I have a Trek 520 and was measuring the seat post tube to get a spacer for my USE post to fit. I measured that seat tube several times and am still not sure if it is 27 or 27.2. Thoughts? I am starting top think that the difference is bunk.

I have a Mieta Lupa frame that I was told was 27.2 and I am having a hell of a time get a 27.2 post down easy enough to make hieght adjustment easy. It is really tight and I even polished it. Is the seat tube clean, yes.

Seat post are driving me crazy

My guess would be 27.2 based on Sheldon's seat post database. It seems to have been a fairly common size for the older Trek's. If you want to be positive you could take it down to your LBS and have them measure it with a special post they ought to have.

Have you gently spread the "ears"?

Yes, on the Mieta I have worked it quite a bit. It is STILL tight
Makes me want to turn down the seat post if i could work my leth better. I have a same leth but i am a beginner.

My 1988 has a 27.2 mm seat post, but that was the last year of the 531 main frame. Trek used several different tube sets over the years, so I can't say if mine is the same as yours.

My wife had a slightly later 520-- we bought the frame only as a closeout in 1991, it was NOS a couple years old by then; and it took a 26.8 post. It was purple, and the tubing was TrueTemper, if that helps narrow the year down.

Ironically, in 1991 the closeout price on the 520 frame was $335, exactly the same as I paid for my 720 frame brand new in 1983.