oh sweet mercy, never force a seat post down. I did that to my old viscount with a hammer, got it 2 inches in, realized it wasnt going to make it, then couldnt get it out. i had to cut the post then saw it out of the frame with a hack saw blade, it took many , many hours as sheldon said:
"The three rules of hammering seatposts:
- Never install a seatpost with a hammer.
- Never, never install a seatpost with a hammer.
- Never, never, EVER install a seatpost with a hammer!"
yeah, i wish i read that first.
you probably don't need/wont find a 26.8mm carbon post, and if it is a vintage bike, a carbon post would be a one way ticket to tacky/lamesville