What are the outer diameters of the seatpost and the seat tube? The smaller the difference between the two, i.e., the thinner the tubing wall, the more likely you have a butted frame (double-butted top tube and downtube, single-butted seat tube). Typical numbers for d.b. Reynolds or Columbus would be 27.2mm for the seatpost and 28.6mm for the seat tube, indicating a tubing wall thickness of only 1.4/2 = 0.7mm. A straight gauge Reynolds or Columbus frame might take a 26.4mm seatpost, for a tubing wall thickness of 2.2/2 = 1.1mm. A plain carbon steel frame may take an even thinner seatpost, with correspondingly thicker-walled tubing.
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"Far and away the best prize that life offers is the chance to work hard at work worth doing." --Theodore Roosevelt
Capo: 1959 Modell Campagnolo, S/N 40324; 1960 Sieger (2), S/N 42624, 42597
Carlton: 1962 Franco Suisse, S/N K7911
Peugeot: 1970 UO-8, S/N 0010468
Bianchi: 1982 Campione d'Italia, S/N 1.M9914
Schwinn: 1988 Project KOM-10, S/N F804069