Adjust Fuel Cap Spring Tension

[TheAv8r]

I had just topped the tanks off with 100LL and was departing on the right tank when I noticed fuel spewing out the back end (tail-side) of the cap. Thinking I must just not have secured it correctly I said no big deal, entered back in the pattern and executed a normal landing. Only when I shut down and got out of the plane, I noticed I had put it back in correctly. What had happened is when the fuel cap (O&N Bladders Shaw cap) lever is pushed down to hold tension against the center spring, there's play. The vacuum created on top of the wings pulls the cap out every so slightly, enough to spew a gallon or two overboard. It won't come out, the rotation mechanism is still working correctly, but the spring tension needs to be adjusted and tightened to get rid of the play. I couldn't find any screw, bolt or anything I could get access to to take it apart... is there a diagram/blueprint anywhere for these or a method to adjust that spring tension?

[carusoam]

Are you saying there is a spring in there somewhere?

Shaw caps are usually held in place with nut pressure squeezing tightly against the rubber Orings…

if the nut isn’t put in place properly… fuel can get sucked out past a loose seal…

 

Look close, it is unlikely to have a spring…

PP thoughts only… not a mechanic…

Best regards,

-a-

[TheAv8r]

3 minutes ago, carusoam said:

Are you saying there is a spring in there somewhere?

Shaw caps are usually held in place with nut pressure squeezing against the rubber Orings…

if the nut isn’t put in place properly… fuel can get sucked out past a loose seal…

 

Look close, it is unlikely to have a spring…

PP thoughts only… not a mechanic…

Best regards,

-a-

It definitely looked like a spring to me... you push the bottom part up and let go and it springs back to where it was. Plus I could see what looked like a spring coiled around the central bolt. I'll take a closer look, but I'm pretty sure it was a spring.

[carusoam]

Let’s invite @Marauder to stop by…

He may have insight on the O&N caps…

Either way… there is a method to adjust the closing tension… and probably a method of testing it….

From a user point of view… if the tab is easy to close… that is a sign of it being too loose to keep the fuel in….

 

The usual Shaw caps… have a cotter pin holding the nut in place… you might want to check if it is still there…

another sign… that your caps are on too loose… when you fly in the rain… you see air bubbles being pulled out passed the cap seals… 

 

PP thoughts only, not a mechanic…

Best regards,

-a-

[TheAv8r]

Thank you. I thought for sure there's a bolt running through it and must be a nut somewhere at the bottom holding the assembly together, but there was none visible. Best guess is there was a rounded black cap at the bottom of the fuel cap and it's under that...? I tried twisting and pulling it off and it wasn't budging so didn't force it.

[A64Pilot]

Had to go out in the hangar and look, I have O&N bladders and this is my fuel cap, the nut is rather obvious but it’s self locking instead of the normal castellated nut and cotter pin, I believe they adjust the same, but I’ve not adjusted mine, I’d start with a 1/4 turn.

I’d tighten the nut as a temp fix, but I’d want new O-rings, they are usually not expensive and most certainly if you ever get any water in the fuel it got there past the cap O-rings.

There is definitely a coil spring in there, it’s visable.

I assume you must have a different cap, I’m wondering if it possible that there is a cover over the bottom that can be removed to access the nut?

 

[TheAv8r]

11 minutes ago, A64Pilot said:

Had to go out in the hangar and look, I have O&N bladders and this is my fuel cap, the nut is rather obvious but it’s self locking instead of the normal castellated nut and cotter pin, I believe they adjust the same, but I’ve not adjusted mine, I’d start with a 1/4 turn.

I’d tighten the nut as a temp fix, but I’d want new O-rings, they are usually not expensive and most certainly if you ever get any water in the fuel it got there past the cap O-rings.

There is definitely a coil spring in there, it’s visable.

I assume you must have a different cap, I’m wondering if it possible that there is a cover over the bottom that can be removed to access the nut?

 

It was very kind of you to run out to your hangar and check for me, thank you!

Affirm, mine don't look like that. They look closer to this one, with the black cap at the end. I kept thinking the cap must come off to access the bolt, but twisting and prying wasn't getting me anywhere and I didn't want to force it too much unless that's just what you have to do.

[TheAv8r]

Looks like this gentlemen is hitting the exact same issue as me... only none of the photos load and he didn't post pictures at the end of how he got it apart and back together . Anyone remember? 

[TheAv8r]

@Joe Larussa Hey there, do you remember how you got these apart or how to adjust the spring tension?

[TheAv8r]

So that there's a documented post on how to do this in Mooneyspace. My fuel caps were Shaw P/N 457-1015. They have a black rubber housing at the bottom. To hold a good seal against the tank when closing, there should be 2" from the top of the cap to the bottom of the cap when in the closed position (approx. 2.25" when open). On mine, it was a little over 2 1/3" when open, and that gave it the extra play in the tank. 

The first step is to take the bottom cap off. This was a little tough as years of being screwed in can seize it a bit, but what you do is hold the bottom rubber ring it's connecting to steady and then twist the cap counter-clockwise. Continue twisting until it comes off, then you'll see the bolt and washer:

From there, disassembly is pretty self-explanatory. You'll see the spring coiled around the central bolt, a plastic washer, and a small o-ring.

To correct the tension issue, reassemble all parts together, make sure the teeth properly align when in the open & closed positions, and then, holding on to the big bottom rotating piece to keep it in the same position, screw the bottom cap back in. Move the top tab to the closed position, and keep screwing the bottom cap in (tightening that spring) until it measures 2" from top to bottom. Test fit the cap in the tank to check for a good seal and you're done!

[PeteMc]

Also remember that sometimes the leaking issue is the O-Ring is just flattened.   For the cost, changing them every year (or two) at the annual is an easy fix.

And not for leaks, but there's also the often over looked basic maintenance of proper lubrication.  I've had a number of mechanics tell me that occasionally after adding a quart of oil before you toss the bottle, take it over to the cap, lift the lock and add a drop or two of oil out of the bottle.  Open/Close the cap lock a few times and then wipe up the excess oil.  I'm sure Tri-Flow would also work fine, I've just always been told that the process of adding a quart of oil was a good reminder and you had the lub in your hand.

[TaildraggerPilot]

Anyone have the o-ring part numbers or the set of 4 (2 small, 2 large - as a kit) part numbers for the Shaw 457 - 1015 - 12 fuel caps? 

[Pinecone]

@OSUAV8TER

[OSUAV8TER]

These Shaw 457 fuel caps already come equipped with fluorosilicone o-rings so they should not deteriorate. PMA Products, Inc. does make a new specialty one for these - I believe.