I have a mid-70s Gitane Tour de France and ran into the same difficulty. I think a too-small post had been used previously and the lug was distorted a bit. I massaged it back into shape until the cross section was round, basing the size on the curvature of the lug where the seat stays are brazed, which seemed undistorted. I ended up using a 26.2 and it seemed to work well. Measuring the ID is confusing because it depends on where you measure. The ID of the seat tube below the lug is somewhat larger - about 27.2 mm if I recall correctly - but the tube seems to be pinched a bit inside the lug. I read somewhere that these bikes are supposed to take a 26.4 post but that seemed a bit tight in mine.
Brian Samson Vancouver, BC, Canada
On 29-Feb-08, at 12:48 PM, Derek Willburn wrote:
> I'm working on an early 70s Gitane and it has a bit of
> damage around the seat lug, I would like to know what
> size seatpost these should have. My 25.8 is too loose
> and my 26.4 won't go in.
> Thanks,
> Derek Willburn
> Long Beach, CA USA