Joined: Jul 2007
Posts: 8,586
Likes: 3,518
From: South shore, L.I., NY
Bikes: Trek Emonda SL7, Cannondale Topstone, Miyata City Liner, Specialized Chisel, Specialized Epic Evo
Research says it should be 27.2 but your measuring tool says 26. I would be questioning the measuring tool and getting a digital caliper. You do not want to force a too large seatpost. FWIW, Sheldon’s site lists every Serotta using a 27.2, so possibly a prior owner used too small a seatpost and forced the tubes together.