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Fuel drain valve leak saga


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So I have a M20C with the screw in fuel drains. I have had a fuel leak for 2 years since I bought the plane. We have replaced the fuel drain and gaskets a few times and tried to reseal the area inside the tanks. I think its coming from the rivets and my mechanic mentioned something about replacing the tube rivets with solid ones. What are some suggestions? I saw something on bucking the rivits? What is that?  Thanks as always. 

CCC21ACB-C487-4D02-9C19-79B4DB64CECB.jpeg

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Solid rivets are called out in the service bulletin for replacing the original tank drains with a screw in drain.  That repair, including bucking those rivet is through the top access hole in the tank.  The tube rivets (pulled rivets, blind rivets, pop rivets) are available with a sealed end, see aircraft spruce ad 42h.   It appears fuel is leaking somewhere other than the those rivets.

https://cdn2.hubspot.net/hubfs/4147179/technical_documents/service_bulletins/sbm20-188a-1.pdf

 

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I had a very small leak after stripping and resealing my right tank. It was 1 drop every 28 seconds, but enough to make a real mess under the belly, and at $4.64 /gal it adds up to a few bucks a day setting in the hangar.  I used my old shop vac to find the problem. Get about a foot of 1/4" ID vinyl tubing, and use duct tape to "seal" it to the end of the vacuum hose. Slip this over the vent tube sticking out the bottom of your wing.  Go to an office supply store and buy a pack of clear plastic 3 ring binder dividers or  any semi rigid clear sheet plastic.  Use your duct tape to place these over all access openings after you use a spray bottle of water with one tablespoon of dish soap.  Spray any suspected areas with a fine mist of the soap water, seal the access holes with the plastic/duct tape, and turn on the vacuum cleaner.  See video.

-mark

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Looks like it is time to fly the fuel out of that tank...

remove the fuel drain...

take a pic of what the surface of the wing looks like under the fuel drain...

There is supposed to be a smooth sealing surface to mount the drain on...

By the look of the circular scratches... it looks like somebody forgot the smooth sealing surface...

 

inviting @Gagarin He may have some insight for this one...

Best regards,

-a-

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7 minutes ago, LucasC said:

Would sealant on the threads be a good idea if the sheet metal surface is the problem?  

I would say no. 

As far as your installation, it is possible that it was installed from below the wing without ever accessing the inside of the tank (why else would they have used blind rivets?). I would drain the tank, pull the fuel sending unit and get a mirror to inspect the installation- it may not have been sealed up properly. I honestly do not know whether the new interior drain fitting can be passed through the new hole...but I am curious about that. If it's small enough to pass through, then it may have been installed that way.

 I have seen pics of a few now that used blind rivets and have been wondering why.....

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Does anyone know if Cherry rivets are acceptable with fuel tank applications like this?  For structural applications, they are acceptable as replacements for solid rivets and MUCH easier to install, and realistically, it's hard to imagine they would leak unless improperly installed.

It's tough to tell from the pic if those are Cherry rivets.  Any other kind of blind/pull rivet would be bad news and would eventually have a risk of leakage...

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My two cents on the subject

To avoid any stains on the valve I replaced the F391-53S valve for the F391- 72 valve used on Cessnas and many others and sealed the threads. No leaks and drains better than me on a palm tree.:)

The - 53S has the drain holes at the very bottom of the threads. It drains through the little holes on the nut plate F391-53- SMP-2. If the nut plate little holes at the bottom are easily plugged with debris or sealant the drain valve will drain fuel but no water from the very top of the stem.

The -72 drain holes are just above the threads of the nut plate. So debris or sealant on the nutplate will not block the drain valve holes. Because of this you can put sealant on the threads (no need for the O-ring) to avoid leaks without impairing the draining ability. These are the same ones used in the Monroy long range tanks.

Edited by Gagarin
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9 minutes ago, PilotCoyote said:

I believe the rivets are supposed to be coated with polysulfide sealant during assembly. Perhaps this wasn't done the first time around...

The three places that seal the whole assembly are the two rivets and the o-ring on the screw-in valve

I recall some debates among RV people as to whether correctly assembled rivets require sealant.  The consensus seemed to be that the shear strength of rivets far exceeded the shear strength of any primer, paint or sealant, so sealant shouldn't make a difference once the rivet is set properly.  I can't say anyone produced any evidence of this, though...

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4 hours ago, Gagarin said:

My two cents on the subject

To avoid any stains on the valve I replaced the F391-53S valve for the F391- 72 valve used on Cessnas and many others and sealed the threads. No leaks and drains better than me on a palm tree.:)

The - 53S has the drain holes at the very bottom of the threads. It drains through the little holes on the nut plate F391-53- SMP-2. If the nut plate little holes at the bottom are easily plugged with debris or sealant the drain valve will drain fuel but no water from the very top of the stem.

The -72 drain holes are just above the threads of the nut plate. So debris or sealant on the nutplate will not block the drain valve holes. Because of this you can put sealant on the threads (no need for the O-ring) to avoid leaks without impairing the draining ability. These are the same ones used in the Monroy long range tanks.

Jose is that you?  If not you’ve read too many of his posts on the subject of fuel drains.  How would it improve safety by raising the drain holes in the valve about 3/8” above the designed drain hole that Mooney installed in the wing?

I hope no one follows your advice.

Clarence

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On 10/23/2019 at 4:05 PM, LucasC said:

So I have a M20C with the screw in fuel drains. I have had a fuel leak for 2 years since I bought the plane. We have replaced the fuel drain and gaskets a few times and tried to reseal the area inside the tanks. I think its coming from the rivets and my mechanic mentioned something about replacing the tube rivets with solid ones. What are some suggestions? I saw something on bucking the rivits? What is that?  Thanks as always. 

CCC21ACB-C487-4D02-9C19-79B4DB64CECB.jpeg

With fuel in the tank use compressed air to blow over the surface to evaporate the fuel.  Roll some tissue paper into a point and probe around the circumference of the valve, the paper will instantly wick up the fuel.

Cherry Max rivets are contrary to the SB, and may be the cause of the leak, but the condition of the skin is also in question. Damage to the skin can tear the O ring under the valve.  It’s normally an MS29513-014.

Clarence

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2 hours ago, M20Doc said:

Jose is that you?  If not you’ve read too many of his posts on the subject of fuel drains.  How would it improve safety by raising the drain holes in the valve about 3/8” above the designed drain hole that Mooney installed in the wing?

I hope no one follows your advice.

Clarence

It is me José N11606. The -72 holes are about 1/4" above the bottom (well below the fuel pickup screen). I found about this on an engine failure on takeoff on my old M20C based at TJBQ on the ramp. The runway was 9,000 ft long so I Ianded on it and taxi out with the momentum. On the taxiway I drained the tanks with the old 53S valves and only fuel came out just like it did before flight when I drained them. I decided to unscrew the valves and plenty of water came out. Once the -53S nutplate drain holes are clogged they will not drain water but fuel at the top of the valve stem. The higher water accumulated  level raises toward the fuel pickup when the plane accelerates and climb causing the engine malfunction. This why the malfunction does no show on the ground run up. Have been using the -72S for over 20 years with no leaks and no engine failure. The nutplate bottom drain holes can easily be plugged during tank reseal or by old sealant debris. If using the -53S make sure the drain stream is greater than yours on a palm tree, otherwise suspect a clogged nut plate drain holes.

Edited by Gagarin
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I had my tanks resealed back in January.  Right before the resea we had a drain that kept leaking despite replacing it a few times.  When Edison opened up the tanks, he saw the plates were pretty severly corroded and so replaced them while he was resealing the tanks.  Between the pop rivets and the mounting plates, that could be your issue.  Shared these same pics just the other day to a similar post.

fuel drain 1.jpg

fuel drain 2.jpg

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18 hours ago, M20Doc said:

Cherry Max rivets are contrary to the SB, and may be the cause of the leak, but the condition of the skin is also in question. Damage to the skin can tear the O ring under the valve.  It’s normally an MS29513-014.

Clarence

If the devil is in the details, @M20Doc is the devil! ;)  How the heck do you remember stuff like that?  I'm glad we have you around!

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At the risk of beating a dead horse even more, I took some pictures of the standard fuel drain valve and the suggested alternate, plus the plate nut which is in the wing tank of many Mooneys. 

In the first picture, the standard(correct) drain valve is on the left, the alternate is on the right and the plate nut is in the center.  Note that the correct valve has holes which align with the arches in the plate nut, this allows water and contaminates to drain off the bottom of the tank.

The alternate valve on the right has drain holes which are above the plate nut and have no means of draining water or contaminates from the bottom of the tank through the arches.  This valve can allow about 3/8” of water and contaminates to collect in the fuel tank never to be drained, hardly and improvement is safety.  Water causes engine failures and corrosion inside your fuel tanks.

The second picture shows the difference in O ring size, the correct O ring should seal against the lower wing skin, the alternate valve will force the O ring into the opening in the lower wing skin.

I hopes this helps.

Clarence

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75808ABF-104A-4527-ABAA-3E084B5498C7.jpeg

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Nice pictures. It shows how little sealant or debris is needed to plug the nut plate bottom drain holes (about 1/16").  While the valve on the right  -72 is immune to having the drain holes plugged by sealant or debris. As the plane ages there is substantial debris build up, not to mention tank re sealing. Even cleaning the tank before resealing can introduce debris into the nut plate. 

If using the -53s drain valve never apply sealant to the thread otherwise it will plug the drain holes, you have to live with the belly stains no matter how many rivets you try.

If using the -72 drain valve you can apply sealant to the thread  without plugging the drain holes. You can also remove the O-ring for better fit. No stains on the belly.

 

Edited by Gagarin
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16 minutes ago, Gagarin said:

Nice pictures. It shows how little sealant or debris is needed to plug the nut plate bottom drain holes (about 1/16").  While the valve on the right  -72 is immune to having the drain holes plugged by sealant or debris. As the plane ages there is substantial debris build up, not to mention tank re sealing. Even cleaning the tank before resealing can introduce debris into the nut plate. 

If using the -53s drain valve never apply sealant to the thread otherwise it will plug the drain holes, you have to live with the belly stains no matter how many rivets you try.

If using the -72 drain valve you can apply sealant to the thread  without plugging the drain holes. You can also remove the O-ring for better fit. No stains on the belly.

 

That...seems like a poor idea.

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