Is there a rule of thumb for determining seatpost diameter for frames based on vintage, country of origin, etc? I've seen frames of similar vintage (mid 80s) with different seatpost diameters. Basso - 27.2; Merckx - 27.0; Pinarello - 27.0. I was trying to determine seatpost size for a Torpado frame that is probably of a similar vintage, but the 27.0 seemed too big. Are there any rules that make determining seatpost diameter for frames easier?

Thanks in advance.

If you know the tubing that can help, but even that's not a guarantee. There's always Sheldon's database, if yours happens to be on the list.