I'm building a bicycle for myself on a steel road frame using primarily used and salvaged parts. I measured the inner diameter of the seat tube to be 26.8mm but I only have on hand a 27.2 seat post. Should it be fine to use this post?

I'm only asking this because, to my surprise, the 27.2 post fit cleanly into the frame. No force was needed to insert it and, there's even a small amount of play when the clamp isn't attached.

  • How did you measure it? With a seatpost measuring rod? Calipers?
    – Batman
    Nov 14, 2016 at 23:07
  • @Batman Park Tool calipers. Nov 14, 2016 at 23:17
  • Please redo your measurement - vernier calipers are easy to read wrong. Or get someone else to measure both and see if your results are close. The seatpost should have its size stamped into the metal, makes for easy corroboration
    – Criggie
    Nov 15, 2016 at 6:19

1 Answer 1


If it fits and you can clamp it, it's fine.

  • Good to know. I don't have a clamp to do a full test with right now but I'll accept when and if it does work. Nov 14, 2016 at 23:26
  • 2
    If the saddle suddenly drops while riding, the seatpost is too small.
    – Criggie
    Nov 14, 2016 at 23:29
  • For a steel frame I agree. What about other materials. I would hesitate to put a too small seat post into Carbon, but don't actually know if it would hurt.
    – mattnz
    Nov 15, 2016 at 3:30
  • Got the bike together and everything works fine. I had a friend look at the bike and his opinion was that the top of the seat tube was crushed/deformed at some point but that shouldn't be an issue with steel. Dec 2, 2016 at 17:22

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