Next on the list was to countersink the spar flange for the #8 screws that attach the fuel tank skins to the spar. While Van's calls out a .007" sub-structure countersink for dimpled skins with rivets, they don't really say much about what is appropriate for something held together by a screw. Looking on the VAF forums, this has obviously been a head scratching moment for a lot of people. Some people just make the depth such that a screw head just sits flush in it, but that doesn't really let a dimpled skin seat well enough. Other people have gone so extreme in countersinking too deeply that they've had to work with Van's to engineer fixes, which don't sound like fun. I finally found a reference to an old RVator article (an old newsletter written by Van's, but no longer published unfortunately) that mentioned the countersink diameter for a #8 screw in the spar flange. The say the diameter should be between .365-.375". They're instructions said to go flush and then 2 clicks of the countersink cage beyond that. As many people noted in response to that though, that 2 clicks doesn't get you anywhere near .365". So the bottom line is I need to stay under .375" and anything under that is fine if I think the skin will seat okay. I finally landed on a happy medium that gives me a little bit of a countersink halo around the screw head. I tested with a scrap of dimpled aluminum, and I think it'll work okay. I measured it and the diameter is around .350"ish. Worst case scenario is that when I mount the tanks I'll find that the dimples aren't deep enough and I'll have to redo the countersinks a little deeper. That wouldn't be the end of the world.
Anyway, after a lot of nervous back and forth with the cage settings, I started. I had set the cage to allow for the thickness of the tape - the countersink cages have a metal foot that easily scratches whatever they are up against. Just like when countersinking the thin material on the elevator trim tab spar, the pilot of the bit doesn't have enough material to guide it through. That makes it chatter and create a slightly oblong hole. Van's calls out using a slightly undersized pilot cutter though, with the idea that it'll actually fit into the threaded hole of the nutplate once it gets deep enough. Sure enough, it worked pretty well. Things would chatter like crazy until the pilot reached the nutplate and then it would center and smooth out the last small bit of the countersink. This still creates a knife edge on the material because the size of the hole is bigger than the material thickness allows, so I used a small round file to knock off the sharp internal edge of each hole. Lastly, I hit each hole with primer, flipped the spar, and then did the exact same thing to the other side.
