Fuel Tank Leak?

[JoeRuscito]

It seems I may have a tank leak. Now that my wings are complete I figured Id pressure test the tanks again with all of the fittings/sight tubes installed. I first tested them connected together at the main fuel supply ports and plugged the air vent line (with a plugged tycoon tube) and fuel caps (with sealing tape). I blew into the tanks enough to get the tape to bubble. I found this configuration leaked enough that the tape wasn't bubbled after about 12 hours. Then I separated the tanks to test individually. They both hold lung pressure for about 24 hrs before I can no longer see the tape is bubbled. So both tanks seems to leak at about the same rate and quite slow.

Im wondering if I should be concerned or if this is what others have observed. Im wondering if the quick drain might leak air but not fuel. Or if the tanks should hold air indefinitely and I need to go searching for leaks at each of the fittings.

Thanks for the help!

[jrevens]

Hi Joe,

It’s difficult to seal the filler openings with tape to make a perfectly air-tight seal. I found that aluminum duct tape worked best for me. So... you may or may not have leaks in the system. You can use a soapy water solution in a spray bottle to leak check, looking for tiny (or not so tiny) bubbles forming. If you have good seals on your filler caps you can connect balloons to the vent tubes as a pressure indicator also. Regarding the quick drain valves - if they leak air they leak fuel.

[jiott]

I also say WOW! What if someone uses it that way on a fuel fitting downstream of the last filter. The residual could easily clog up the very small jets and orifices in the carburetors. Its been awhile since I read thru the FAA publication on recommended practices for aircraft construction and maintenance, but I believe pushing sealant into the fuel system is totally against their directions.

The stuff Kitfox recommends is Permatex Permashield 85420 Gasket Dressing and Sealant. When you read the application directions it says nothing about threads, but talks only about gasket and flange surfaces. It is specified as being fuel, ethanol resistant.

Personally, I will continue to use it by applying to the male thread only and leaving the last couple of threads bare.

[JoeRuscito]

Well it was the main pickup line. The fitting was leaking at the threads where the finger strained threaded into the tank. After talking to the factory my only option was to cut the elbow to unscrew and redo the sealant.

For anyone out there that doesn’t have the banjo fittings. Get them. It would have been a simple fix if they were installed.

Also, my suggestion would be to leak test the tanks for a second time just before covering and with all of the fittings installed. Just my two cents.


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[Shadowrider]

So you just removed fitting and it needed more sealant?

[JoeRuscito]

Yes. And also I may not have tightened enough. I was pretty conservative in tightening so as to not crack the tank. So when the elbow was level and what I thought was tight enough I stopped. Rather than going for a whole additional rotation of the strainer and risking cracking the tank. I hope that makes sense.

[Shadowrider]

Yes thanks. Crossing my fingers for no leaks. I did test mine after installing fittings but not overnight.

[JoeRuscito]

I have two successfully tested tanks! Both held air while connected together for 24+ hours. I think Im good to go. Retrofitting for the banjo fittings was a bit of a project but got it done... highly highly suggest doing this from the start... Also the new sealant the factory is shipping, its blue, Needs to be added to both threads male and female to form a tight seal and before inserting needs to sit for quite a while (read 30 mins or so) for the solvents to evaporate. I originally only put the sealant on the male threads and avoided the last thread with the thinking this would keep from contaminating the tank/fuel... it seems for this stuff to work that thinking needs to be abandoned.

Just my experience in case it helps anyone.

[jiott]

I built my whole kit using 9AR on every tapered threaded fitting including fuel, brakes, oil and coolant and have had zero leaks for 740 hours; I like the stuff. However I recently used it up and bought the blue 85420 which I have not used yet. My understanding was the 9AR is no longer available and Kitfox factory has been selling the 85420. I always used the 9AR by putting it only on the male thread, but don't remember what the actual instructions on the tube said. I believe the 9AR was also described as a gasket/flange sealant rather than a thread sealant.

I understand from this forum that many folks have been using Loctite thread sealants on their projects with apparently good results (those thread sealants instruct you to apply only to the male thread). My own hesitation in using them is that they cure to a hard non-flexible condition which is fine for metal to metal fittings torqued to proper tightness. But for a metal fitting into fiberglass where the torque value is extremely low I want the non-hardening sealant that remains flexible and of course is compatible with ethanol containing fuel. My 2 cents.

[jrevens]

Interesting. My instructions specifically specified Permatex 80938 (9AR) for this. I recently had to remove one of my tank quick drains and asked John at Kitfox what they now used. He said it was the 9AR. It is creamy white in color as opposed to the blue 85420 you guys are talking about. I think that the 9AR has possibly been discontinued by Permatex, but I was able to get a new tube on Amazon, at a ridiculously high price. Threading and sealing a metal fitting directly into a tapped fiberglass hole is potentially problematic, and there should be a better way. If I was starting over, I believe I'd drill and tap the tanks oversize, then bond brass threaded reducer bushings into the tanks with a structural epoxy. A metal to metal joint on fittings that may have to be removed would be a much better design, IMHO. Better still would be to redesign the tanks slightly to allow that to be done while also providing additional clearance to better accommodate the metallic bushings.