Thanks Andy. I am going to drill and dimple in the same way as the F1018. Page 29-17 shows that they have to be done at some time and leaving till after it's riveted in place means you would have to use a pop rivet dimpler and probably have to grind down the die to fit against the flange. As for the intercostal, those holes can be countersunk through the 2 layers.
Right - build on!
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