I have an iron horse tandem (Tango model), and wanted to replace the stocker seat post with a straight post with upside down clamp, for my daughter. The seat post looked like it had 26.8 stamped on it (although it was not very legible), so I ordered a seat post of that diameter, and it definitely isn't right. The original seat post fits in snugly but not tightly, while I can barely force the 26.8 in an inch or so. Do any of you have any idea what size I should be looking at? Would 26.6 be right, or do I need smaller? I could ride to the LBS, but it's not terribly convenient, and I don't know when I'll get around to it.

The old post is the right diameter, take it to the LBS so they can match it.

26.6 seatposts exist though they're rare, unfortunately 26.4 also exists, so it's not like you can simply guess next size down with any assurance. the only clue you have is that while it's easy to misread 26.6 for 26.8 if badly marked, 26.4 should look different enough.

If you have access to an accurate dial or vernier caliper, measure the original to confirm the size.

If not you can compare the sizes with a 2 steel rules and a flat table. Lay the posts next to each other on the table with the heads hanging off. Hold the rule edgewise across the tops at one end extending out over the table. If the posts are of the same size the rule will be horizontal, if not it'll slope. Now take two height measurements 5-1/2 or 11 inches apart. The slope will be 1mm/5-1/2" for every .2mm in diameter. Comparing the two heights will tell you what you need.