Fuel cap keeps popping open

[kenpilot]

My RV-6A has what appears to be the standard fuel caps:
https://store.vansaircraft.com/t-406...bly-t-406.html
or maybe this:
https://www.aircraftspruce.com/catal...?clickkey=7373

On the LH tank, when I close the cap, it pops right back open. RH tank doesn't do that. Both feel a bit "spongy", though. The cap doesn't click down like I remember the Cessna 310 caps doing. If I rotate the lever a bit, so on the way down it scrapes against the sides, sometimes the friction of that can keep the fuel cap on. But I'm hesitant to just rely on friction like that; one of these days it'll pop open in flight.

Has anyone else experienced this or know why it happens?

My best guess thus far is that the edge of the "lever" is rounded instead of squared off. But I'd have replace it, to fix that, I think.

[Sam Buchanan]

I used a file to "square" the corner of the lever and that made the cap close more positively.

[RV8JD]

Usher Fuel Caps

Adjustment instructions and parts shown below. Instead of fuel lube for lubrication, many, including me, use Krytox GPL 205 grease with good results. Expensive, but the small 0.5 ounce tube will last a long time.



Parts are available from Van's:

https://store.vansaircraft.com/catal...p+parts+t-406a

[RV7ator]

Sam has the right idea. It's a wear problem you can fix. The over center shoulder rounds with wear, and compensating by increasing the seating pressure just wears faster. I also carry a spare cap. I needed it on the ramp in Ketchikan, learning after returning home what went wrong.

John Siebold

[DavidAhrens]

Some vans fuel caps had plastic type material under latch instead of the current stainless washer, makes difference in feel and operation.

[gasman]

As you can see in the picture above, as the cap is closed, the O ring must slide on the taper to expand and hold the cap tight. If the O ring can't slide, it acts like a rubber band and pushes back.

Lube it with this..... https://www.harborfreight.com/85-gra...5a232e08734092

[wilddog]

You may also need to replace the pin the latch pivots on. A piece of 1/16” SS welding rod fits perfectly.

[MacCool]

Pretty easy fix…replace both O rings, lubricate them well with a silicone grease like Krytox or Superlube, and adjust the tension when reassembling.

I do carry a spare cap because a missing gas cap on a cross country can ground you while you try to find a replacement.

[kenpilot]

Wow, what a lot of insight into this issue. This is the first time I'm posting on the site; quite impressed with the support!

Yes, I saw that parts diagram and guide, but neither the lubrication nor the tension adjustment made much sense at the time. I thought lubrication was only when the cap was hard to close; it's not hard to close, it just won't stay closed. lol. And I thought it was just squeezing the o-ring to deform it. I see now that it's actually pushing it entirely out of position, which would require lube. Lastly, in hindsight, when I found the cap mechanism "loose" a while ago and "tightened it up", I likely inadvertently made the problem worse by maxing out the tension adjustment.

Now, it all makes sense what's happening. Thank you for the great explanations. So, to summarize here are all the things I should check next time I'm at the airport:
1. Check if the washer under the latch is metal or plastic
2. Add lube to the o-ring
3. Loosen up the tension adjustment
4. Use a file to square off the corner, if there is enough material remaining to do so

@MacCool, I like that idea of packing an extra fuel cap on cross countries. I do typically keep a roll of duct tape in the plane (the heavy duty stuff with the threads inside). I wonder if that would suffice in a pinch, to seal off the fill opening if I lost a cap.

@gasman and @RV8JD, I see your recommendations on grease. But is there anything I should definitely avoid with a grease, since in theory some could get into the fuel and therefore into the engine combustion? I've been trying to figure out exactly what is "fuel lube".

[kenpilot]

Oh, I see some of this mentioned in an old post:
https://vansairforce.net/community/s....php?p=1551441

fuel lube gets sticky/tacky, and as it gets cooler, the resistance of the o-rings to sliding between the two cap halves goes up. This increases wear on the latch.

Krytox is a very light grease and only a thin film is required. It is chemically insoluble in nearly all solvents.