Estimating seatpost diameter
 

I've had an itch to replace the Origin8 pro fit seatpost that came with my Kona Cinder Cone from 1989, and when I went investigating, determined that the seatpost is a 26.6, and not the 26.8 expected. My calipers didn't give me an ID greater than 26.58 though, in any direction.
the bicycle uses Tange MTB, though I'm not sure of the grade. Velobase lists the 1988 Tange catalog, which specifies that there were two thicknesses for a 28.6mm seat tube (ignoring the single butt at the BB), 0.9 and 0.7. Do the math and the ID is either 26.8 or 27.2, and I've seen no indication that the early Konas used a seatpost as large as 27.2. Am I right in thinking that the quick release ever so slightly crushed the clamp?

I'll bring it to the Co op to check with the seatpost gauge, then maybe use a 26.8 and a 27 to stretch the clamp back to 26.8.

I recall early Konas used Kalloy seat posts and I know they made and probably still make 26.6mm posts. If that is what you measured for the post and clamp why are you trying to stuff something else in there? The clamp on my old Lava Dome is plenty robust and up to the task.

Have you pulled the seatpost completely out to see if the diam. is stamped on it. Many were (are?) stamped with the diam.

The calipers measure 26.58, seatpost is stamped 26.6. It's 26.6, like the original post said.

https://www.sheldonbrown.com/seatpost-sizes.html
Sheldon Brown's seatpost diameter database lists the 1995 Cinder Cone (the earliest one listed) as having a 26.6mm post. And a number of other pre-mid-'90s Kona bikes as 26.6.

Because the bike is specified to use a different diameter, and using a smaller diameter can stress the seat tube. I looked at it again this morning, and there's a tiny gap between the seat post and the tube at the back where the clamp is. That's all I'd expect to see if it's just a tiny bit ovalized. I know it's a sin to try and fix something that's working, but an undersized post is something that should be addressed, just maybe not this week.

I've seen that. He also stated that the measurements are volunteered and should not be blindly trusted because he can't verify them. There's an old thread about an early Trek MTB, and truing to use the tube specifications to figure the seatpost diameter. The consensus there was that (mostly brazed) bikes can have a .2mm tolerance for distortion. My frame is welded, but I don't know if that's better or worse for tolerances. .2mm is on the edge of what's considered an acceptable variance for a bicycle, isn't it?

Older thread link
 

Trek seatpost discussion on teh biek frooms!

Doing the math will rarely give a useful answer. The problem is heat distortion. From your numbers it seems you are talking about a 28.6mm outside diameter seat tube. From that era the premier tubesets would have been Columbus SL and Reynolds 531. The SL had wall thickness of 0.6mm at top. Two times 0.6 subtracted from 28.6 would give a 27.4 seatpost. That happened but it was very very rare. The standard post for that frame was 27.2. Builders at the level of Chris Kvale or Al Eisentraut on his best days controlled the heat so perfectly they might get it at 27.4, no factory ever did and most top builders never did. The popular Reynolds tubeset had 0.56mm at top of seat tube and that also used 27.2. Of the handful of frames from that era that used 27.4 the most common way to do it was to ream the seat tube. That gave superlight bragging rights.

Thanks for the help everyone. I'm concluding that the bicycle is capable of accepting a 26.8 diameter seatpost (confirmed last night) but would benefit from a reaming to do it (as it was helpfully explained). Moreover, there are a handful of less trustworthy examples that arrive at the same conclusion, including Sheldon's database. Since the 26.6 post moves well enough in the tube, I'm going to conclude that that's the intended size!