Originally Posted by
levoz
Surely seat posts are not an interference fit. Reynolds 531c seat post is single butted 0.8 mm and 0.5mm. according to you the seat post diameter should be 28.6 - 0.5 + 0.5 therefore 27.6 and not the actual size which is 27.2mm. the difference is 0.4mm . The same applies to the tubing that I have described, except that the difference is 0.6mm.
I have two Peugeot Perthus 753 frames. Seat post diameter is 26.6mm. It seems that french bikes have metric tubing, so 28mm od. 28.0 minus 2x0.5 mm does not equal 26.6 mm.
I never stated that seat posts are an interference fit. I said they were a "CLEARANCE FIT". That's why the correct post size in my examples are 0.2mm (one post increment) smaller than the nominal inner diameter of the seat tube.
I don't know why you're digressing to Reynolds when we were discussing Columbs but Reynolds introduces it's own complications. Being British, the wall thickness is actually based on Standard Wire Gauge, which doesn't translate into a nice, even metric figure. The non-butted wall of Reynolds 531C is 24 SWG, which is 0.559mm and closer to 0.6mm than 0.5mm, however it's advantageous for Reynolds to round down and market the tubes slightly thinner than actual. Since posts don't come in odd increments, you go down closer to 1-1/2 sizes and the correct size post for Reynolds 531C is 27.2mm.
French bicycle came with metric standard tubing into the early 1980s. Around the mid-1980s most French manufacturers started converting to the imperial standard. Not everybody converted at the same time.