Dan - Before you go shimming (you won't find a thin enough shim anyway) or reaming the seat tube, try something simpler. Often a frame's seat clamp ears will get squashed in, if they are thin unsupported stamped-lug ears, or if they are some of the Italian cast-lug ears that stick out too far, or if there had been a seat post in there that was too small. Maybe the 26.2 is too small but they used it rather than ream the frame when it was new. Anyway, try just gently wiggling the seat post back and forth, and side to side, with it in there only about 2", so that you are slightly expanding the seat lug area. You might have to go out and find a 26.4, but if that is what it takes, that is what it should have. What kind of frame is it anyway?