7.7.58 Mega fix left fuel tank and prepare wing walk doublers

Well this left fuel tank is really getting on my nerves now! After yesterday’s failed pressure test today I decided to do all I can to fix the tank one last time. If this doesn’t work then the only option left is to cut a hole in the tank and fix it from the inside. Of course, I really want to avoid that level of drastic action.

I did have a thought about why this is happening. I think that, because I didn’t fix the original leak from the inside, the very first leak I plugged from the outside is causing the air to route in between the baffle, sealant and skin and find any rivets that aren’t completely sealed. I’ve effectively created a kind of drinking straw with one end being in the tank. The only way to plug that end of the straw is to cut a hole in the tank and put sealant on the inside.

Before I take that drastic action I first mixed up a large batch of sealant and covered every rivet head on the skin side. I’m now completely out of tank sealant and need to get some more ordered.

Once that was done I hooked up my manometer to my vacuum pump so I could accurately measure the vacuum in the tank and then created a -1 PSI vacuum in the tank by pumping the air out using the red handles vacuum pump.

I also screwed the dust cap on the schrader valve and sealed up the fuel filler cap using an off cut of a plastic carrier bag.

Then I (repeatedly) checked the rivet heads and every other possible place on the tank for leaks. Wherever there was doubt I put some more sealant. However small it seemed, I piled on the sealant.

In between watching the pressure and topping up sealant I also prepared the right wing walk doubler skin by scuffing and dimpling it ready for primer.

Then I went back to checking the left fuel tank, topping up sealant and checking pressures again. Once I’d done as much as I could on the tank I went back to preparing the wing walk doublers only this time it was the turn of the left. First job was to trim it to size using my giant shears.

Then I deburred the edges of the doubler on the scotchbrite wheel and deburred the rivet holes using the hand tool. Next I scuffed and dimpled it so it too is ready for primer.

I went back to check on the tank pressures one last time and made a note of the fuel tank pressure, air temperature and atmospheric pressure on the tank. Unfortunately the pressure seems to be still dropping (very slowly) and for the life of me I can’t see where else it could be coming from. Oh well, it looks like I’m definitely going to need to cut a hole in the tank later this week. I decided to leave the vacuum on the tank and leave it to cure overnight before making a final decision.


  1. I’m not sure if this helps but…
    The fuel tank sealant is trying to keep pressure in. I therefore tested my tanks by applying pressure into the tank, not vacuum. I made up a manometer with a length of plastic tubing and dropped some water into it. Then pumped air into the tank until the water rose up the manometer. I marked the level in the tube and left it for 48 hours. I came back to the same mark. Perhaps this would give the same result but I’m not sure if creating a vacuum would cause the leak in the first place.

    • Thanks Robert, I did the same as you to test the fuel tank. Once I found the leaks using 1 PSI in the tank and some soapy water I decided to use a vacuum to suck sealant from on top of the rivet heads. I think I’m probably going to end up having to cut a hole in the baffle and seal it from the inside. I’ll test it again with positive pressure in a few days but I rather suspect that I’m going to need to resort to cutting a hole in the baffle to fix the leak from the inside. 🙁

      • That’s a shame, frustrating I guess. If my tanks had leaked I was going to do the same using the spare blank access plate I had. Fortunately I I didn’t have a leak.
        Best wishes with the fix.

    • Haha, very funny! I’m still convinced that, due to the size of the holes (2″) in the ribs and the volume of air in each chamber (10 litres appx.), it will take a few seconds for air to travel from the inlet chamber (inboard chamber) to the vent tube outlet near the cap in the 6th chamber. That’s why we see air pressure rise and fall as we fill the tank with air for testing. Basic aerodynamics buddy! 😉

      A discussion that will rage on for some time I think! 🙂

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