Every build log I've read calls cutting these holes an "iterative process" Vans instructions are equally vague, and just say something to the effect of "hole size as needed so that he pushrod doesn't rub throughout it's full range of travel."

So, armed with these less than precise instructions, I started by attaching the heim joints into the inboard flap rib. Then I rotated the flap up until I could see where the slot in the fuselage side needed to be to accommodate the shaft that screws into the rib, as well the approximate size of the hole that needed to be cut into the fuselage bottom to allow the body of the heim to tuck up inside the belly. The location includes the slot already existing in the side skin and the rivet hole you are instructed to leave open in the betty during fuselage assembly, so they are good starting points, but at least in my case the holes didn't end up centered on these pre-existing locations.

I marked my crude dimensions on the skin with a sharpie, then swung the flap back out off the way and with combination of drill bits, unibit, and a chainsaw file, crept up on the lines, pausing often to swing the flap up and check for clearance.

Once the flap would retract fully, I installed the pushrod and slowly extended the flaps a few degrees at a time. The hole in the belly needs to be sort of an odd oval shape because the pushrod moves laterally as well as forward as it extends. Every time I could see where rubbing was imminent, I would unhook the pushrod, swing the flap out of the way, and file a little bit away.

This went on for, no joke, probably 3 hour or so.

a few details to mention;

I started the process by installing the wingtips and ailerons so I could confirm that when the flaps were nested up against the belly they were approximately in line with the ailerons.

Van's standard part for the pushrod is a length of aluminum tube that the builder cuts to length and taps threads into for the heim joints. All well and good as long as they are made correctly. However, there have been a couple of instances of the tubes failing either because the relatively thin tube wall was rubbing on a too-small hole in the fuselage, or because they were drilled and tapped crooked leading to a thin wall and a failure at the threads. A failure of one of these rods can lead to flaps binding up or worse. According to folks on VAF, there has been at least one instance of an aircraft wrecking on the runway during landing because of a split flap condition caused by an improperly made/installed pushrod.

I believe I could make and install these rods properly and not have a problem, but a couple of years ago, I happened to be ordering something from I think Cleveland tools, and saw that they had the more robust hex shaped pushrods on clearance. They are beefier, and are pre-tapped on both ends. So for $16 bucks or so, I picked up a set and they had been in a drawer in my tool box ever since. It was really nice to be able to skip making this parts and not have to worry about it. However, they're also are a bigger diameter than the simpler fabricated pushrods and require a bigger hole.

Anyway, I finally got a hole that allowed about 1/8" of clearance with the pushrod thru it's full range of travel from the flaps zero position (where it's trailing edge is aligned with the aileron), to a full 45 degrees down without rubbing, as measured with a cheap digital level taped to the flap at mid span. This isn't as precise as I'll rig these before first flight, but good enough for this stage.

Vans says the max flap travel should be 40-45 degrees. Even though I have good travel all the way to 45 degrees I will likely limit these to 40 degrees to prevent the other problem I've seen personally on my buddy's rv6.

When the flaps are fully extended and somebody steps on the wing walk too far aft, it can pop the trailing edge of the wing skin down behind the forward edge of the flap. When this happened to my friend, he didn't notice it, retracted the flaps and bent both the leading edge of the flap and the trailing edge of the wing. Limiting full down flaps to 40 degrees allows more overlap of the trailing edge wing skin over the flap leading edge and should eliminate this possibility.

Like most things with this project, if you have to make two of something, the second one goes a lot faster than the first. Once I understood what the hole should look like and the approximate size, in addition to my arsenal of hand files, I broke out a 1/2" rotary file chucked into an electric drill and it only took about 1/2 hour to knock out.