Question on wing skin prep

I'm getting ready for a marathon dimple/deburr/edge prep session tomorrow and have been looking over the drawing to make sure that I dont make any assumptions. I've tripped over something that doesn't make any sense. On DWG-12 around 6F is a notation that points to various holes on the inboard edge of W-702.

Do not rivet. Leave open for root fairing attachment (see dwg-38)

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Of course I dont have dwg38 yet, but I do have my preview plans. On DWG-38 there it says at approximately 1B that I should:

Machine Countersink

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and it points to the wing skin W-702. I have no idea what material makes up the wing root fairing, but the wing skin is to thin to machine countersink. Is this notation meant to point at the fairing for countersinking and leave those empty holes in the skin un-dimpled and un-countersinked?

Gil,

If I understand your description, you are referring to the W-702 inboard (top) skin. These holes are left un-dimpled. There is a transition strip made of aluminum that fills the gap between the wing and fuselage late in construction. That strip lays over this area of the wing and is countersunk into both pieces. This way it's not too deep for a single skin. So yes...undimpled and uncountersunk at these locations of the wing.

Thanks for the clarification, Sean.

I dimpled both

For the wing root fairings I dimpled both the fairing strips and the wing skins for (IIRC) a #8 flathead screw. I don't remember the exact part number for the screws. Throughout the airframe I also dimpled for the flush rivets (AN426) used for attaching nutplates. This causes the nutplates to stand off the inner surface of the skin maybe .010" or so, but doesn't cause any assembly issues and doesn't remove any metal around the hole (vs c'sinking). The nutplates used for the #8 screws have an adequate c'sink built in for plenty of clearance for the dimples in the skins. Hope this helps.

+1 for dimpling both.

However I did not do the dimpling during wing construction. Just do what the instructions tell you now. Leave the holes open.

Its easy to dimple and use C-Sunk nutplates when working on the fairing strips later.

would like to dig out this old topic...

is there any specific reason not to dimple the W-702 top wing skin to F-799 wing root fairing holes? drawing 38 clearly states to machine countersink these holes using K1000-08 platenuts. The guys at Van's engineering usually do such things for a reason but i don't see why. I think that to dimple is always the better option as long as you can. Any thoughts...?

Not dimpling serves as a good guide for which holes to use to attach the wing root fairing. It doesn't really matter if you dimple the holes though because the number 19 drill you'll use for the k1000-08 nutplate and the coutersinking will take away all of the dimpled material.

If you dimple the rib flange, wing walk doubler & top skin, use K1100 nutplates.
If you dimple top skin, countersink wing walk doubler & left rib flange flat, use K1000 nut plate.
If you left rib, doubler & skin flat, you could countersink through and use K1000 nutplate, would work too, not optimal situation for top skin but workable as double rivets for nutplates provide strength.
Vans didn't treat these screw attach spots on the inboard tank ribs the same way as they needed max strength & sealing properties there.