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Discussion Starter · #1 ·
I purchased a '85 Bottecchia frameset as a retro reclamation project. Last night as I was putting the finishing touches on the build (Campy and Modolo), I tried unsuccessfully to insert a Thomson 27.2 seatpost into the seat tube. I measured the ID with a pair of calipers and came up with 27.2. I then re-lubed the post and tube, but the post would not insert more than a couple of inches. I also tried a American Classic 27.2 without any luck. Any chance that a Bottecchia owner could provide some insight? Does anyone know if this vintage Bottecchia seat tube is actually 27.0?
 

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What tube set is used?

Here's an explanation of seatpost/seat tube diameters from a practical POV. I don't know who the original author is and would certainly credit him/her if I knew.

"27.2mm became the standard seat-post size because most high-end road frames in the 1970s and 1980s were lugged and were almost universally made out of Columbus SL or SLX or Reynolds 531. These seat tubes were 1-1/8” in diameter, or 28.6mm. The single-butted seat tube was 0.9mm thick at the bottom and 0.6mm thick at the top. Well, 2x0.6mm = 1.2mm, which, when subtracted from 28.6mm, yields an I.D. of 27.4mm. However, the tolerance on the wall thickness and roundness of the seat tube made it so that you rarely could fit a 27.4mm post inside, even before brazing. And then, the seat tube always got distorted during brazing, making it even less possible to fit a 27.4mm in there, but a 27.2mm fit nicely. The same goes for why a 27.0mm seat-post was often used on the bigger sizes, which were made out of Columbus SP or SPX, whose seat tubes had 1.0 X 0.7mm wall thicknesses. Because of tolerances, the predicted 27.2mm post (28.6 – 2x0.7 = 27.2mm) never fit, but a 27.0mm fit nicely."

If yours is a large frame (say 60cm or bigger), the seat tube is likely Columbus SP or SPX, and the seatpost should be 27.0.
 

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Thanks Scooper

That article is interesting. My Bottecchia is made from Columbus Aelle, near the bottom of the Columbus hierarchy. I'm guessing that the exact ID varies down the length of the tube. I'll try a 27.0 and see what happens.
 

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MerckxMad said:
That article is interesting. My Bottecchia is made from Columbus Aelle, near the bottom of the Columbus hierarchy. I'm guessing that the exact ID varies down the length of the tube. I'll try a 27.0 and see what happens.
Seat tubes are almost universally single-butted, which means that most of the tube will have the same I.D. as the I.D. on the seat post end of the tube. The end near the bottom bracket will be thicker.

Aelle and Gara are the two eighties Columbus tube sets that are not butted, except for the steerer tube. The Aelle seat tube wall thickness is 0.8mm over its entire length.

28.6 - 2 x 0.8 = 27.0.

You can try 27.0, but because of wall thickness tolerances and heat distortion caused by brazing the seat cluster, you might have to go to 26.8.

 
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