I’ve removed and resealed the fuel tank drain and fuel line bung, three times. 1st with yellow PTFE tape, 2nd with an older product I got from an A&P friend, 3rd Permatex 80633 w/PTFE. Next I’m going to try Locktight 567. I’m in denial that I have a crack in my fuel tank so I’m trying every other thing possible.
Question: has anyone had a crack in the fuel tank, possibly by the bottom drain? Has anyone removed the tank with the wing still attached? Is it best to cut open the top skin to get to the tank?
Dennis M

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Here are a few ideas to sustain your denial and maybe avoid/postpone any major surgery :-)

You're probably already sure that it's a fuel leak if you've done all that work, but I had liquid dripping from a drain valve and it turned out to be rain water that had gotten into the wing and was exiting there, so I thought I'd mention it as a possibility just in case.

You didn't mention how much fuel is coming out, but I do know that in some certified aircraft a slow fuel leak in a wing is not considered a safety of flight issue and it's fine to keep flying it like that indefinitely unless it gets worse. I'm not recommending it, just sayin'.

Could it be the valve itself that's leaking? When replacing the valve with a new one you could clean the threads in the tank a bit with an NPT tap (I think it's 3/8" NPT) to make sure the valve's threads are sealing against nice new threads in the tank. At the same time make sure you get all the PTFE tape fragments out of there... at least one fatal crash has been caused by bits of PTFE tape ending up where they shouldn't be.

To find the leak you could try draining the tank, closing the shutoff valve, and putting an inflated balloon over the tank's vent to lightly pressurize the tank. Then apply some soapy water around the valve, bung, and anywhere else you can access. If air is escaping, bubbles will form in the soapy water. That would potentially show you where the leak is and it would help you decide if you need to remove the tank to fix it.

I'm no expert and haven't had to remove a fuel tank, but if you do need to remove the tank, the plans show an optional opening in the top wing skin for removing the tank, so if your wing doesn't already have that hole I don't see why you couldn't cut it now (very carefully). See pages 7-V-7 and 7-V-11. You might need an additional access hole in the bottom of the wing to unscrew the fuel line from the tank in order to get the tank out.

I think opening the wing could be done without removing the wing from the aircraft... I'm not sure which would be easier - removing the wing so it's more convenient to work on, or having to work on it from an elevated platform. I've removed my wings a couple of times and it's not fun and when you do it there's a risk of damaging something, so I'd be inclined to try working on it with the wing on, maybe removing the slat and the flaperon so you can stand closer to the middle of the wing. Someone else might have experience removing a tank though.


Great advice. I didn’t think of using the tap, good idea. Also checking with for leak with soap, I’ll try that. Also guess I gotta check the plans for removing the tank in case it comes to that.

In the early days the 701 wing tanks we’re bolted on to the wing. Every thing was flush, even the cap is flush. I had to work on the tanks, and I didn’t want to remove the wings. It is easier to make an elevated platform to work on the wing. Just take your time. Removing rivets is easy, it’s the first one thats hard to do. I would use an Alfa knife to cut the skin, yes it’s slow but makes a beautiful cut. I would do the balloon and soap procedure to verify that you need to cut the wing. Perhaps design a way to make the tank cover removable. The wing already has a lot of built in drag. Good luck. 

Hi Dennis,

It has been a few months now, but as Matt suggested, It was necessary for me to run the proper tap up the fuel drains on my new 701 fuel tanks to get the drain valves to seat properly. It’s probably due to distortion of the weld-o-let (female nozzle) when it was welded in. 

Also agree with all on making access to remove tank. I made some measurements and took some pictures before closing up my wings in case I wanted to do it in the future. I can send you pictures if you get to that point.


Thanks, looks like my fuel sender is leaking. I’ll replace that and hope that’s the end of it. I’ve found equivalent sending units on line but I think I’ll just order it from Zenith.
Thanks for all the good advice. I’ll post when I’ve completed the sender replacement to verify that took care of the problem.
Dennis M

Removing the tanks is not difficult if the wing was built with the access panel in the earlier plans.  The wing root fairings can be partially removed as well to allow access to fuel line connections.  Whatever you do, do not slosh your tanks.  See photos


Hi Mike, can you explain what you mean when you say "do not slosh your tanks"? And was that a rock in your wing??


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