fuel cap gasket dimensions

GeorgeC

GeorgeC

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GeorgeC
Last fall, I noticed a few drops of water in one tank. The fuel cap gaskets were looking a little old, so I replaced them. They fit snugly but were otherwise okay.

Fast forward a couple of seasons, my plane gets out of annual, and the caps are stuck on. Had to use channellocks (with plywood taped to the jaws) to remove them. Applied some DC4, flew home.

I go to fly the next week, and they're stuck again. Channellocks again. This time I follow the advice of @Dan Thomas:

Dan Thomas said:
Greasing the seal's filler neck contact face just attracts dirt and it gets washed away by the fuel. Put the grease under the seal, against the cap so the cap will rotate on the seal instead of the seal rotating on the filler neck.
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I tried to give them a twist after flying yesterday and they were stuck again. Do these gaskets swell over time or something?

(punch line: I still occasionally get a couple of drops of water in that tank...)

My caps look like this:
8f9c534e96e1b4d18cc3d18a8790a4db.jpg
 
Are the gaskets OE or aftermarket and if they are aftermarket, what material are they make out of? Sounds like a swelling issue to me.
 
If your running mogas this can cause some rubber gaskets to swell (happened on mine).
 
Let'sgoflying! said:
Don't those have an allen screw to adjust the tension?
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No. That screw holds the cap's components together. It's not an adjustment.

The OP could try putting a bit of DC4 on the central cylinder of the cap, around which the gasket rotates. When the gasket is compressed, it is extruded both outward and inward a bit, and if its center hole is too small it grabs that cylinder and puts the brakes on.
 
GeorgeC said:
(punch line: I still occasionally get a couple of drops of water in that tank...)
Click to expand...

On the underside of the center of that cap is a red silicone rubber umbrella-type vent check valve. They get old and will sag away from the seat and let water dribble into the tank. Rain will splash into the vent openings in the sides of the cap's handle flange and run past the valve into the tank. Melting snow on the top of the wing is even worse; the snow acts as a dam around the cap and the water really piles up there.

Take the cap off and hold it upright (normal position) at eye level and see if that little thing is still seated. Pick at it with a finger and see if there's any tension on it against the seat. The repair is to replace the cap. Ugh. I was never able to find a replacement rubber, either OEM or aftermarket, and I worked for a Cessna dealer.

17314587282_6491335a23.jpg
 
Your gaskets are a little too thick. Easy to twist them on with the cam ramps in motion, hard to get them started the other way when they've taken a set. My Cub is the same way. I lube the gaskets with a little fuel to get the caps on, then pay to price trying to get them off.
 
You can remove the cap, remove the rubber seal, and replace it with a home made leather gasket, leather will not stick to anything.
 
I use chap-stick on my fuel caps. Dramatic improvement in the ease of opening and closing.
 
Tom-D said:
You can remove the cap, remove the rubber seal, and replace it with a home made leather gasket, leather will not stick to anything.
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... except cows. Leather seems to stick quite nicely to cows. :D
 
FORANE said:
I use chap-stick on my fuel caps. Dramatic improvement in the ease of opening and closing.
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Use it sparingly. It has wax in it, stuff that won't dissolve in the fuel and which could plug something.

Same for Fuel Lube. Sparingly.

I have found waxy contaminants in fuel strainer screens, sometimes a lot. Maybe that's where it came from.
 
The tension of the cap can be reduced by bending the stainless steel lock tabs downward a little. Just a little, or the gasket won't have enough pressure to seal.

You know, there's probably a big opportunity for an aftermarket STC-PMA'd cap for Cessnas and Pipers that would use a soft O-ring (50 Durometer, not the usual 70-Duro) as the seal, and would cost less, especially where Pipers are concerned. Cessna's caps cost much less than Piper's. And they could have a replaceable check valve, too. An O-ring's smaller contact area would seal with a lot less pressure. With a bit more work, the cap could have a mechanism that would tighten it once it was in position, rather than all that friction as it rotates.
 
spray silicone works great use on both sides of rubber.go to your local or online Honda motorcycle parts place same rubber part as used on the cb100 gas cap
 
Dan Thomas said:
The tension of the cap can be reduced by bending the stainless steel lock tabs downward a little. Just a little, or the gasket won't have enough pressure to seal.
Click to expand...

Tension adjustment instructions:
https://www.mcfarlaneaviation.com/media/documents/doc-7093.pdf

Dan Thomas said:
On the underside of the center of that cap is a red silicone rubber umbrella-type vent check valve. They get old and will sag away from the seat and let water dribble into the tank. ... I was never able to find a replacement rubber, either OEM or aftermarket, and I worked for a Cessna dealer.
Click to expand...

I confirmed that McFarlane does not offer replacement umbrella valves.

I'll try swapping the caps and see if the water ingress moves with the cap.
 
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Stewartb said:
Your few drops of water could easily be from condensation.
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I'd expect condensation to form in both tanks; I've only ever observed water in the left tank.
 
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That may be a product of venting, or lack of it. I get more water from my right tank, but I've always parked on a slope with the right wing low. I never thought much about it. Sump it and go.
 
Dan Thomas said:
On the underside of the center of that cap is a red silicone rubber umbrella-type vent check valve.
Click to expand...
It won't be silicone; silicone is not compatible with gasoline. Some other red tinted rubber, probably buna or neoprene.
 
Dana said:
It won't be silicone; silicone is not compatible with gasoline. Some other red tinted rubber, probably buna or neoprene.
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Yup. It won't be the usual silicone. It could be fluorosilicone, which is resistant to gasoline. It doesn't harden like Buna eventually does. Could be Viton, too. At any rate, we can't buy that part, which is a shame. Maybe MacFarlane will fix that.
 
Stewartb said:
FYI
https://realgaskets.com/product-category/aviation/

Your few drops of water could easily be from condensation.
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About a year or so ago there was an extensive discussion on POA regarding condensation in tanks. Some of the engineering types did some figuring and said that it would take a very long time to get any significant water in the tank solely from condensation. I did some figuring myself and confirmed it. The amount of water in saturated air (fog, or close to it), the volume of the empty tank, and the amount of air exchange at some extreme thermal variations just didn't introduce much water at all, especially in only a week or so. Most water arrives in the fuel or via leaky caps.
The Monarch STC fuel cap folks did some tests on the typical Cessna fuel caps. Simulating a pounding rain had plenty of water getting past the caps, even caps in fairly good condition. I can't find it on the web anywhere. It was in some printed format some years ago.
 
Not much volume in an aquarium and they condense plenty. Wing tops warm and cool faster than surrounding surfaces... it ain't rocket science.
 
Figured it out. Spruce advertised the OEM gaskets as 1/8" thick. The RG225s at Spruce are also advertised as 1/8" thick, and are cheaper, so I bought them.

OEM gaskets mic'd at 0.105, RG225s mic'd at 0.130.

I put the OEM gaskets back on and they are indeed visibly thinner.

The umbrella valves were not sagging, but weren't exactly taut either. Otherwise they appeared to be in good condition. I swapped the caps.
 
Dan Thomas said:
I was never able to find a replacement [umbrella valve], either OEM or aftermarket, and I worked for a Cessna dealer.
Click to expand...
I found some sources, don't know if they still work:

Dan Thomas said:
That [hex] screw holds the cap's components together. It's not an adjustment.
Click to expand...
Unrelated question: can the "phase" (of the handle relative to the tabs that engage the filler neck) be changed by disassembling the cap?
 
GeorgeC said:
Unrelated question: can the "phase" (of the handle relative to the tabs that engage the filler neck) be changed by disassembling the cap?
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No. The cap is a casting:

1701030616909.png

Those two cast lugs that stick up are part of the casting that also has the handle on the topside. The screw goes through the locking tab and into the aluminum cylinder that holds the vent check valve.

I know the problem. The older Cessnas had the filler neck screwed into the tank. It was left in a position that would have the cap handle streamlined with the airflow. Mechanics would remove it for whatever reason, without marking its orientation, then screw it back in with some sealant or threadlocker, and now it's nearly impossible to remove or relocate. The cap handle would end up crosswise to the wind.

1701030937158.png
 
Dan Thomas said:
I was never able to find a replacement rubber, either OEM or aftermarket, and I worked for a Cessna dealer.

17314587282_6491335a23.jpg
Click to expand...
The Cessna P/N for that umbrella valve is B100145, #5 in this diagram.

image001.png

edit: Just noticed this was a recently revived old post, oh well... but still good for future reference for someone hopefully
 
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Dan Thomas said:
The older Cessnas had the filler neck screwed into the tank. It was left in a position that would have the cap handle streamlined with the airflow. Mechanics would remove it for whatever reason, without marking its orientation, then screw it back in with some sealant or threadlocker, and now it's nearly impossible to remove or relocate. The cap handle would end up crosswise to the wind.
Click to expand...
Sigh. Sounds like a loud and clear "don't mess with it", at least while the tank is in the wing.
 
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