Seatpost sizing for old Schwinns - '78 Le Tour, '80 Traveler
Just for the record, once these forums become searchable again...
Lots of old Schwinn road bikes came, production, with a seatpost shim and thus a narrower seatpost than would fit into the frame's seat tube if no shim were used.
There is some thought that shimmed seatpost was so that Schwinn didn't have to clean out any burs on the inside of the seat tube, although it doesn't seem as if that would be very difficult.
Anyway, I have recently worked on three older Schwinn models, using fine sandpaper to smooth the metal on the inside of the seat tube. Here's what they take:
1978 Schwinn Le Tour III - 26.0mm seatpost fits perfectly, slides in with no resistance but also has no play once inside the seat tube
1980 Schwinn Traveler - same as the 1978 Le Tour. Before sanding the inside of the seat tube, a 26.0 post would go partway in and then get caught on whatever burs on the inside of the tube.
1983 Schwinn Le Tour Luxe - 26.4mm seatpost fits cleanly after sanding off the inside of the seattube. <well biked> reports the same for his 1983 Le Tour Luxe.
Some people say the 1978 Le Tour and 1980 Traveler should take a 25.8mm seatpost, but I found that even before sanding the inside of the seat tube, a 25.8mm post had too much play inside the seattube. I'm pretty sure the "correct" seatpost size for the frame is 26.0mm, once the inside of the seat tube is cleaned up.
Maybe it varied some from year to year, model to model, but FWIW, my '82 Super le tour with 1020 tubing (very similar frame to the '78 le tour III and '80 Traveler you mention, I would think) had a 25.8 SR Laprade seatpost, with no shim, and it seemed to fit perfectly.
And yep, the '83 le tour luxe uses a 26.4 post, although I think we've determined it came with a shimmed post of a smaller size originally, as you say. I couldn't find the original seatpost or shim from my '83 when I rebuilt it, though, so I took the 26.4 SR Laprade from an '84 le tour I also have. I really like those old Laprade posts anyway. Both the '83 and '84 le tours are 4130 frames.
Vintage Schwinns, Classy Cannondales, & an '83 Santana Tandem
I've had seatpost challenges of my own on my '79 Traveler. When I wanted to replace the original steel and chrome straight top post, it took a long search before I found a 25.8 alloy one. I ended up bidding on a NOS from an ebayer in Australia. It took over a month to arrive but only cost around $25 with the shipping. With the shim it has been tough to tighten the clamp enough to keep it at the correct height. I gave up using the Sugino recessed allen nut and bolt. I now use a modified MTB quick release lever from a "dump find." It doesn't look the best, but makes for easy on the road adjustments and lets me remove the Brooks in a hurry if a storm pops up. Tim, I wish I had known about your experience of moving up to a 26mm.