There are a LOT of holes to countersink in the spars, I couldn’t face counting … but there are over 1000. You have to carefully study the manual to ensure they are machined to the correct size. But it really boils down to 4 types:
- Flush for a AN326AD3 rivet – for nutplate rivets & holes at some of the rib locations where the tanks fit.
- Flush for a AN326AD3 rivet + 0.007” deeper to accommodate the 0.032” skin
- Size to accommodate a #8 screw dimple, max outside diameter/min inside diameter specified in the manual. These are for wing tank attachment.
- Size to accommodate a #6 screw dimple, max outside diameter/min inside diameter specified in the manual. These are for access panel attachment.
Skin & Nutplate Countersinks
The skin & nut plate rivet locations are straightforward, just loads of them! This picture shows a countersink allowing for the wing skin, which is flush + 7 thou, as advised by Vans in Section 5 of the manual. I used the micrometer countersink tool to do these, checking each hole with a rivet head to make sure the depth wasn’t wandering. You rapidly attain an eye for the correct diameter countersink : )
I had spot primed the nutplate countersinks in anticipation of riveting the nutplates prior to countersinking, but I changed my mind as you’ll see below!
Screw Countersinks
Vans recommend these are done by first riveting on the relevant nutplate, then using a microstop countersink tool. The idea is that the nut plate thread acts as a locator for the countersink pilot. I’m sure this works, but I didn’t fancy it much. The problem is the countersink has to be quite deep before the pilot engages in the hole. There is a lot of metal to remove, and I could see it being full of jeopardy. Other people have suggested making a template and using this as a guide for the countersinking. That’s what I did too, using some scrap aluminium angle. 
I made templates to replicate the #8 & #6 nut plate geometry, drilling the pilot hole locator with a #19 & #27 drill respectively to fit the countersink pilots. The spar holes also had to be final drilled with these to allow the countersink pilots to fit.
Aligning the Jigs
I aligned the jig in each location by centering with the relevant drill bit, and also inserting rivets. Once happy, clamps held it in place.
Finally I again used the microstop countersink tool to machine the spar. I carefully set up the depth by machining a scrap piece of aluminium angle, and then measuring the diameter of the hole to keep within the tolerance specified in the manual.
I ended up deciding on diameters of 0.360 & 0.298 … this seemed to allow a good fit of the screw size dimples in a piece of scrap 0.032″ skin. The inside diameter is automatically governed by the outside diameter of course, and these values ensured the inside remained well within the tolerance. I set my digital calipers to the required diameter and checked the diameter of each contersink as I machined. I also used suitably sized drills to double check the inside diameters.
Machining
I used the microstop tool in an ordinary cordless drill, using a low speed to start the cut and speeding up at the end. Applying a higher pressure at the end also seemed to help stop chattering and produce a consistent depth. It all took quite a long time because of having to check the max diameters were not wandering! Eventually all were done … phew!
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