Here I’ve drawn out the rough cut of the end ribs of the AUX tanks.

Drill stress relief holes at the corners of the tabs where the bends come together.

I rough cut the rib blanks on my old band saw with a metal blade.

Next I finished to final size and smoothed all of the edges.

To begin forming I used a brake to bend the bottom flange. This gave me a straight and consistent flange to register the form on for the rest of the forming process.

Here I have clamped the rib form block onto the rib and bent the aft flange over to give me a solid vertical edge to register the form block against.

Now to the vise to finish bending the fore and top flanges.

Here are the four AUX tank ribs after bending all the flanges. As with the wing ribs, the top curved flange caused the rib to potato-chip in shape. I used a shrinker to flatten the top flanges and straighten the rib.

Once I had the ribs finished, I used them to calculate the dimensions of the tank bottom and end bends to achieve a close fit of parts. You can see here that I made the bottom as a one-piece part to eliminate seams at the ends of the tanks.

Here one tank is held together with clamps for further deliberations on how to rivet the parts together.

I inspected the plans and the tanks of a RV-8 for guidance. The RV used -4 rivets at a 3/4 inch pitch on the tank end ribs. I used the 3/4 inch pitch but I staggered the line a bit. Since I formed the flanges 3/4 inches wide to increase the surface area for sealing, I thought staggering the rivets would help spread out the gripping forces a bit. I reduced the pitch a bit a the corners where I assumed the stresses were more concentrated by the straps holding the tanks in the wing.

Here a tank is clecoed together before enlarging the holes to #30 and dimpling.

For the AUX tanks I will probably use AN bulkhead fittings to plumb the lines to pump the fuel to the main tanks. However, for the main tanks, I will use the bungs as specified by the Bob for the gravity system. To facilitate riveting the bungs to the main tanks I turned the flanges off of the bungs to have enough space for the rivets. Here is a before and after picture of the modified bung.

That’s it for now. I’ll add more when I get the Pro-Seal off of my fingers, hair, nose, etc.