Wrong seat post size, now what to buy as replacement?

I have a 1990’s steel Giant frame. I bought it about 15 years ago, and the seller had installed a 27.0mm seat post. I have over time adjusted that seat post in height and always noticed how hard it was to move it within the seat tube. Now I need to replace the seat post. Without giving it sufficient thought I had bought a 27.0 post, which doesn’t fit.

After some research (Sheldon Brown’s seat post size database included) I suspect the actual seat post size should be 26.6. When I look at my current seat post I can tell that the years have worn down the diameter of the original seat post: I can start to see the aluminium “silver” color come through the black. I wonder if the steel seat tube inner diameter has also worn down a bit.

Now I have a dilemma: do I buy a 26.6 seat post as I suspect that is the intended size, or should I expect some wear on the inner diameter of the seat tube and buy a 26.8 instead?

Additional comments:

  • I tried to measure the the inner diameter of the seat tube, but my non-digital calliper isn’t accurate enough to pick up on 1/10mm.
  • I am risk averse in this, so: if the 26.8 is too wide I have to force it in, if 26.6 is too narrow I have to over tighten the integrated seat post collar. Which would be worse from fatigue perspective?

Your non-digital caliper will probably have a vernier scale on it, that’s how measurements to fractions of a mm were made before digital calipers were invented… Vernier scale - Wikipedia

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Awesome for teaching me that, thanks!

Unfortunately I don’t feel this is going to solve my problem immediately: firstly the calliper is a cheap plastic thing that I wouldn’t trust with anything more precise than a mm and secondly I am only able to measure the immediate entry of the seat tube and this can even fit the new 27.0 seat post for about 2cm deep.

26.6 and a strip of beer can. Beer cans are generally about 0.1 mm thick so ~40mm will make up a 0.1mm difference in diameter and ~80mm will make up a 0.2mm difference.


I used my newly gained knowledge to measure the original seat post with my plastic calliper. It measures 26.8. Seeing it moves very tightly, I am going to try my luck with a new 26.6 post. Worst case it would be time to open a can of beer.

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Find a good bike shop that’s been around for a while. Ideally, they’ll have a set of seatpost sizers (like these) that will confirm for sure what the proper seatpost size is.

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Thanks James, I should have thought of that! Seat post has already been ordered :crossed_fingers: