Originally Posted by
coelcanth
it's not a given that one specific tubing will always yield the same seatpost size..
most 28.6 tubes with .6mm walls at the seatpost end will use a 27.2 seatpost, but if there was a bit too much heat during brazing (or some other variable) maybe the builder had to ream out the tube a bit more to take care of ovalization or maybe somethin else happened during the bike's lifetime...
likewise most 28mm tubes with .7mm walls will use the ubiquitous 26.4, but it's probably also possible they just pulled some lighter gauge tubing off the shelf that day, or maybe there was even a special model with thinner butting (for smaller frames, perhaps ?)
French will use most often the metric tubing 28mm seat and downtube and 26mm top tube, wal thickness varied, well before there was the unique transfer, there was the light tubing, often called 3-10, also 5-10 and 7-10, .3, .5 , .7mm in the unbutted portion. Seat tubes are normally speced with a single butt, otherwise WAY too many custom lengths of tube to stock, now some custon builders would use a downtube for a seat tube, but not often. Oddball bikes like Cinelli's had internal sleeves, more work.
The 3-10 stuff is VERY fragile, if the frame is mounted in a Park workstand, grab the seat post, I have seen the damage otherwise. .3mm was really beyond the design limits of the base material, back in the pre air-hardening days. But in the mid size (56cm) without too much trouble 18lbs was possible fully built. Stays were also lighter in wall thickness too.