Sealing Leaking Deluxe Fuel Caps

[David Paule]

There are two leak paths - the large perimeter o-ring and the stem. There's a smaller leak path, too, through the lock, but I haven't had to mess with that one yet.

The leak path past the large o-ring is easily cured simply by tightening down the center nut in the bottom of the cap. Be sensitive and try it often, since the sweet spot, where the latch works nicely but the leak is stopped, isn't very large. It's a fraction of a turn to go from too loose to too tight.

I didn't use any lubricant on this large o-ring.

Here's how I dealt with a small but definite leak at the stem.

I used EZ Turn Fuel Lube. There might be a generic version of this fuel-resistant grease available - look for this specification: Mil-G-6032D. It's a very sticky, thick paste and you certainly won't need much.

Here's a photo of a disassembled cap. The arrow points to the stem. Just to the right of the arrow is the o-ring. I added the grease in this area, paying particular attention to the o-ring.



Mostly hidden by the upside down cap casting is a washer on top of the bore, you can barely see it through the bore. This washer is the bearing surface for the heels of the cam part of the lever. It's also the outer race of the o-ring bore in the casting, so I added some of the grease on that area too, before reinstalling the washer.

The order of reassembly was to install this washer on the stem, insert the stem in the bore, slip the coil spring over it, then the black plastic retainer, and finally the small washer and the nut.

My RV-3B has a long way to go before I get any fuel in the tanks, so be aware that this was tested with air pressure, under 1 psi, not fuel in flight.

Dave

[Vlad]

Thanks for Deluxe cap anatomy Dave. After several years of flying I never did any maintenance on them till recently. One cap was very difficult to close/lock. I was far from home and used a drop of oil from engine dipstick on the stem. It cured the problem and now I know why.

[krw5927]

These O-rings are easily replaced as well. I went with Viton replacements. Size documented here:

http://www.vansairforce.com/communit...ad.php?t=69354

[flightlogic]

Second the new O-rings at annual. Oddly, I posted once that Fuel Lube was a thread sealant, not a lube. After some time and experimentation, I have to admit it is the appropriate stuff for those O-rings. Both of them.
Play with the adjustment now and then, to get the fit right. The self locking nut does not have to be tight. The main body of the cap will not usually turn. If not torqued down, then you don't need tools while on the road... to make slight adjustments. BTW, the fuel lube will keep most of the rain out of your tanks too. I do flip my caps over after a shower though... to get any puddled rain from the low spot.