Originally Posted by
T-Mar
I don't believe the seat post is undersize by much, if at all. Looking at the picture, the problem appears to be overtightening of the cinch bolt, to the point where the mounts have deformed/bent and are touching each other. If you look at the cinch slot itself, there is still a very noticeable gap at the top, about 2/3 the width of the slot at the bottom. Typically, cinch slots are cut about 3mm wide, so this one appears to be about 1mm narrower at the top. Even with a correct size post, proper tightening will result in a cinch slot narrowing by about 0.6mm.
So, the frame might accept a 27.4mm post but it doesn't look like it would accept 28.6mm (and it definitely won't take 31.6mm), which are the most common sizes for oversize frames. 27.4mm isn't a common size, so it was most likely designed for 27.2mm. The current post might be slightly under its listed size or the lug may have been aggressively reamed.
I've had two of those Nishiki Internationals with the oversized tubing and both took a 27.4mm. The catalogue states they are 27.2 but I suspect some overly excited reaming in the factory has caused them to be too big. Both frames had cracking in the lugs/tubes from the clamps being tightened too much with the stock 27.2 in there. One I had repaired and later sold, the other I got used and fitted a shim right away so the cracks haven't spread far enough to weaken the clamps.
Would you like the serial no. of the second of my frames? I still have it hanging in the garage awaiting better days.