Originally Posted by T-Mar

During the period question, there were two standard seat tube outer diameters, 1-1/8" (28.6mm) which was the imperial standard and 28mm, which was the metric standard. When comparing two frames using either metric and imperial seat tubes, and assuming the same seat post clearance (0.2mm diametral clearance is standard) the seat tube requirng the larger post wil have thinner walls and therefore be lighter and typically made of higher strength material. This is the basis for using seat post diameter as an indicator of frame strength. The assumptions are that the outer diameter of the seat tube are the same and the diametral clearance between the post and seat tube are the same.

In your case, all the frames are late enough that I would expect imperial diameter tubing and 27.2mm post. Even if byy chance, some were metric standard, I'd still expect a 26.6mm post. Assuming all the posts use the same outer diameter, the Reynolds 531 uses the largest post and therefore has the thinnest walls and is the lightest frame. The Gitane and Motobecane, by comparison would be the "gas pipe" frames, with Carerra being in the middle.

Both Reynolds and Columbus typically used single butted seat tubes. While manufactures could spec a custom seat tube, I doubt they'd spec a double butted version. The theory is that the seat post itself effectively acts as the top butt and reinforces the joint. I've never across a fame with a double butted Cromor or SL seat tube.

Having said that, it is possible to have the effective diameter of the seat tube reduced by scale, burrs or distortion during brazing. Normally, a respectable manufacturer will ream/hone the inner diameter to return it to spec but there have been cases where manufacturers simply installed a smaller post to compensate. I would hope that wouldn't happen with a Cromor or SL frame. Very rarely, we see a case where the seat tube was inadvertently installed upside down, with the butt at the top.

Yes, you could buy Record post in 26.2mm and even 25.0mm. There are always exceptions to the rules, such as the Columbus MS seat tube and the aluminum tubes which had to use much thicker walls to compensate for lower material strength and metal fatigue. However, I think you find that most popular size for high grade vintage seat posts would be 27.0-27.2mm (for imperial seat tubes) and 26.4-26.5mm (for metric standard seat tubes).

As far as I'm concerned, the post sizes for your Cromor and SL frames are atypical. It seems strange that they are all under size. If you're using calipers to determine the size, I wonder if there is a calibration issue? It would also be interesting to know if the outer diameters of the seat tubes are 28.6mm or 28mm. Finally, photos of the cinch slots would aid in determining if the posts are undersize for the seat tube. Something isn't normal.