First step…getting things set up in the shop. Here are the spars, laid out for countersinking.

Oooh…pretty!

Like a lot of other new wing builders, I agonized over how best to make nice, clean countersinks for the tank and access plate screws. Some folks claimed that the Van’s approach – attach the nutplates first, then use a #30 piloted countersink that eventually engages the nutplate hole – seemed to result in chattering and countersinks that weren’t smooth and concentric, because the countersink starts cutting with nothing to center it.

My friend Jim used an alternative technique – a jig with a #30 hole to center the countersink pilot. With this approach, the countersink pilot is supported all the time rather than just when the countersink is deep enough to engage the nutplate hole as in Vans’ technique. I decided to use the jig approach for the tank attach screws. Here’s what it looks like…

Tank attach screw countersink jig

How to make this jig? First drill the #30 hole in your scrap Al, then center it under one of the tank attach screw holes. Once the hole is accurately centered, clamp it in place and match-drill to the #40 rivet holes on either side of the screw hole. You’ll need two sets of holes in the jig – one for nutplates parallel to the spar, and one for the angled nutplates at the spar’s root end.

To use the jig, attach it to the spar with #40 clecos, then clamp the plate in place and remove the clecos. Then, countersink the hole to the appropriate depth. The positioned plate and countersunk hole look something like this…

Clamped countersink jig

The first 2005 issue of the RVator has some improved instructions on how to countersink these screw holes to the proper depth without going too far. I strongly recommend you read up on this procedure before countersinking your spars…you might just save yourself some serious heartache.

The finished product. Looks good!

Finished countersinks