M20J Fuel drain sump slow leak

[DC_Brasil]

Hello everyone. 

My M20J is in the avionics shop finishing a panel upgrade. When the guys went to drain the fuel tanks to calibrate the fuel sensors with the G3X, the fuel drain sump valves on both wings had cracks.

Since they are hollow screws and rather fragile, I'm guessing the previous guys that worked on the plane may have over-torqued it when  screwing the valves in place.

These were SAF AIR 53S fuel drain valves. The SAF AIR 53S are direct replacements for the F-391-53S original fuel drains.

I bought a couple of F-391-53S from Aircraft Spruce and they already come with new o-Rings. When installed these drain valves were very slowly leaking. It is a very slow and subtle leak, not even dripping. The drain valve gets wet with fuel. 

The shop tried to use two o-rings but the slow leak remained.

Anyone has any ideas how to mitigate it? It is the correct piece, correct PN with a brand new o-ring. Should be a good fit, right?

[PT20J]

You can't double up o-rings. The o-ring on these valves seals against the underside of the wing. You need to figure out what's causing the leak. Make sure the o-ring is not damaged and there is no damage to the area where the o-ring seats. The valves need to be tightened snug but not overtightened, You could try putting some fuel lube (EZ-Turn) on the o-ring before assembly.

[Marc_B]

@PT20J Are these sumps only “sealed“ with the o-rings or are they sealed on the inside of the wing with sealant from factory or with tank reseal?  I’ve always been curious if these were made to be easily replaced if needed…

[PT20J]

1 minute ago, Marc_B said:

@PT20J Are these sumps only “sealed“ with the o-rings or are they sealed on the inside of the wing with sealant from factory or with tank reseal?  I’ve always been curious if these were made to be easily replaced if needed…

The o-ring is the seal. They are designed to be replaceable.

[EricJ]

The o-ring is the seal against the bottom of the wing and the nut plate that is riveted to the wing skin inside the tank.   The nut plate and the rivets that hold it are sealed with tank sealant on the inside just like the rest of the tank.   If the drain valve gets overtorqued on installation or it gets broken or stuck and needs a lot of torque to remove it, it can move the nut plate enough to slightly unseat the rivets and/or lift some of the sealant inside the tank.   That can lead to some annoying leaks (both of mine did this).

On mine the leaks are around the rivets that hold the nut plate in.  Both of mine responded well to liberal application of oyl-tite seal, swiping it across the rivet heads to try to drive the seal stick material under the rivet head.   One side I've had to re-apply it once after a couple years.    Other than that they've stayed pretty tight and leak very little.  

Otherwise a "proper" repair requires resetting the rivets, which requires removing the seal inside the tank around the nutplate and resealing it after either replacing or resetting the rivets.   It's the sort of thing that is done during a tank reseal or repair but I think is otherwise not the first option.   

[Z W]

I had the same experience with fuel seeping out of one of the rivets near the fuel valve.  When tightening/loosening the fuel valve you could actually see this rivet move slightly.

I applied Oyltite to the rivet while turning on the fuel sump drain to move the rivet and tried to really work the stuff in there.  It stopped the leak. That was in January of 2024, so it's held up for almost a year now. Link to buy the stuff:

https://www.amazon.com/Co-11475-Oyltite-Stik-Temporary-Low-Pressure/dp/B002C3L5NO

The other fix is apparently to drain the tank, open the top access cover, drill out the rivets, and replace them "wet" with fuel tank sealant.  A much more involved job.

I have had fuel valves go bad internally - there's another O-ring inside them that can fail. So I now keep a spare in the plane. More often though, it's just a small piece of debris inside the valve that's keeping it from sealing, and vigorously working it up and down with the sumper will dislodge the item and let it seal back up.

Be careful installing new valves - the torque required is shown on the packaging in inch pounds, not foot pounds, and they are delicate brass that can snap off. They're barely snugged in there, just enough to seal the external O-ring against the wing. 

[N201MKTurbo]

My left tank drain was leaking around the rivets. I removed the top cover plate, drilled out the rivets holding in the modified nut plate the drain screws into. I cleaned everything super clean, then with the help of a retired American Airlines mechanic, I set two new rivets. I coated the rivets with tank sealer before setting them, then let it cure before cleaning it up.

It is leak free now. The hardest part is by far cleaning and resealing the cover plate.

[Fly Boomer]

11 hours ago, EricJ said:

Both of mine responded well to liberal application of oyl-tite seal, swiping it across the rivet heads to try to drive the seal stick material under the rivet head.

Did you use the yellow Oyltite-Stik Sealant?

[EricJ]

21 minutes ago, Fly Boomer said:

Did you use the yellow Oyltite-Stik Sealant?

It's grey, but it has a yellow wrapper/label.   It's the same stuff that Z W linked above.

[DC_Brasil]

Thanks everyone for the very useful insights.

I'll get right on it with the guys at the shop.