Countersinks and Plate Nuts in Spar Flange

This is a wing spar with the attach points on the flange for the fuel tank. The fuel tank skin will be dimpled to fit into the shiny countersunk holes with screws holding to the plate nuts behind the holes. This is one of those steps that everyone worries about and tries to improve on Van's technique. I have to admit I was nervous drilling into that beautiful gold anodized spar. I followed Van's instructions exactly and they came out near perfect. I used the micrometer that Peggy and Stephen gave me to measure the outside diameter of the holes with the target being .365-.375 and they were .370! The more I build, the more I realize that Van's instructions work great and that I'll need a compelling reason to deviate from them.

This is a plate nut that's behind one of the countersunk holes you see in the picture above. A plate nut is simply a nut for screws or bolts that can be permanently attached to the underlying surface. This one has two wings on either side of the threaded nut with holes in them that are riveted to the flange. There are quite a few different flavors of plate nuts like ones with one long wing with both rivet holes on the same side or 90 degree corner ones but this is the most common style.