I am in the process of building up a Celeste green Italian-made Bianchi frame, "Campione del Mondo 1986-87", that uses "Columbus Formula Two Cromo Molibdeno" tubing. The frame did not come with a seat post. So, I measured the seat tube and my calipers measure 27.2. A local bike shop said they thought that 27.2 seemed right, after having used a cylindrical measurement tool inserted down the seat tube.

Based on that information, I bought a vintage Campy Chorus 27.2 aero seat post for the frame. However, when I start to insert it I can feel that it requires some force to push it in; twisting it works to insert it, but it leaves twist marks on the seat post. I have only inserted this post about 1.5” because I do not want to mar the post.

Questions:

1- What would a Bianchi spec book say? Have we mistakenly thought the seat post should be 27.2 whereas it should actually take a 27.0 seat post?
2- If the Bianchi literature confirms the correct size post is 27.2, then what can be done to prevent the marring of the post I have? There may be a burr or something down the tube that is causing the marring, but in any case, the appropriate word for the fit, with or without a burr is “snug”.