Originally Posted by randyjawa

When measuring a seat post cavity, I measure in five locations, add the fire measurements up and divide by five. This method helps to account for any ovality that might have occurred during years of use. When installing a seat post, I install one that is tight, not loose at all. A slightly loose post will usually alloy the clamp assembly to distort and, more often than not, the post will slip.

Before measuring, I use emery cloth to clean the inside of the seat post cavity. Up and down, round and round, and repeat as often as necessary to get things clean. If there is any ovality, I try to eliminate it as best I can. Usually, the distortion is around the clamp ears. Finally, I use a small smooth round file to clean up the slot in the seat tube. I try to slightly camphor the edges of the slot, reducing the chances of scoring the alloy seat post. With all that done, I find the tightest seat post and install it. A proper sized seat post done not have to be super duper snugged up. Super duper snugging will, often times, just bend the seat clamp bolt and/or distort the clamp ears

It all might sound like a lot of work, but a half hour's effort will, usually, do the trick. When ever attempting to fit the post, always ensure that it is greased, even for trial fits. Best of luck.