Mark Stonich wrote:
> Even "Standard" 531 seat tubes would accept a 27.4 post before brazing.
> Brazing, even with silver, would always shrink the ID somewhat. A
> really skilled builder like Chris Kvale would often get so little
> shrinkage that a 27.2 wouldn't work, so he always kept 27.4 posts on hand.
It's not "shrinkage" but distortion. When you braze the top tube and seat tube into the seat lug, the heat softens the metal and causes it to expand. Since the top tube is held at both ends, this expansion pushes the top tube into the seat tube, causing a dent that effectively reduces the inside diameter. You also see this at the other joints, but since they don't have to accommodate a close-fitting insertion like the seat post, you don't notice it as much. If you remove your fork and headset you can often see a similar effect from the head tube milling to accommodate the headset cups -- more metal tends to be removed from the side of the head tube where the top and down tubes attach than on the front of the head tube.
BTW, it's fairly common practice to run a straight reamer in the seat tube to shave a little metal off the distorted area, making it a better fit for the seat post.
--
-John Thompson (john@os2.dhs.org)
Appleton WI USA