I don't know what the factory spec seatpost size was. 26.8 is probably correct, or possibly even 27.0 or so. The reason I say that is because how tight the PO had cranked the binder bolt to hold the seatpost, and the way the seatpost dropped right down under it's own weight when I loosened the binder bolt - no seat on the seatpost either.. I've NEVER had a seatpost do that before! I mean the one in there that I removed could have been a slightly too-small seatpost from his parts bin. I'm hesitant to measure the seatpost opening with calipers since it may have been distorted by the seatpost binder bolt.

So, maybe the question I should have asked last night was: What is the factory seatpost size for a '84 Miyata/Univega with CrMo Double butted tubes???

I see from the '84 Miyata catalog that their CrMo double-butted tubing has wall thickness of 0.9/0.6/0.9mm. So would I be safe to measure the OD of my seat tube and subtract 1.8mm to get the ID?? Or did they do a finish-reaming of the seatpost tube to accept a certain spec-size seatpost?? What about paint adding thickness... Aaaauuuggghhh!!!

I never had this problem when I changed seatposts on my Fuji years ago - because I bought that bike new so I knew no parts were replaced with wrong-size parts before I bought it.