Cracked fuel tank repair

[8475]

I'm writing to document a leak repair on the port tank of my 140. I pulled the tank, capped off all the fittings, and then pressurized with a bicycle pump to 0.5psi. Soapy water identified the leak source as being under the rib. Also in the same vicinity, the end-cap of the rib had cracked free. Normally spot-welded to the rib flange. (I believe that these two failures are likely related. The cracked end-cap doesn't contribute to the leak, obviously, but the failure does suggest that this is a high-vibration location.)

N76415 fuel tank crack check 1024.jpg (68.29 KiB) Viewed 676 times

[8475]

Harbor freight sells a spot-weld cutter for $5. Cheap and effective. I turned down the shank of the tool so that I had more clearance to the holes I cut in the top flange of the rib. This was painstaking work. But at the end I managed to get the rib removed without cutting into the tank.

N76415 tank rib removal 1024.jpg (99.55 KiB) Viewed 675 times

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Here is a pic of the fully removed rib. Now we can see that the crack is located at the furthest spot weld. I theorize that this is likely the focal point of vibrations fed into the rib by the end-cap.

N76415 tank rib removed 2, 1024.jpg (70.67 KiB) Viewed 674 times

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Here is a close-up image of the crack. Runs right through the center of the furthest aft spot weld.

N76415 tank spot weld crack 1, 1024.jpg (66.77 KiB) Viewed 674 times

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Here is an image of all the constituent parts.

N76415 tank spot weld crack 2, 1024.jpg (67.09 KiB) Viewed 674 times

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Note how the crack runs through the center of the spot-weld.

N76415 tank spot weld crack 3, 1024.jpg (85.44 KiB) Viewed 673 times

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Here is the welded repair. After consulting with my IA, I opted not to reinstall the rib. That rib was originally intended to support stitching for the fabric cover. But my airplane has a metal tank cover so the rib isn't used for any purpose. (Unless feeding vibration into a spot weld is considered a purpose...)

N76415 tank weld repair, 1024.jpg (65.16 KiB) Viewed 673 times

[6843]

I am in the middle of replacing my right tank. I replaced the left one a few years ago. Both leaked in exactly the same place as shown in your photos. I thought it was suspicious my leaks were in the same place. With your experience, I now believe its a manufacturing flaw. Mine were in metalized wings, by the way. Wonder if that makes a difference.

[8342]

If all three tanks had metalized wings and cracked in the exact same place, I would bet that is the cause.
Fabric would be more flexible in that area and absorb any vibration instead of transmitting it to the tank rib and then the tank.
My two cents.

Rick

[6277]

Just noticed the fuel cap might not be the right kind. No vent holes?