Fuel cap keeps popping open

[MacCool]

An O-ring that looks good still might function poorly. I think that replacing the two O-rings empirically is a good idea (I’m assuming you have the usual Usher caps).

https://www.aircraftspruce.com/catal...lickkey=903068

[kenpilot]

Yep, I already added those o-rings to my next ACS order. Shipping is expensive, so I try to batch stuff together with oil/filters.

[Mile High Relic]

Originally Posted by kenpilot

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.

I'd suggest splashing some fuel on the duct tape adhesive and observing the results.

My experience with duct tape and gas (not on an airplane) was the the gas made the duct tape adhesive turn very soft and gooey.

[Sam Buchanan]

A trick that has been around a long time is to dip your finger in the fuel and spread it around the big o-ring. This quickie lube can often make the cap close better.

[gasman]

Originally Posted by Sam Buchanan

A trick that has been around a long time is to dip your finger in the fuel and spread it around the big o-ring. This quickie lube can often make the cap close better.

I remember working in a gas station in the late 60's, this old guy was fueling his truck and stuck his finger in the fuel neck, and then looked over at me washing his windshield, and said "oh! feels Ethyl".....

[gasman]

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

Super Lube has the same ingredients, is way cheaper and is found locally even in many ACE hardware stores and Lowes.
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[RV8JD]

Originally Posted by kenpilot

I've been trying to figure out exactly what is "fuel lube".

Fuel Lube is now called EZ Turn, but don't use it:

https://www.aircraftspruce.com/catal...ezturnlube.php

The O-rings are slightly cheaper from Van's and they will ship USPS for cheaper shipping costs. Link to Van's parts is in post #3.

[BillL]

Originally Posted by 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

Attachment 34235

OK for short term, but longer term get Krytox 205 lubricant. It is the only lube that will not get dissolved by gasoline. It will stay lubricated so it will always be easy to operate and seal better.

[kenpilot]

So lubricating the o-ring and adjusting tension did not seem to help any. The metal shoulder was already too worn out. But with the pin out, it was quite easy to file the shoulder down... not to a sharp point, but definitely squared. Now both caps have a nice click closed as I'd expect. And I have a new appreciation for fuel caps being just barely tight enough to stay in position and not over-tightened.

Thanks, all! I'll consider this resolved.