The seatpost size for this frame is 25.4, and it is held in by 2 set screws in the back of the collar. The collar is integral to the frame. This seatpost is the Corima Ellipse, which is about 5mm thick all the way up (a blade, essentially) and was intended for both this frame and the Cougar, which was their track frame offered until a couple of years ago. Their new track frame is really nice, but has been redesigned extensively from the old Cougar it is still based on. The seatpost was a really difficult piece to find, as it didn't come with the frame and the bike had been out of production for several years when I found it. I actually had the post sent from a shop in Greece that actually still has some.