I replace an incorrect sized seatpost with a correct size seatpost for the frame. I bought a nice Koga Miyata bike, but it had a dirty old steel seatpost which was obviously not the correct post. And it looked like the diameter was too small since the seatpost clamp look a bit tight. I measured the old one (25.8mm), and looked up what size seat post should be in the frame (26.8mm). I expanded the seatpost clamp part of the tube to accept the correct sized seatpost.