This is one part of the project a lot of people dread. The sealant used, usually referred to as 'Proseal', is also known as the black death. Sealing the tank is a rather messy job, and at first is completely frustrating. After working with it for a few days, it is definitely getting easier to work with.

This is the inboard rib and accessories, capacitive fuel sender plates, and capacitive fuel sender tank wiring. The picture on the right is of the right tank internal rib and flop tube, part of the inverted tank modifications. I am only installing these modifications in the right tank. They help keep fuel in the most inboard bay during aerobatics.

The first step was to rivet the stiffeners in place. It takes around 40 grams of sealant to do a complete row of stiffeners. This can be done in one batch if you first tape all the rivets in place and apply sealant to only the stiffeners. The fuel cap and drain flange were next. The the tank was then clecoed together to move onto the ribs. Note that the drain flange and inboard stiffeners need to leave access for water to flow to the drain.

We only had enough time to do one tank rib at a time before the sealant became difficult to work with. It also took 40 grams of sealant for each internal rib. Some tricks we learned:

- Add rivet tape over the rivets similar to back riveting. This keeps the sealant off the rivet gun and makes it a whole lot easier to shoot.
- Another trick we learned was to clean up all the goop on and near the rivets inside the tank before bucking. This makes it much easier to buck and see the shop heads. Once finished, just add some goop back.

These pictures are a bit out of order as far as the timeline goes, but here are the tank internals from inboard to outboard. The fuel pickup was purchased from Van's, and is much nicer than the one I would have created from the plans. The inboard rib has the fuel vent line attach fitting, and BNC connector for the capacitive plates.  

* Dave Pohl, fellow RV-7A builder, informed me that there was an issue with the black insulators I received from Vans. It turns out I never received a letter that warned builders about an issue with them, even though Vans system suggested one was sent. Luckily, I had not attached the rear baffle and had time to fix it.

Vans support explained that some of the black insulators may have been contaminated and can cause a short between the plates and the tank. I never noticed an issue with the resistance, but went back and re-checked to find a high resistance short of around 1M ohm. This was curious, as I also realized that the capacitance measurement I had made earlier was significantly higher that it should be. Once I replaced the insulators to the white ones Vans sent, the highest setting on the meter detects no current flow. They are fully insulated now, and the capacitance measures out around 200pf. This is still high based on the parallel capacitance formula, but much closer than the 700pf I was reading before.

The flop tube has a couple of locations that it can hang up on, so there are two recommended anti hang-up strips to install. The first strip is across the inboard rib opening to minimize damage with the platenuts. I offset the lower hole from the recommended position to provide more access for my arm. The other strip prevents the pickup from getting trapped behind the stiffener as seen in the second picture. 

I was also a bit worried about the resting position of the flop tube as sits in the center of the drain hole. Once I installed the drain valve, I realized it pushes the flop tube up during a drain and will not be a problem.

Based on other builders recommendations, I added proseal to the flange as well as the skin before dropping the baffle in place. The bead inside the tank looks sufficient and it isn't blocking the water drain access near the back of the ribs, so it doesn't appear too much was added. I also added a big glob in all four corners. I would recommend help here, as it took a while to get all the proseal on everything, plus the inboard and outboard z-brackets couldn't be squeezed, so they got riveted using a gun.

I didn't get any pictures before installing it on the wing to cure 'in position'. Some builders have reported no leaks until mounting the tank, so letting it cure there may help prevent that problem. I added Saran wrap underneath to keep any excess Proseal off the spar. Once I give it enough time to cure, it will get leak tested.

I attached the tank access plate and fuel pickup with the cork and fuel lube paste. The paste was as messy as proseal, but it appears to do the trick.

I used a balloon attached to the fuel pickup fitting to check for leaks. I also put an AN929 cap on the vent line (although another balloon would have also worked), and a balloon around the fuel cap prior to installation to help prevent leaks many have seen there. I then took my bicycle pump, removed the hard plastic end, and held the rubber bushing to the tank drain to inflate the tank/balloon. It has been about 45 mins so far, and there appears to be no leaks, Woo Hoo!!! It is pretty sweet to know that all this work was successful. 

If you noticed my watch, its actually measuring the temperature of the spar, and barometric pressure in the garage. This allowed me to track external changes in pressure so I could account for a deflated balloon due to either change. Even a couple degrees of temperature change affects the pressure enough to notice.

The SB requires adding safety wire to the fuel pickups. I started with the right tank flop tube. The SB suggested removal of the bulkhead fitting, but I was able to safety the flop tube internally without removal. That saved me from having to re-seal that fitting again, though it is a bit tricky doing this one-handed and blind. The left tank was much easier and took only a few minutes to complete.