It is 26mm for sure! I remember measuring it to approx 26,1 including the paint. That is only 0,6mm bigger than a 1" , 0,3mm "on each side". You say we can not just fill it up with soldering? Where the toptube hits the headtube and seat tube (innside the lugs) the hole going trough the head and seat tubes into the toptube is only approx 10mm (+/-) so the top tube is kept in its place by the two other horisontal tubes.