Seat post nominal and actual size

Question

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?

Andrew · Accepted Answer

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.

Correct answer by Andrew on January 25, 2021

EarlGrey · Answer

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.

Seat post nominal and actual size

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