Actually I would have to say that the seat post is 27.2mm. I just didn't have a set of calipers handy so I used a ruler. It was just wider than 27mm so 27.2mm sounds about right. It sounds like everyone has had some fun trying to figure this one out. I am very greatful for your interest as well as seemingly infinite knowledge! I have always been very curious as to what this bike was since I bought it about 7 years ago, but no one seemed to have an idea beyond the Bianchi stickers.