Wing countersinking...

[kbalch]

Question regarding Page 13-03, Steps 4-6 (due mostly to my countersink cutters having full-diameter pilots):

- Is it permissible to drill the screw holes to #27 & #19 (as appropriate, of course) prior to countersinking and then riveting the nutplates to the spar?

If it’s necessary for some reason (?) to do it exactly as described in the manual, I’ll acquire a couple of new countersinks with reduced-diameter (#8/#30 & #6/#40) pilots.

[flion]

I am guessing that these countersinks are in the spar or similar where a nutplate will be behind and dimpled skin over the countersink. I don't have the -14 plans but the -10 does this. The purpose is that the smaller pilots fit into the installed nutplate and center the countersink perfectly. If you attempt it with a large pilot by the time you have made your countersink deep enough, it will have thinned the material around the hole to the point that it may wander. You are probably going to need those smaller pilots sooner or later (like the countersinks for the nutplate rivets), so I'd go ahead and get them and then follow the directions.

[kbalch]

Originally Posted by flion

I am guessing that these countersinks are in the spar or similar where a nutplate will be behind and dimpled skin over the countersink. I don't have the -14 plans but the -10 does this. The purpose is that the smaller pilots fit into the installed nutplate and center the countersink perfectly. If you attempt it with a large pilot by the time you have made your countersink deep enough, it will have thinned the material around the hole to the point that it may wander. You are probably going to need those smaller pilots sooner or later (like the countersinks for the nutplate rivets), so I'd go ahead and get them and then follow the directions.

Nutplate rivet countersinks are #40 and the #40 countersink cutters have the right size pilot. What I want to do is simply drill the screw holes to their final size and then countersink them before riveting the nutplates. That?s how I?ve always done and never had an issue. I simply didn?t know if there were non-obvious reasons for doing it differently here.

[jibby212]

If I remember correctly, you will get a knife edge as the countersink is slightly deeper than the spar which allows the countersink to wander when the the pilot hole dissapears. The nut plate keeps the pilot centered.

[flion]

Originally Posted by jibby212

If I remember correctly, you will get a knife edge as the countersink is slightly deeper than the spar which allows the countersink to wander when the the pilot hole dissapears. The nut plate keeps the pilot centered.

That's right, and the smaller pilot keeps the countersink centered in the nutplate, so you don't need a backing plate or anything. The spar material is just not quite deep enough but it doesn't matter since the screw bears in the nutplate and the dimpled skin bears in the countersink and it all holds nice and tight. This would not work if you were putting a rivet there; you'd need thicker material to countersink in.

[kbalch]

I just spoke to Van's and I've got it now. My misunderstanding partially stemmed from the wording in the manual - I thought that it was calling for a #8 countersink with a reduced, #30 pilot. It's actually (perhaps even obviously, now) calling for an ordinary #30 countersink and never actually drilling the screw holes out to full size, but allowing the angle of the countersink to open up the holes for the screws.

So, my nice set of full-size screw countersinks will sit on the bench. Onward!

[flion]

Don't worry, you'll have use for the larger pilots.