teperman

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”.

JiveTurkey

Sheldon Brown listed the 1981 Campione del Mundo at 27.2mm. Assuming that's correct, it would be hard to imagine the same model would go to 27.0mm a few years later. https://sheldonbrown.com/seatpost-sizes.html#b

Seat posts and seat tubes are not perfect. In your case, the seat post may be near the upper limit of tolerance and the seat tube at the lower end. If you suspect a burr, you should be able to feel it (since you can only get the post in 1.5"). You are putting a little grease on the inside of the tube and/or on the post, right?

Iowegian

Try to find a bike shop that has a 27.2 seat tube reamer and have the tube reamed out. When you install the seat post be careful not to over-tighten it. As long as it doesn't move it's tight enough.

BCRider

Before you go reaming it if the seat post is being gouged like a cutter is scribing it then check around the split area where the seatpost clamp goes. I've seen some bad edges in there that need to be dressed back with a file or something like a Dremel with a ball grinding bit. Some work at the top tube clamping split with this and you may find that it goes in just fine. After the file or grinding some fine sandpaper by hand to polish it a little can't hurt either.

I'll bet the post slips in just fine after that.

Deanster04

If the seat is tight to start with rust can reduct the diameter slightly. A reamer will clean out any rust...and don't forget to grease the seatpost slightly.