Working on the spar begins with the installation of nutplates for the fuel tanks. Each nutplate position needs three countersinks: two for the rivets and one for the screw. There are 150 nutplates.
The countersinking for the rivets goes very smoothly, fot the screw it's a bit different as the aluminium is not very thick, the pilot wanders on the other side. So I built a small jig with a steel bar and two rivets, it's visible on the picture and just held in position by a clamp.



The good depth of countersinking is reached when the outer diameter is 0,370 (+ or - 0,005) inches. I think I got his one right.