One can make measurements to the seat tube, but sometimes it's useful to measure the post you want to use also.
I used a digital caliper to measure the post that came out of my '72 Gitane TDF, and tho it was steel (thus less than round) I easily concluded it was 26.4mm. Needing a longer post, I took an SR Laprade post which happened to be labeled 26.6mm, but actually measures only 26.46mm. It fits rather snugly and that's perfect. I won't have to tighten the pinch bolt so much and there's less of a gap for water to enter. Undersized posts are common: even Campy undersized them. I had to switch one Super Record post out for a "same-sized" (actually a bit bigger) Avocet post in order to cure post-slipping on my Raleigh, lest I shear the pinch bolt right off trying to get sufficient grip. Even with the Avocet's slippery anodized finish, a slightly better fit really helped with the grip.
I've seen these digital calipers selling for as little as $10 lately and they're quite handy for eliminating guesswork when working on bikes as well as scoping the swap meets. I could hardly do without them when re-spacing axle assemblies, and it's nice to know what's the actual diameter of handlebar stem quills and clamps.
David Snyder Auburn, CA usa
> To avoid purchasing more than one seat post to discover the size needed,
> you could purchase or borrow a seatpost sizing rod, like Stein makes. See
> biketoolsetc.com or perhaps your local bike shop ....Peter
> Peter Brueggeman