Making sure the fuel sender is sealed

Hi Builders,

I wanted to make very sure that my fuel senders were sealed before putting the tank in the wing and then filling it with fuel in the future!


The builders guide says the screws should have fiber washers under the head for sealing. These are no longer provided in the supplied VDO install kit. It sounds like a very good idea to have a fiber washer to help seal or a copper washer - like the seal washer used in banjo fuel fittings. I am also using the polysifide rubber sealant.


After sealing, here is a way to test for leakage. Get a shop vac, leaf blower or low pressure / high volume air source from a HVLP sprayer. Use duct tape to connect this to the fuel tank vent tube. Seal the pipe threads and fuel cap with duct tape - no need to completely seal these areas. After the sender sealant cures, turn on the blower to lightly pressurize the tank with air. Fill a spray bottle with water and a few drops of dish detergent, mix well and spray on the outside of the sender. If you see any foam bubbles forming, you need to re-seal. This is the approved method for checking a low pressure gas line for leaks - it should work great on your fuel tank.

Happy building,  Larry Zepp  Zodiac 650 B builder

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Comment by John Austin on June 26, 2013 at 7:22am


One thing I forgot to mention - the glove will sometimes be more "leaky" than the tank! They're mass-produced, so you occasionally get a defective glove. I wound up double-gloving by putting one inside another just to be on the safe side - I didn't want to be chasing a tank leak that wasn't there!


Comment by Larry Zepp on June 26, 2013 at 6:02am

Hi, Many thanks to John A., John M., and Don for your wise and helpful comments!


The VDO sender and install kit is intended for a sender flange that is in the top of the tank. Zenith uses this sender in the tank sidewall, so I believe the screws do need extra sealing for this change to a submerged application. John Austin, I liked your idea of using the screws with fuel resistant seals bonded to the underside of the heads. Following this idea, I purchased #8 size Stat-O-Seal washers from McMaster Carr that provide a pressure proof seal between the screw head and the sender body. This is their PN 93783A009. A bag of (10) seal washers is $ 9.61. With these, the screws from the VDO kit can be used and it has a reliable pressure seal.


I have checked the 3M polysulfide rubber sealant (from Aircraft Spruce) and this has a good fuel and alcohol rating. A am using this sealant on the gasket and screw holes as extra insurance. I am glad I waited to get a better plan to do this sender install. The rubber glove idea is a good safety valve for excess psi! I will also make sure not to over torque those screws. Best Regards,  Larry Zepp 

Comment by Don Herbel on June 19, 2013 at 9:22pm

Good advice from John Marzulli.  I talked with Caleb as well, couple of years ago.  I filled both tanks with fuel while they were still outside the aircraft and let them set overnight.  No leaks detected.  My 701 passed airworthiness and first flew last July 4.  I now have 60 hours on her and no leaks yet.

Comment by John Marzulli on June 17, 2013 at 10:29am

A couple of notes from my recent re-installation of the senders:

1) I would used washers again, but this time replaced the fiber washers with a thin flexible plastic washer. An additional lock washer was added between the screw head and the plastic washer as over two years of flying a few of the screws had loosened.

2) Use Permatex PermaShield #85420. It is ethanol safe. The previous brown permatex I used was not.

3) Make sure you add access panels to the "shoulders" at the wing roots to get to your senders again.

4) If you plan on using MoGas, do a full and true fill test with E10. Let the tanks sit with the E10 for a few days to make sure.

5) Use this chance to calibrate the fuel senders. There will never been an easier time.

6) Be careful to not over torque the screws. 20-25"\LB should do it.

Good luck!

Comment by John Austin on June 16, 2013 at 4:49pm


Caleb told me that the fiber washers were no longer used and to just seal the threads with EZ Turn. I still felt uneasy about that, so I got some screws from McMaster Carr that had fuel-resistant rubber seals bonded to the underside of the screw heads ... sort of like having a permanent washer attached. I applied a little EZ Turn to the threads and used these and they seemed to seal well.

My variation of pressure testing was to tape a nitrile rubber glove over the fuel filler, temporarily install a Schrader valve in the fuel outlet, and put a couple of psi air into the tank - enough to inflate the glove. Let it sit for a day or two to make sure the glove didn't deflate. As you well know, it only takes a little "oops" of too much pressure to rupture the fuel tank, so the glove is a pretty good safety valve - it would probably rupture before the tank does. Worked for me!



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