As I was recently seeking advice on a new seat post, a number of list members suggested that my bike (a PX-10) should require a 26.4 mm post rather than 26.2. It turns out you were right. The seat lug opening was egg-shaped; too small fore and aft, and too wide across. The existing seat post was obviously making contact front and rear but not on the sides, and a micrometer confirmed the facts. The original dealer had evidently selected the post to fit the smaller dimension rather than addressing the real problem.
Merely prying the ears apart would not help since this would tend to further widen the transverse diameter, while doing little or nothing to increase the longitudinal diameter. The proper solution would no doubt be to ream the seat lug, and it may still come to that, however, I devised another method that proved to be at least a good first step. I took a piece of 1-inch oak dowel, about seven or eight inches long and cut it in half lengthwise at a slight angle to centerline, thus forming a pair of wedges. This split dowel was inserted into the seat tube about four inches or so, and one side tapped down with a hammer, thus exerting force from within to expand the lug in the desired direction. I was hoping, of course, to reshape the opening without increasing its overall size and without damaging the exterior surface of the lug. Here is a link to some pictures:
http://www.flickr.com/
Once the wedging was complete I simply tapped down the opposite wedge and removed the pair. The operation was conducted with a good deal of caution, but in the end I was able to insert a 26.4 mm post. The micrometer shows the opening to be still a bit wide (nearly 26.9 mm) and just barely adequate longitudinally (26.4 mm). The seat post I was using measured slightly less than 26.4 mm. I might repeat the treatment to gain a little more improvement. I also applied the Dremel tool with various sanding and grinding attachments to eliminate burrs inside the lug.
I've never had any problems with seat post slippage, but some of the seat post problems described in recent postings may be due to out-of-round lugs, and the unwitting use of posts that are actually too small. If the post was highly polished and well greased, the interior of the lug very smooth, and the rider pretty hefty, I could see how post slippage could be a problem.
John Hurley
Austin, Texas, USA