Originally Posted by
VintageSteelEU
That's some interesting information there. Of course, it's a bit naive of me to assume the seat posts would be all manufactured with total precision. The reason why I decided to ask people for the measurements of their seatposts is primarily so I can make an "educated guess" as I couldn't find official information anywhere. Looking at the seat post today, I think the top of the seat tube slot / cut out goes a bit wider than the bottom (will have to measure that later). That would mean the seat post itself might be a bit too wide. Attached the photo (red bike).
...the other thing that happens, is that sometimes people will take a seat post that is a little oversized (in this case 26.8), and reduce the diameter of the inserted portion by sanding. I have done this myself, either because the only seat post I have handy is the slightly larger size, or to achieve a better fit in a seat tube that feels a little large, and the slip fit is a little loose, using the smaller size.
But certainly many bicycles get something tooo big jammed in there, by owners who don't know any better. Best of luck in the removal. The best penetrating oil for this is a 50/50 mix of acetone and automatic transmission fluid.
And if you have, or can access, a large, firmly mounted bench vise, clamping the post in the vise and twisting the frame off (two people is better...one twists the frame, while the other pulls on it in the direction off extraction), works pretty well in many cases. The post will be ruined, but you wouldn't want to reuse that one anyway.