I began countersinking the right spar top flange. The aft edges of the fuel tank skins screw into the wing spar. Basically you've got #8 screws, which sit flush in the tank skin, which is dimpled. The screws go into nutplates which rivet to the inside of the spar flanges. The skin dimples need to sit flush against the spar flange, so you have to countersink the screw holes.
Here you can see a scrap of .032" sheet with #8 and #6 dimples in it. I used the #8 side to make sure the dimpled tank skin will sit flush against the spar flange.
The instructions called for using a #30 pilot countersink with a microstop. Problem is, the #8 screw holes are already much bigger than #30, so the countersink bit has no reference for the "center" of the hole. And as you countersink deeper, it gets worse.
So, I decided to drill out the holes with a #21 drill bit first, then use a #20 pilot countersink. That worked "better", but it still leaves a lot to be desired. The countersink for the #8 dimple needs to be so deep that the bit loses a hole center reference. I'm not sure what the best way to handle this will be. Might just have to do with how I'm holding the drill or the microstop or something. I'm gonna check the RV List archives and ask around to see how to make these machine countersinks perfect. Until tomorrow...
Nope...until tonight... As it turns out, this is a very common issue. I searched the archives and found two suggestions that sound viable: