seat post fit?

His best guess is that the seat tube was fitted upside down, so that the butt is at the top!

Isn't the butt supposed to be at the top of a seat tube? (sorry, awful dad joke)

anyway, on to the fettling.I now have callipers and I'm getting

• 26.5 as the inside diameter of the seat tube
  
• a range of 26.4-26.7 as the diameter of the main body of the seat post
  
• a range of 26.25 - 26.4 around the fluted area, varying wildly with the flutes.
  
  
  

all these are approx as the tubes are worn/scratched in places

It makes sense that the seatpost body is actually wider than the seat tube in places, because when I took the seat post out it had to be zig zagged hard out of the tube and scraped a noticeable amount of metal off the post.

I can't find a lip like Scilly's bike had so perhaps these old 531c Orbits really are 26.4. It's certainly not 27.7 unless the Maplin callipers are wildly out of whack, and we got similar measurements using grandad's old brass micrometer screw gauge.

Right now I'm leaning towards

a) it's supposed to be a 26.4 seatpost
b) the existing seatpost is either the right size or a 26.6 which has been forced in
c) if I can find a position where the existing post is ok, I'm probably best off using that while I look for a second hand 26.4
d) there's a good chance I'm not going to ride this bike to work on monday after all :(