Originally Posted by
Iride01
Okay, but to me that's not much material to be reaming. The out of roundness the OP suggested might easily be more than that amount if they are seeing it by their EYE2020's. Especially if this bike has ever had a reamer in it before.
Seat tubes are often distorted during the brazing process. The top tube is fixed at both ends, and when heated to brazing temperature, the seat tube/top tube joint gets soft. The top tube expands (lengthens) when heated, and pushes into the soft seat tube, distorting it. A reamer can remove this distortion without removing material elsewhere.