There were a lot of reasons for different seat post size. Fist off was tubing wall thickness.
The seat tube wall thickness for Columbus SP was 0.7mm, SL was 0.6mm. Reynolds produced seat tubes in 0.5mm, 0.6mm, 0.7mm, 0.8mm and 0.9mm. Super Vitus was 0.5mm or 0.6mm plus Durifort and Vitus 172 were 0.7mm. Tange and Isiwata offered the same range.
All of these manufactures also offered special light weight tubing with even thinner seat tube wall thickness. Add to this the European tubing manufacturers offered tubes in both Imperial(inch)and metric outside diameters.
For example, 1 1/8" OD = 28.575mm - 27.2mm = 1.38mm /2 = 0.69mm per side
Theoretically a 1 1/8" diameter seat tube with 0.6mm wall thickness would take a 27.2mm seat post that would have 0.09mm per side for fit.
A 0.7mm (Columbus SP)seat tube would take a 27mm seatpost.
In reality, the tube inside the seat lug frequently bulged plus the lug itself distorted out of round from the heat required to braze on the seat stays and build up a filet.
A special tool "should have" been driven into the seat tube to round out the top and remove any bulges then the tube reamed to the proper size. Most of the production bikes that I've seen that were built between the 60s and 90s appear to have only been reamed during final assembly to remove excess paint!
I think that some bike makers had an assortment of seat post sizes that they used during assembly rather than correct the problem: if a 27.2mm didn't fit, use a 27mm.
I bought a ~1972 all Campy Gitane Super Corsa at the beginning of the year. As I was going over the bike, something didn't look right about the seatpost. I checked the size stamped in the rear a found it was a 25.8mm instead of a 26.4mm! It appeared to have come that way from the factory because the seat lug was oval so I fixed the problem.
Chas. Colerich Oakland, CA USA
Strickler, George M wrote:
> Since most of the frames that this list are concerned with were made of 531 or Columbus, what accounts for the many different seat post sizes? Even in my very limited collection I have 5 different sizes ranging from 26.8 to 27.2. Did the tubing people make tubes of the same kind (for example, 531 double-butted) with different internal diameters? Certainly would be more convenient if all British stuff was 27.2 and all Italian 27.
> George Strickler
> New Orleans