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If seatpost has nominal size of lets say 27.2, what is its actual size? Or other way around, if frame nominally accepts post of 27.2 what is the actual size of the bore in the frame?

Obviously both can not be 27.2 since it would not be possible to insert the seatpost, so what are actual dimensions?

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  • How would it not possible to insert the seatpost? Physical materials are always somewhat elastic, and experience says sometimes inserting seatpost needs some force.
    – ojs
    Jan 21, 2021 at 9:33
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    @ojs: But I hope there are some more-or-less official tolerance specifications. At least in engineering fittings are always properly specified.
    – Michael
    Jan 21, 2021 at 9:59
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    Partially OT: about tolerance and so on in the bicycle world: have a look at Hambini videos about bottom brackets on high-end bicycles. Yes, there are tolerances, yes, there is some form of Quality Control, no, they are not meaningful.
    – EarlGrey
    Jan 21, 2021 at 10:08
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    Also consider the frame is not an exact size either - it is not unknown for older bikes to be either squashed or stretched at the seatpost collar.
    – Criggie
    Jan 21, 2021 at 11:18
  • It's also not unknown for bikes to have manufacturing defects such as internal bulges where the seat stays are welded to the seat tube. That can happen on even high-end frames and can prevent seat post insertion. And it can be fixed in about 5 seconds with a cylinder hone. :-D Jan 21, 2021 at 13:08

2 Answers 2

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There is typically a ~0.2 mm difference between the post size and the inner diameter of the tubing. For example, a common setup in good quality steel alloy frames is a seat tube with 28.6 outer diameter and 0.6 mm wall thickness at the top, giving an inner diameter of 27.4. These frames take 27.2 mm seatposts.

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  • If the OD is 28.6mm and the wall is 0.6mm wouldn't that make the ID 28mm ? To get an ID of 27.4 then surely the wall has to be 1.2mm
    – Dan K
    Jan 21, 2021 at 13:08
  • Also, 27.4mm is an actual seatpost size. I know that Thomson still offers its Elite post in numerous sizes including 27.4. So, I suspect that this particular tube would not have accepted a 27.2mm post.
    – Weiwen Ng
    Jan 21, 2021 at 13:45
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    @DanK There are two walls. Jan 21, 2021 at 13:49
  • @WeiwenNg - although there is a 27.4 seatpost size, 27.2 is the standard for the 0.6 mm butting on the seattubes of classic steel tubesets that are extremely common on vintage and some modern steel bikes. Eg Tange Champion, Columbus SL, Reynolds 531c all were often (but not always) 0.6 mm thick at the top of the seat tube and took 27.2 posts.
    – Andrew
    Jan 21, 2021 at 17:18
  • @Andrew True. However, if a seat tube had an internal diameter of 27.4mm, I would normally expect a 27.2mm post to be far too loose ... although see post #9 here: bikeforums.net/bicycle-mechanics/…
    – Weiwen Ng
    Jan 21, 2021 at 17:38
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Please check which one refers to internal diameter and which one to the outer.

If the seat tube (the one on the bike) has an inner diameter of 27.2 and the seat-post has an outer diameter of 27.2, obviously they will match. You can expect tolerances (variances, as more correctly mentioned by @weiwen-ng ) on the order of +-0.2mm, unfortunately.

Don't forget grease, especially when installing aluminium seat-post in a steel seat tube.

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  • Thank you, but if both of them are exactly, 27.2 they will not fit, there has to be the difference, and my question is how much, and if seatpost is expected to be smaller by how much, or if tube is expected to be larger and how much, or both seatpost is expected to be smaller and tube to be larger. Jan 21, 2021 at 11:42
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    Actually the dimensions can be exactly the same and the post will still fit in the tube, because the tube has a cutout at the top so that it can stretch a bit, and once you get the end of the post inside, you can just push it in because the rest of tube will stretch a bit as well. If the tube were larger than the post, you would feel very little, if any, resistance as you move the post in the tube, and that (in my experience) is very rarely the case. Only if the tube were a lot smaller than the post would it be impossible to fit the post.
    – Mick
    Jan 21, 2021 at 11:59
  • Tubes are generally produced defining a certain external diameter. Internal diameter then is filed, so it should be internal diameter = x or more mm.
    – EarlGrey
    Jan 21, 2021 at 12:03
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    My impression is that tolerance is something a manufacturer specifies. For BB shells, I know that tolerances are asymmetric, e.g. +0.0 / -0.2mm (if the shell is too big, you get creaking). I would suspect that if there's a tolerance on seat tube size, it's specified by the tube manufacturer. I don't know if this is what engineers say, but variance is what I'd call the actual variance in size - you might have some units out of tolerance and not caught by QC.
    – Weiwen Ng
    Jan 21, 2021 at 13:43
  • @WeiwenNg good point
    – EarlGrey
    Jan 21, 2021 at 14:43

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