It sounds like the ID of the seat tube has been distorted. The two common causes are fitting a too small post and heat from the brazing. Any burrs add to the challenge of finding the correct post, but help hold a post like studded tires help on ice.


So now I would let someone with real experience have a look at this frame. Sure you can try different posts but until you fix the root cause you're just treating symptoms and not the real issues. There's a good chance the binder ears will get damaged from trying to keep a wrong post tight. Andy.