Originally Posted by
vpiuva
I don't have that on my early 70's Gitane TdF, but I've wondered why my metric 531 tubing is different. The standard 531 metric TdF frames take a 26.4 post. Mine, just to be odd, takes a 26.6 post, and I've heard from some Gitane Super Corsa owners that also use a 26.6. Someone out there knows I'm sure.
The theoretical inside diameter for a French 531 seat tube is 26.6mm. During the framebuilding process there is a natural build-up on the inside of the tubes from the plating and painting operations. There may also be some distortion from the brazing process. Normally, the tube is restored to round and any build-up removed by reaming and/or honing operations. However these operations also remove metal and can enlarge the tube.
Some manufacturers will try to bypass these operations and simply substitute a smaller post. Even if the tube is a perfectly round 26.6mm the question is whether to use a 26.4mm or 26.6mm. Ideally, you want the post very slightly undersize relative to the tube so that it provides a good sliding fit when greased. Both 26.4mm and 26.6mm are reasonable sizes depending on the amount of build-up and extent of the reaming/honing operations.