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Fuel Cap -- How Tight?

S

StuBob

Well Known Member
There are 10^6 threads here about fuel caps, but any suggestions about tightness is buried deep in the text or absent entirely.

RV-7 with stock Usher fuel caps when I bought it. Trying to replace the O-rings, I broke the tab off one of them.

So I bought 2 new ones, had them engraved, etc. But I'm not sure how tight they need to be. Most people I've seen open their fuel caps by digging a screwdriver or something in there, but there are those on VAF who claim they can open them with a pinky. I'd like that. I know one guy who can't get a finger in that little slot, but his caps pop open when he slides a credit card under them. I'd prefer the pinky, but a credit card will do.

How is tightness properly gauged? I read somewhere (so it must be true) that it should be tight enough that you can't rotate it when it's latched. If it has to be that tight, there's no way I can get it open without a tool. I know how to adjust them, just not how tight to make them.

My only reference is my previous Bonanza, which had similar caps. In that instance, they should always be set up to release easily with finger pressure.
 
In my experience, if you can rotate them by hand when latched down, they WILL leak. As in rain water into the tank. Not desirable.
 
I can't tell you how much the fuel caps should be tighten but when I was water-leak testing my tanks, I screwed the cap until the water seepage became miniscule. At this setting, the cap lever will pop open when I slide the ignition key underneath it.
 
PhatRV said:
I can't tell you how much the fuel caps should be tighten but when I was water-leak testing my tanks, I screwed the cap until the water seepage became miniscule. At this setting, the cap lever will pop open when I slide the ignition key underneath it.
Click to expand...
If it's locked, can you rotate it?
 
StuBob said:
If it's locked, can you rotate it?
Click to expand...

I think you may have misunderstood what I said or I may have misunderstood your original post. I screw the flip lever so the cap is compressed more tightly, and thus the flip level is harder to flip open.
 
From insert that came with a fuel cap I bought from Vans a few years ago: “...lubricate both o—rings and all moving parts...A small rotation of 1/8” makes a large adjustment in the tension on the latch. Adjust the tension in the latch so that the latch just snaps over center. Do not overtighten! Cap is properly adjusted when it does not rotate when radial pressure is applied in the latch cut-out. A key or pen can be used for easy testing.”

Excess tightness can cause the dowl pin in the latch to break. I found that the dowl pin can be easily replaced with a small section of drill bit shank cut to size. The drill bit shank can take a lot more pressure than the dowl pin.
 
Before adjusting the fuel cap, consider the type of lube you apply to the "O" rings. They come with a lube that will cause excessive stress on operating.

The best lube for fuel cap "O" rings is expensive.... $25 per 1/2 oz. tube !!!:eek: but it works, and a tube will last a lifetime. Krytox found on Amazon. It only takes a drop or two for both caps. The "O" ring will slip with ease and the cap lift out and drop in without effort.

To reduce damage to the hinge pin as you adjust the nut, have the lever in the down position.

If EZ turn (fuel lube) was used, the only way to remove it is by wiping it off with a rag..... very sticky stuff...
 
Tim Lewis said:
From insert that came with a fuel cap I bought from Vans a few years ago: “...lubricate both o—rings and all moving parts...A small rotation of 1/8” makes a large adjustment in the tension on the latch. Adjust the tension in the latch so that the latch just snaps over center. Do not overtighten! Cap is properly adjusted when it does not rotate when radial pressure is applied in the latch cut-out. A key or pen can be used for easy testing.”
Click to expand...

I guess that’s where I read it. So, apparently the answer is to keep tightening the bottom plate — 1/8 turn at a time — just until “radial pressure” no longer causes it to rotate. And to buy some Krytox.

Thanks, everyone.
 
Yup, that's pretty much it. If you can rotate the cap, it may let water in (and fuel out when the tank is full).

To prevent damaging a key when prying up the lever, I made a little tool out of 1/4" aluminum bar stock to use as a pry bar. It's shaped kind of like a "J". It slips right under the lip but won't damage the paint. I drilled a hole in one end and put it on the key chain for the plane so I always have it with me.
 
They need to be tight if you don't want water past them. If you can lift it easilly with a fingernail, I promise you that it WILL allow water in. My 6A sits outside and have replaced the pin several times; I know how tight it needs to be to prevent rain water from entering your tanks and it is well beyond just enough to stop rotation. Here is how it works: the lever is rotated 90* and this action pulls up on the shaft. The shaft is attached to the bottom piece fo the cap. As this bottom piece moves up, it forces the O-ring outward into the housing and also must compress the large O-ring. If there is not some effort required to open or close the lever, you are not getting much o-ring compression and therefore not getting a good seal. Lubrication of o-ring makes the force less.

If you are one of those with a plane in a hanger and wouldn't even consider flying somewhere if it could possibly rain, then it is just fine to tighten just enough so it doesn't rotate. Just BE SURE to sump the tanks if it does rain. Mine, when set with enough tension, can sit outside for years and never have a drop of water in the tanks. Once I was in a rush replacing a pin and didn't measure before replacement and set what I felt was pretty firm and started getting water in the tank. If you set them just past where they stop rotating, you will get a LOT of water.

Larry
 
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New RV Owners : Listen Up.

StuBob said:
I guess that’s where I read it. So, apparently the answer is to keep tightening the bottom plate — 1/8 turn at a time — just until “radial pressure” no longer causes it to rotate. And to buy some Krytox.

Thanks, everyone.
Click to expand...

Get the Krytox on there first - -or the adjustment will not be consistent enough for a proper adjustment.

For new owners with these caps, if it has ever closed easy and is hard to open, remove the caps, clean with alcohol and use Krytox. Why Krytox?? Because it is pure PTFE and nothing to dissolve or wash away in gasoline. The purchase price is more but the long run cost of not using it is more. These caps are a dream to use with krytox or a PITA without. They will ultimately leak, stick, and/or break the pivot pin w/o Krytox 205.

Vans should ship a starter tube inside every box. Just one of those Vans things - like taking 5 yrs to fix the 6-7-9A step failures.
 
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Vans should start using fuel caps like they use on Cessna's...at least the older ones anyway, those quarter turn type things. There really isn't much question if it's closed all the way or not!
...and no need for a torque wrench!
 
If you tighten them too much, this can happen (see pic); thankfully, it broke while I was on the ground.
 

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With respect to opening the cap I use one of those plastic beer bottle openers, have it on the ignition key ring. Dual purpose, it works well for opening beer bottles as well :D
Figs
 
After reading this post, I decided to RE-lubricate my fuel caps with my Krytox because it’s been a while. For opening my caps, I use a part supplied to me from my friend Steve Melton (rvplasticparts). He sometimes includes these in an order for other parts.

IMG_3018.jpg

I modified my Sporty’s fuel tester to make things easier on pre-flight.
 
StuBob said:
So, what am I applying Krytox to? The large gasket, the post, or both?
Click to expand...

All lube points, the large o-ring, the shaft o-ring, the pivot and cam rubbing surface.

Most of the trouble is that the large o-ring is expanded by two conical surfaces that require it to slide to expand and seal. It is the typical area that causes sticking and hard closing. The vertical post does get fuel splashed on it so it seems prudent to use Krytox there as well. The cam rub area gets the same lube just out of convenience.
 
where to apply Krytox

StuBob said:
So, what am I applying Krytox to? The large gasket, the post, or both?
Click to expand...

I put a tiny dab in the lever arm to stainless washer interface. That's where all the friction comes from when opening/closing the lever.

fuel cap.jpg
 
StuBob said:
So, what am I applying Krytox to? The large gasket, the post, or both?
Click to expand...

90%of the drag of closing the fuel cap comes from the big "O" ring that you see looking at the bottom of the cap.

With the cap in the open position, grab the o ring and remove it. Clean it off and lube it a drop or two with Krytox. Doing both caps at the same time can be done with less Krytox.

The "O" ring on the shaft only needs lube during replacement.

The remaining 10% drag during cam over of lever is meaningless after the big O ring is properly lubed with Krytox.

If the cap is hard to remove from the tank due to lack of maintenance, with the tab in the unlocked position, just press down on the tab.
 
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Preventative anti-breakage question

My fuel caps appear to be sorely in need of maintenance. What (if any) precautions should I take to avoid breaking things when disassembling them & replacing o-rings etc.?

Looks like the pin in the handle is a problem area, for example.
 
Dugaru said:
My fuel caps appear to be sorely in need of maintenance. What (if any) precautions should I take to avoid breaking things when disassembling them & replacing o-rings etc.?

Looks like the pin in the handle is a problem area, for example.
Click to expand...

Use WD-40 on the nut and let it set 10 minutes. Keep the handle in the closed position.
Use a 1/4" impact.... short bursts only. Hold the cap in a gloved hand to reduce the chance of hinge pin damage.
impact.JPG
 
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