3alarmer 

...if you can't see from reading that thread that seat post size varies from the marked standard by as much as .1mm, any link I provide won't help. Is that enough to cause some catastrophic, life threatening failure ? Of course not. Will a seat post that is undersized by as much as .1mm require higher torque on the seat post clamp ? Probably, but the length of insertion also comes into play in this. Will, as in the OP's problem, a very slightly undersized seat tube (because it has been reamed slightly undersized, or there is some other distortion issue from the frame construction process) fit "tightly" on what ought to be the correct seat post size....well, yes, yes it will.

You are absolutely horrified by my suggestion that some light sanding will probably save money (no new post required), and quite possibly provide a better overall fit here. In your absolute conviction that you are expert in all things (not unusual for you here on this forum), your suggestion is to buy something that is .2mm smaller. Because that's how you get stuff to fit at your co-op. I'm not suggesting you're killing people, only that possibly you are not as educated by all your experience as you seem to think. The reason is your dogmatic approach to just about every topic I've ever witnessed you hold forth on.

Anyway, this discussion has gone on waaaay too long. Which is like most of the ones I've had with you.

If you want to continue stalking me on the mechanics forum suggesting I don't know what I'm talking about when I try to help people here, feel free. But don't expect me to continue to respond to you. It just brings out the worst side of me, and I end up regretting it.

FrenchFit 

Thanks all. 26.6, perfect fit.

zacster

I installed my newly acquired Kuota 30.0 post into my Kuota Kredo and it fits perfectly tight as I expected. The ridges seem to be intact and much sharper than the ones in my original Kuota post. I haven't had a chance yet to ride, but later today.

The fit is tighter than the fit of the 27.2 with the 27.2 to 30.0 shim that I had in there, so maybe it just was never tight enough and that's why it slipped. And when I took out the old I saw that it was bottomed out again after a 40+ mile city ride. This is another instance on a bike where the decimals of the millimeter mattered to me. I built a Campy compatible 10sp cassette from a Shimano one by inserting .2mm spacers, cut from plastic sheeting. It works as good if not better than a real Campy cassette on my trainer.