The floor stiffeners need to be countersunk. the F772 forward bottom skin is 0.040" thick and will be dimpled. Important on those is to make nice and 'coin' dimples as the material is that thick.
That also means that the countersinks in the material will be a bit deeper than usual because the skin dimple is bigger. I made a sample dimple piece out of 0.040" material and hit some crisp dimples in them to be able to test the countersinks I'm making. The test part should not wobble in the countersink but also, the minimum criteria is the inability to slip a .002" feeler gauge between the mating surfaces of the skin and the stiffener.
All holes get countersunk except for the most forward one on the firewall and the most backwards one on the F704. These overlap the 704 and the firewall so you don't need to dimple or countersink those.
I've setup my countersinking location with some blocks of wood and a single clamp.
After 3 hours work, finalized all 4 stiffeners. They all came out very nice with no oval "moon" holes or enlargements.
Usually if you see this amount of shavings on your work desk, you can tell yourself you had a good days work :)
