I continued countersinking the wing spar flange anchor nut installation. Van's suggestion to use the anchor nut itself to keep the countersink centered did not work well. The threads of the anchor nut are too far down for the countersink pilot to reach properly. After the first countersunk hole turned out poorly I switched to the Dan Checkoway method as shown in image 1. A 1/8 inch piece of aluminium angle drilled as a template worked well. After a further 5 hours all 120 anchor nut that will eventually hold the fuel tank on are installed. In image 2 you can see a top and bottom view.
