What model '85 Pinarello is it?
The only '85 model that didn't use SL or SLX tubing was the Catena Lusso which used Aelle R tubing and took a 26.8mm seatpost. All the other models (Montello 85 SLX, Super Record Special, Treviso N.R., Treviso Victory, and Triathlon Victory) took 27.2mm seatposts.
Surface rust inside the seat tube can make it damn near impossible to get the proper size post inserted. Every few years, I use a Cyclus seat tube reamer on the seat tubes of my frames that take a 27.2mm seatpost to clean up the internal surface rust and ensure the seat tube is round.
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- Stan
my bikes
Science doesn't care what you believe.