This post was supposed to be titled countersunk HS spar. But in fact I screwed it up so badly that I had to order a new part from Vans. The issue was that I needed to back the spar with something ore substantial in order to keep the countersink guide pin from wandering. As a result the hole became elongated and too large.
I was, shall we say, perturbed. Not really any guidance in the manual about this. Oh well, as long as the piece was a write off, I experimented. Friends who built RV-7s used a dimpler for this application. So I tried one, but it was just too thick to work well. I did try backing the spar with a piece of metal, and some holes in the workbench top. Much petter, but not perfect. Will experiment some more when I have time. However the next evening or two need to be dedicated to day-job projects.
You can see the issue in the photo below. The hole with the rivet is clearly not the appropriate size (used to be), and the one just to the right of it is oval-shaped. You can click on the photo to see a much more detailed view of the issue. (Warning, 4 MB)