Countersinking the spar

I have started countersinking the main wing spars for the nutplates that hold the tank.

In the RVator, they says #8 (8R8) screws should have a outer countersink diameter of .365" to .375"
I did one to .370", but boy... what a crater !

First of all, the hole gets enlarged way too much. Not that this is critical as the hole just serves as 'pass through' for the screw to fit in the nut. But still, I consider more material being left behind stronger.
Secondly, I dimpled a #19 in a similar size skin as the tank skin and fit the skin in the countersunk hole. If you have .370, the skin will wobble a little left and right. In other words, it doesn't sit tight in the countersunk hole.

I started testing by myself on different dimensions and figured out that .350" was about the maximum size before the tight fit would be lost. Later on, I found some engineering and military specs that say that for a 3/16, the countersink outer diameter is 0.351 which confirms my findings.

Is there a reason why in RVator they say .370 ?

A second question.
The enlargment of the hole creates a knife edge. Do you guys use sandpaper of deburring tool on these or will this not be a thread for the screw? My fear is that using the edge deburring tool will even more enlarge the hole.

The caliper that I had at the time only went to hundredth's, but I used .37 for mine...flies just fine.



You can deburr the hole with a deburring tool to get rid of the razor sharp edge. No harm done there. The strength comes from the screw going in to the plate nut, not the material you are cutting away.

All of mine were .365-.370".

If you don't countersink deep enough, the dimple pushes the material up away from the spar. Consider: the countersink has a sharp edge at the top while the dimple has a slightly rounded edge. So you need to countersink a bit wider so the dimpled material does not ride on the radius instead of settling onto the underlying material. Similarly with dimpled and countersunk material for riveting: you want to try a couple of tests so that the countersink allows the dimpled material to "pull in" as the rivets are set rather than either being so shallow that the material doesn't nestle well or so deep that the dimple doesn't seat well in the countersink. Countersinking for screws is a little deeper than for rivets because the bearing surface between the screw and dimple is what's important while for rivets you want good mating surfaces all the way through.