Originally Posted by TejanoTrackie

Actually, just 0.2 mm can make a big difference. That's why seat posts are sized in 0.2 mm increments. Also, just because the manufacturer specs say the seatube ID is a certain number, doesn't mean they are always correct. As stated in my post above, I got a frame that was spec'd at 27.2 mm and turned out to be 27.4 mm. No matter how hard I tightened the seat clamp on the 27.2 mm post, it just kept slipping on me. The 27.4 mm fit perfectly (snug but not binding), and the clamp did not require excessive tightening.