation for frame prep.
However, it's also possible to distort the tube by installing and clamping an undersize post. This typically results in a cinch slot which is narrower at the top than the bottom. In such cases, I pry the cinch slot open with a wide, flat bladed screwdriver, so that it's wider at the top. Then I see what size seat post will install past the bottom of the slot,as that usually represents the true inner diameter of the tube.