There are several possibilities.
The wall thickness for a Columbus SL seat tube is 0.6mm at the top. An SP seat tube has a 0.7mm wall thickness at the top.
1 1/8" = 28.575mm (seat tube outside diameter)
SL 28.575 -1.2mm (2 x 0.6mm wall thickness) = 27.375mm (theoretical seat tube hole size)
SP 28.575 -1.4mm (2 x 0.7mm wall thickness) = 27.175mm (theoretical seat tube hole size)
Theoretically an SL seat tube should take a 27.2mm seatpost and an SP Seat tube a 27mm seatpost.
In reality many bikes made with the heavier gage SP tubing used 26.8mm seatposts while most SL frames came with 27.2mm seatpost, why?
Colnago could have used an SP seat tube instead of an SL tube but not likely. I don't recall ever seeing a 'Nag with SP tubing; Bianchis, Botteccias, Legnanos, Fejus and even Cinellis but no Colnagos.
More likely, here's the reason for the under size seatpost:
Brazing the seat stays to the seat tube lug and building up fillets takes a lot of heat. It's very easy to overheat the lug causing it to warp and the seat tube inside to bulge.
Ideally the seat tube should have had a pin driven down into it to reduce any bulges and round out the lug. It should then have been reamed to the proper size before painting but that wasn't always the case. Sometimes on production frames the seat tube was only reamed during assembly and just to remove enough paint to force a seatpost in.
I've seen out of round seat tubes on some of the best name brand frames. My 1974 Raleigh Illkeson Team Pro would hardly take a 27mm seat post until I removed the bulge and reamed it to take a 27.2mm seat post.
Bike Boom era production bikes were worse. I recently acquired an all Campy 1972 Gitane Super Corsa. It should have had a 26.4mm or 26.6mm seatpost. I was surprised to find it had a 25.8mm Campy seat post that appeared to be factory installed. The seat lug was almost .5mm out of round from heat warpage.
It's possible that your Colnago frame could have been repaired and the seat tube/lug not properly resized after the brazing was done.
I think that improperly sized seat tubes was one reason for better quality seatposts being readily available in 0.2mm increments. It was easier to grab a smaller sized seatpost during assembly than properly fix the problem.
Chas. Colerich Oakland, CA USA
Hello Vintage Cycling Friends,
I have a 78 Colnago Mexico and the frame is so light with the Columbus lightweight thin walled tubing. The issue, which may sound strange, is that the seat tube is a tube that accepts the 26.8 post only. After many emails and people questioning me, there are numerous comments about this frame really being a Mexico. The frame is much lighter than the Super in the same size, matched frame for frame on a scale. Everyone I have confronted with the information has stated that Mexico's have a 27.2 post. I have a panto Mexico post and it is a 27.2. Could anyone raise an explanation why this 26.8 is or how it is possible? There has got to be some Colnago intelligence with this CR site. All the indicators are there for this being a Mexico. 1) The extra light weight 2) The non crimped chain stays 3) The faint remains of pieces of a Mexico decal (this is a joke and is no indicator of proof because decals are everywhere)
If anyone could please help with some data or recommendations, I would certainly appreciate it.
La Grange, Texas