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Fuel Selector Valve Leaks

suupercub

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1976 PA-18 "150" fuel selector valve leaks. Drained fuel completely and installed new (tiny) O-Ring and reassembled. Flew the aircraft and did not note any leaks. But when the engine was shut down and the aircraft sat for approximately one hour I noticed a "wet" finger when checking the fuel selector valve again. Is this a special O-Ring or is there some kind of back pressure which is causing it to leak? The new O-Ring was obtained from an auto supply outlet. Is there a special O-Ring for this? I didn't note any special O-Rings in the Univair catalog.
 
Picture of your valve??

if original, replace with a univair or Dakota cub valve new style valve.


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That's what I did! Ordered a couple of Fuel Resistant O-Rings (have a spare now) and a tube of Fuel Resistant Grease from Acft Spruce. Rather than spend $400 on a new valve and $$$ labor on cutting and patching the fabric surrounding it, I thought I'd try the O-Ring and Grease first. So far so good.

 
You don’t have to cut any fabric to do fuel valve maintenance or replace a valve but it is a bit of a pain to get the left interior panel out. I had my fuel system apart recently for maintenance and the o-rings in a 3 year old Univair valve have rotted out already. Avgas isn't what it used to be...according to the paperwork a Univair valve isn’t field repairable or something to that effect, anyone ever rebuild one? I ended up replacing with Dakota Cub but am having a hard time believing a basically new Univair valve is toast.
 
So if it’s a CAR certified airplane, why not use that AC on obsolete airplane parts and install a Moller valve as an owner produced part?
 
So if it’s a CAR certified airplane, why not use that AC on obsolete airplane parts and install a Moller valve as an owner produced part?

Doesn’t matter if it’s a CAR certified or FAR 23 certified airplane, parts requirements are the same. That said, the fuel valve is a commercial part, and replacing it with the same commercial part is acceptable. It’s an Imperial 108 HD valve.


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I've wondered why not put an inspection hole or cover in the fabric over the valve location? I asked about that once and two good mechanics nearly bit me for even mentioning. If it leaks it gets like doing dentistry though the colon but yes the panel can be removed.

Gary
 
You don’t have to cut any fabric to do fuel valve maintenance or replace a valve but it is a bit of a pain to get the left interior panel out. I had my fuel system apart recently for maintenance and the o-rings in a 3 year old Univair valve have rotted out already. Avgas isn't what it used to be...according to the paperwork a Univair valve isn’t field repairable or something to that effect, anyone ever rebuild one? I ended up replacing with Dakota Cub but am having a hard time believing a basically new Univair valve is toast.

Stoddards in anchorage has parts.

It’s gonna shoot the bb’s out when you pull it apart, so hold a baggie over it. I also use a pice of pvc pipe coupling to hold them in for reinstating valve

Drain gas first..........



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Univair valve-

MS29513-114 orings on the plug ends

MS29513-012 orings inside

Not hard to do with the valve in the plane, just make sure to catch all the detent balls and springs. It will take longer to defuel the plane than reseal the valve.
 
Univair valve-

MS29513-114 orings on the plug ends

MS29513-012 orings inside

Not hard to do with the valve in the plane, just make sure to catch all the detent balls and springs. It will take longer to defuel the plane than reseal the valve.

Are you sure on the -012 oring?

The Allen valve and Piper valves use the MS29513-011
 
The-012 seals around the individual ports on the plug. These are the parts I stock for the univair valve. Other valves,or if univair ever changed the design, your results may very.
 
I've wondered why not put an inspection hole or cover in the fabric over the valve location? I asked about that once and two good mechanics nearly bit me for even mentioning. If it leaks it gets like doing dentistry though the colon but yes the panel can be removed.

Gary
I had an access panel framed into the side of my '12 at rebuild about 6 years ago. It paid off because I (stupidly) kept the twin Imperial valves and rebuilt them. After a year or so they started seeping again and tightening the packing to stop the leak made them hard to turn. Having the access made installing the Dakota valve not too bad.
IMG_3423.JPG
 
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The-012 seals around the individual ports on the plug. These are the parts I stock for the univair valve. Other valves,or if univair ever changed the design, your results may very.

Maybe they use different orings. This is the part list for the 6S122 Allen, it lists a -011. Maybe the -012 seals better than that 011 if it fits, its just a hair bigger. I thought the Allen and Univair were the same valve, they might not be.
 

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Doesn’t matter if it’s a CAR certified or FAR 23 certified airplane, parts requirements are the same. That said, the fuel valve is a commercial part, and replacing it with the same commercial part is acceptable. It’s an Imperial 108 HD valve.
That part number appears to be a three way valve, rather than a simple shutoff. Would you have a different part number perhaps? Thanks.

Edit: fwiw, my -12 has two valves that are the same style as that part number - i.e. they are look-alikes, except that they are simple shutoffs. I believe, but can't be positive, that they are original.
 
That part number appears to be a three way valve, rather than a simple shutoff. Would you have a different part number perhaps? Thanks.

Edit: fwiw, my -12 has two valves that are the same style as that part number - i.e. they are look-alikes, except that they are simple shutoffs. I believe, but can't be positive, that they are original.

Forgot the -12 had a different fuel system. I don’t have any -12 drawings, so I’m not sure what valve Piper used.


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Buy the replacement Dakota cub valve for the -12. The STC then gives you a both position and you just plumb it to one valve like a -18


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