Frame tubes are normally described by the thickness of their butted sections. Thickness is represented in 10ths of a millimeter. Reynolds 531 double butted ("competition") frames use, typically, 8/5/8 db tubing with a 27.2 seatpost. The 8/5/8 moniker refers to the both ends of the top tube, and downtube. the seat tube is typically not butted at the top (e.g. it's 5/5/8) because the seatpost creates the missing butt.
Reynolds 753 by the way is 7/4/7, so it's 0.1mm thinner, hence it uses a 27.2 + .1 + .1 = 27.4mm seatpost. Reynolds 531 Professional frames have similar dimensions and presumbly much more noodly handling (?)
You'd expect a plain-gauge frameset such as a Raleigh Super Course to use thicker tubing, the equivalent of 8/8/8 since the seat tube is not butted. This means the seat tube would be 0.3mm thicker at the top, and hence, 27.2 - 0.3 - 0.3 = 26.6 mm.
On the other hand, heavy-duty frames ("531 super tourist") are often 9/6/9, and so 531 plain gauge might be 9/9/9, which would give 27.2 - 0.2 (0.9 mm thick) - 0.3 - 0.3 = 26.2.
So you kinda shoulda maybea mighta been able to guess from the above information, that 27.0 would be an impossibly small change, going from butted to unbutted tubing, for the seatpost.
- Don Gillies
San Diego, CA, USA