Originally Posted by
FBinNY
It's VERY simple. If a 27.2mm post fit at all, it's the right size and a 27.0 will be too small. It is physically impossible to insert a post which is 0.2mm oversize.
The tightness may be because of ovalizing at the seat clamp area, or simply a tolerance issue. FWIW- the tolerances for seatposts are that the tube should be the nominal plus 0.00-0.05mm, and the post should be the nominal minus 0.00-0.05mm. This kind of tolerance is designed around reasonably achievable tolerances, ensuring that the post will fit without jamming or being sloppy.
Think about this as a 2 lane road with a double line down the center. The nominal is the double yellow, and the fog lines are the extreme limits. As a practical matter makers try to stay centered in their "lanes" without crossing the centerline,or running off onto the shoulder.
In your case, it's a tight fit because one or both are crowding the double yellow.
If you are uncomfortable with the fit, you can consider reaming the frame or trying another post, but do not ream the frame before confirming that the post is on spec. I see too many instances of oversize seat tubes, which is a far more expensive issue than buying another seatpost.
Very clear and logical explanation! In the meantime, I did an online search and found in one site that states "A rule of thumb is that the widest post that slides in without being forced (other than pushing or twisting by hand) is the right one. If it drops in, with play, before the pinch bolt is tightened, the seatpost is probably too narrow." This supports your logic. I am guessing 27.2 mm is the right size for me. Either the seat tube has some corrosion or impurities inside, or perhaps the tolerances of the seat post is not that great. I bought it from Amazon for about $20 (FOMTOR Bicycle Seat Post). I will check inside the seat tube as well as measure the seat post diameter to confirm. Thanks very much!