Countersink or dimple

Question on the sixteen screws on the bottom of the wing that attaches the F976 center section bottom skin to the W904 l and r bottom skins and the inboard root rib.are these holes dimpled or counter sunk?holes are for the 8r8 screws and plate nuts that get riveted to the wing skins and rib.Thanks for the help

32 or less, dimple, over 32, countersink. but, be sure to back it up with material otherwise the pilot will chatter and the hole will look like it's been carved out with a chisel.

Dimple! To countersink for those screws you would need vey thick material. The .032 miminum is for -3 rivets I believe!

Future RV 9 Flyer said:

Question on the sixteen screws on the bottom of the wing that attaches the F976 center section bottom skin to the W904 l and r bottom skins and the inboard root rib.are these holes dimpled or counter sunk?holes are for the 8r8 screws and plate nuts that get riveted to the wing skins and rib.Thanks for the help

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I'm building an RV-6A and it's the same design, overlap of fuselage bottom skin to wing inboard skin and wing inboard-most rib so I understand what you're asking. My answer is to dimple for the screws and for the platenut rivets. You can dimple the platenut wings to match the dimpled rib; you may have to grind material off the female dimple die to clear the center of the platenut where the threaded portion is. You'll need dimple dies for the #8 screws in addition to those for the 3/32 rivets. I don't know about the RV-9 but for the RV-7 there's a detail view called "Wing Root Fairing Installation" on dwg 38.

Here are a couple pages from the construction manual circa 1997... I circled some paragraphs on pages 5-2 and 5-6 having to do with dimpling.

Here is a snip of RV-7 dwg 38.