The trailing edge holes in the top and bottom rudder ribs are extremely difficult to dimple simply because the flanges are so close together, thereby making it hard to find a dimpling tool that can get into the tight space.
In their wisdom, Vans have made it easier on us simple builders by providing factory dimples in the four aft-most holes each side of the top rib.
Awesome. Pity they didn't do the same on the bottom rib!
The first picture below shows how I dimpled the very end holes of the bottom rib. Using my pop-rivet style dimple die, and a heap of washers as spacers, I managed to get a half decent result. The only problem is that because the flanges are not parallel, then the die doesn't pull up square with the flange face.
I didn't want to do the same for the next two holes along, so instead I took my 10mm steel back-riveting bar and drilled a countersunk hole in one corner. This formed a fixed female die. I then inserted a short nail through the male pop-rivet style die and placed this through the hole to be dimpled, and the newly drilled hole in the back-riveting bar. A sharp blow with a mallet then formed a nice neat dimple.
