For a 27.2 seatpost what is typically the inner diameter of the insert that sits inside the seat tube? The seatpost, when clamped, is touching that insert…
Are 27.2 seatposts actually 27.2mm outer diameter and then the inner diameter of the insert is slightly larger than 27.2? How much larger would be appropriate?
In our experience, seat post diameters vary greatly, which is why some will slip and creak while others get stuck.
If you provide some context on what you need the dimension for, maybe we can help you out more.
Interesting and a bit…sucks to hear.
By variance do you mean with different models or even within the same model?
If it’s the former, i hope it’s not bad enough that we have to start a mini table of what and what doesnt slip. For instance, @Duanedr told me that this seat tube slips: Columbus seat tube 28.6 dia .8/.6/1 ext. butted - 635mm – Framebuilder Supply
If so, does anyone have experiences with these?
I have little experience with seat tube inserts, when I’ve used 31.8OD seat tubes, I get the correct seatpost diameter, but I mostly build with 28.6OD seat tubes in road and gravel, not hardtails.
But in answer to your second question, the actual outside diameter of 27.2 seatposts varies widely from brand to brand and model to model, from 27.00 to 27.15. Which is certainly annoying. The norm, if there is such a thing, seems to be 27.1mm.
Oops only realized OP was talking about seatpost and not seat tube just now.
Sorry i didnt get it at first, must have been the caffeine crash…And good to see you here Mark!
I’m wondering what inner diameter the insert should have given my goal of using a 27.2 seatpost. Maybe 27.3mm? 27.25mm?
it doesn’t really matter as long as the wall thickness is "correct, since the insert is most certainly slotted 
27.2. You can always make it bigger by reaming. You cannot make the hole smaller.
I recently used a seattube that seems to be out of spec. With a bore mic I measure it at 27.35 and it’s looser than I would like. My 27.2mm reamer didn’t take off any material, it just slide down into the tube. 
With carbon paste it should be ok but the binder requires more torque than would be normal.
There are 27.4 seatposts out there, Duane. I have a couple of American Classic’s from back in the day, when Klein used 27.4 as a stock size, and a couple of vintage Campagnolo Record’s and Super Record’s, in my stash for just this eventuality. I did have some Reynolds 753 tubesets way back when that took this size. These days, I make sure my seat tube choice prevents this from happening!
Thanks for all of you input! I will go with 27.25mm. Should this be insufficient I can always ream it to make it bigger.
I measured as Redshift Dual Position seatpost which is 27.2mm.
I did a search and found those American Classics. Thomson has one but not with setback. And I don’t know if it’s big enough for 27.4mm so I’d have to remove material and I hate adjustable reamers. A 27.2 fits, I just don’t like how much torque has to go into the bolt.
I haven’t had a chance to dig through my tubes to see if others are similar or if this is a one off.
Lesson: Don’t trust tubing specs.