I have a Coker Big One that came with 2 seat posts that both have 25.4mm stamped on them.
I also have a sun uni and 2 nimbus unis. all 3 unis came with seat posts with 25.4mm stamped on them.
All 3 nimbus/sun seat posts and frames fit each other perfectly. But the Coker seat posts are too big to fit into the nimbus/sun frames and if i put Numbus/Sun seat post into Coker it won’t quite tighten down properly.
Is what i’m explaining normal? any ideas why the seat posts and frames aren’t the same size?
The easy answer is “tolerance variables.” But it is also a bit more complex than that. Steel and aluminum tubing is measured and specified by outside diameter and wall thickness. So you get an inside diameter by mathematically combining the two. Both of these numbers has a manufacturing tolerance, but in my limited experience, the outside diameter is the more accurate of the two. Wall thickness varies a little more.
My Coker has a 25.4 seatpost, and it fits nicely. I purchased a KH adjustable seatpost, and it fits nicely also. That means that Coker did a good job of matching their upper frame tube diameter to the expected seatpost.
I also have a Nimbus 26" that I’ve never changed anything around on, and your post makes me interested to go home and measure that seatpost and see how they compare.
Bicycle seatpost diameters are all over the map, and you can get lots of shim sizes because of this. You can probably get shims to help your sun/nimbus posts to fit the coker, but you’re probably out of luck going the other way.
It’s not a bad idea to get a cheap pair of calipers (like from harbor freight or similar) to have on hand to check out this kind of thing. Then you know who’s on first, so to speak.