Main spar countersinking

I am finally diving into the main spars. The kit plans have us install nutplates for the fuel tank skin and access panel attachment points, and then countersink the screw holes to fit the dimpled skins/panels. They call for using a #30 or #40 pilot countersink, which will center well enough in the nutplate to get the job done.

My question is this: Why can?t I use my larger-pilot countersink to countersink the screw holes before I put in the nutplates? I think that they would center better and give a cleaner countersink. Is there a reason against this?

the depth of the countersink is such that a larger size pilot won’t remain centered when complete, as the spar thickness leaves no ‘extra’ material to hold it.

Wunderon said:

the depth of the countersink is such that a larger size pilot won?t remain centered when complete, as the spar thickness leaves no ?extra? material to hold it.

Click to expand...

Just as he said. The pilot on the countersink cutter engages nicely in the funnel/threaded portion of the nutplate to act as a centering device. Some builders have opted to create a jig to accomplish this. I followed van's guidance to use the nutplate and got quite good results. Push hard and spin the drill slowly for chatter free cuts.

Don't be alarmed when you see how much material you'll be removing. Those countersinks cut quite deep into the flange.

I was concerned about the knife-edging but then I realized that these are not taking rivets, just screws, and the countersunk hole is not really the structural part of the assembly. I did the top side of one spar tonight and I'm pretty happy with the results. Thanks for the quick replies!