So today i went to look at a bike i found on craigslist. It was listed as a schwinn continental but ended up
being a supersport. I got it home and proceeded to look for a seatpost( bike was missing
this.)I have a seatpost that appears to fit from a miyata 312( sr campy copy), but im not positive if the
post is 26.8 or 26.7(frame is 26.8 iirc and seatpost is too scratched to tell). Can anyone confirm the miyata post size? Will anything earth shattering happen if i use a post that is .1 mm too small?

I have 6 SSs and all are 26.8 Roger

Do they even make 26.7, I thought sizes were always by .2

Well, I can tell you that a seatpost that is slightly too small will cause you to be able to clamp the seatpost in, but allow just enough give so that no matter how tight you try to get it, the seat will still rotate from side to side while being ridden & pedaling.My fixed gear build has a bugger of a seatpost/tube size(mine also came sans seatpost), and I had 2 bike shops measure it. Both confirmed it measures a 25.5-6(is a bit oblong in a spot) but sold me a 25.4 saying it would work fine.
I have to clamp it just right, and even then, sometimes I have to have my hubby rewrench it tighter to work reliably. I think the most frustrating part is that .1 is too small to make a shim for. Only thing I have sortof gotten to work is a layer of electrical tape wrapped around once, with a slight overlap in the oblong area.

Miyata used some odd seat post sizes, on some models. Check with a digital caliper.

With a $10 digital caliper, one can readily confirm that most seatposts are nearly .1mm to the small side of what is stamped on them. Even Campag posts (no better than most, and worse than others).

Certain posts are true to size, and I often measure and select from my ~100 posts for more of a custom fit.

The proper size for a particular frame is the biggest post that will fit without real force imo.
On my GT LTS MTB, for example, I replaced the stock 27.0mm post with a Suntour XC Pro in the 27.2mm size, and all is well.

One weird exception occured when I pulled out a nearly 2-foot long, 27.4mm post from my recently-acquired Richard Moon steel frame. It was almost torture getting it out, and I didn't need all that length and weight, so I found an older 27.2mm American Classic post that was quite light, and oversized at exactly 27.3mm diameter, for a perfect fit.
I believe that the AC post was actually a 27.4mm post that got the wrong size stamp at the factory.

BTW, I've had 3 Super Sports and all came with 26.8mm (chromed) straight posts. I always replaced with alloy posts, but these bikes are HEAVY, even as they have quite sporting geometry that allows aggressively fast riding. Like a Continental on steroids!