M20K 231 Fuel Caps / Filling the Tanks.

[231DF]

Anyone have any suggestions as to fitting the fuel caps properly?  

I've had 2 instances where fuel has leaked from the tank in flight from around the outside of the cap. Cap appeared to be fitter correct both times. 

Also any advice of filling the tanks? Seems like I have air stuck in them and I can't get the tanks to fill fully. 

Thanks All!

[Pinecone]

You may need new o-rings.  Cap seal no is supposed to be pressure tested, IIRC at annual. 
 

if you change the o-rings get the blue floursilcone ones. @OSUAV8TER carries tham

and the Marsh Bros washers are nice, but not cheap for what they are

[gabez]

1 hour ago, 231DF said:

Seems like I have air stuck in them and I can't get the tanks to fill fully. 
 

what's the delta in fuel you see? meaning how many gallons are you putting vs how many gallons can the tank take? I never had a problem with air, I did have problems with the fuel caps. If fuel is coming out, also water could get in

Do you have aux tanks? Do you see the delta in fuel in the Aux? 

Edited April 12 by gabez

[231DF]

1 minute ago, gabez said:

what's the delta in fuel you see? meaning how many gallons are you putting vs how many gallons can the tank take? I never had a problem with air, I did have problems with the fuel caps. If fuel is coming out, also water could get in

Do you have aux tanks? Do you see the delta in fuel in the Aux? 


 

No aux tanks.  Tanks are 36 gals a side. With 9 gals indicated reminding I can only get 21 gals in (a side). If I keep filling (brim the tank till its practically spilling out and the watch it settle) I can get more fuel in but I don't know how much more is feasible (Usually I give up after a while of playing that game)

Never had an issue with water ingress, and I sump before every flight. 

[Fly Boomer]

14 minutes ago, 231DF said:

With 9 gals indicated

Unless you have the Cies aftermarket fuel senders, the factory gauges are only telling you the tank is below half.  A calibrated stick is usually more accurate.

[gabez]

21 minutes ago, Fly Boomer said:

Unless you have the Cies aftermarket fuel senders, the factory gauges are only telling you the tank is below half.  A calibrated stick is usually more accurate.

is it true? like the gauge will go below 18 and stay there?

[PeteMc]

One of two things.  O-Rings could be worn or have a nick in it.  Change them frequently or go for the more expensive ones (and you still need to check them for nicks, etc.).

The other thing is to make sure the cap is REALLY seated correctly. Assuming you have a syphon cap inside the filler hole, and if you fill up where there is fuel above the syphon cap, it will get sucked out if the main wing cap is not on straight and sealed.  Learned that shortly after I got my plane when I *thought* the cap was flush, but it was just slightly off and didn't seal completely.  Also learned then that if the locking flap is even slightly harder to push down than normal, the cap is probably not flush.  

[Ragsf15e]

Definitely you should check the o rings.  There are two, one big one you can see and one tiny one you can’t (it’s inside, on the shaft).  There might be one other issue… the caps have a nut with a cotter pin on the end of the shaft holding the whole thing together… the nut needs tightened just right or the cap will be too loose or too tight when you put it on the tank and close the lever.

I just took mine apart and put new O rings on.  The caps had the blue flourosilicone large O rings, but surprisingly, my IA found one of them was a little loose and might not seal properly. Then when I disassembled the caps to clean them and install new o rings, I found that the inner ones were the old style.  No idea when they were last changed.

[OSUAV8TER]

You need these o-rings. For $17 it is the cheapest bang for the buck and favor you can do for your airplane. I also recommend the Marsh washers too. You have a Shaw 431 fuel cap.

https://www.gallagheraviationllc.com/Gallagher-Aviation-Shaw-431531-Fluorosilicone-Fuel-Cap-Kit_p_161.html <-- Shaw 431 fuel cap o-ring kits

https://www.gallagheraviationllc.com/Marsh-Brothers-AeroTough-GF-Fuel-Cap-Washer-Kits_p_252.html <-- Link to Marsh washers

[Fly Boomer]

7 hours ago, gabez said:

is it true? like the gauge will go below 18 and stay there?

No.  What is true is that our ancient factory gauges are unreliable and inaccurate.

[PeteMc]

6 hours ago, Fly Boomer said:

What is true is that our ancient factory gauges are unreliable and inaccurate.

As I understand it they only need to be accurate for Full and Empty. 

 

[Pinecone]

On 4/12/2025 at 11:22 AM, 231DF said:

No aux tanks.  Tanks are 36 gals a side. With 9 gals indicated reminding I can only get 21 gals in (a side). If I keep filling (brim the tank till its practically spilling out and the watch it settle) I can get more fuel in but I don't know how much more is feasible (Usually I give up after a while of playing that game)

Never had an issue with water ingress, and I sump before every flight. 

Are you reading the gauge in flight or on the ground?  The fuel gauges are calibrated to in flight.  If you have the wing sight gauges, they are calibrated to on the ground.

It would be best to calibrate a dipstick to check your fuel level on the ground.  I did mine by running one tank until the engine sputtered.  I wrote an article on doing it for Mooney Flyer.  Late 2022 or early 2023.

If you want to deal issues trying to get the tanks full, the Monroy tanks will give you ALL THAT.   Why they used such a small size fitting/hose I will never understand.  But at home, if I want them FULL, I have the fuel truck top them off.  Then the next day I can add some 5 more gallons a side.

[Marc_B]

On 4/12/2025 at 11:35 AM, Ragsf15e said:

I just took mine apart and put new O rings on.  The caps had the blue flourosilicone large O rings, but surprisingly, my IA found one of them was a little loose and might not seal properly. Then when I disassembled the caps to clean them and install new o rings, I found that the inner ones were the old style.  No idea when they were last changed.

ABS had a great article about fuel caps and o-ring maintenance.  I also found it interesting that there was a comment "On our Baron, the specified force is 12–18 pounds applied perpendicular to the handle. To quote the Baron 55/58 manual, this force is “… applied 0.20 ± 0.03 inches from the edge of the handle at its center.”

I don't believe I've ever come across a spec on the opening force of the Shaw fuel cap and I'm curious if this is found in the Mooney Service Manual somewhere??  I have actually had a fuel cap that was super topped off and leaked a slight amount of fuel from the cap in flight.  A quick tightening to one more "notch" on the castellated nut and no more leak!

ABS May 2017 Article.pdf

[Ragsf15e]

2 minutes ago, Marc_B said:

ABS had a great article about fuel caps and o-ring maintenance.  I also found it interesting that there was a comment "On our Baron, the specified force is 12–18 pounds applied perpendicular to the handle. To quote the Baron 55/58 manual, this force is “… applied 0.20 ± 0.03 inches from the edge of the handle at its center.”

I don't believe I've ever come across a spec on the opening force of the Shaw fuel cap and I'm curious if this is found in the Mooney Service Manual somewhere??  I have actually had a fuel cap that was super topped off and leaked a slight amount of fuel from the cap in flight.  A quick tightening to one more "notch" on the castellated nut and no more leak!

ABS May 2017 Article.pdf 645.31 kB · 0 downloads

Interesting, I didn’t even think there was a spec for that.  I used TLAR (that looks about right) and it seems to work.  The spec must be pretty forgiving, but you’re right, if it’s off too much in either direction, it won’t work.

[Fly Boomer]

1 hour ago, Marc_B said:

ABS had a great article about fuel caps and o-ring maintenance. 

Oh boy!  A justification for a new tool!