i bought a size 268 seatpost for an old road bike. the original seat post did not have the diameter stamped on it so i kinda had to measure and guess (didnt have calipers i just used a ruler). turns out its barely too small. when i went to put it in, it looked like it was gonna work but the clamp didnt hold it. is it obvious that i need a 272 then? just making sure before i pay shipping charges.