After finding my red cube fuel flow transducer leaking for the third time, I decided it was time to do something more than just goop up the fittings again and torque them up tight. I just wanted to share with everyone how I fixed this on my sender in case it helps someone else.
Noting that the fittings were bottoming out in the hole, my first attempt was to chamfer the bottom of the NPT fitting and use an o-ring in the bottom of the hole to seal it. This worked, but I do not recommend this as a long term solution as the fit at the bottom of the hole is not well controlled and I did cut a few o-rings getting it seated properly.
Modified NPT fitting and o-ring.
Not being fully comfortable with the "make shift" o-ring option, I decided it was time to put a real o-ring fitting in so I don't have to deal with the leak again. I bought the -6 o-ring port cutting tool and a 9/16-18 bottoming tap set.
Tools used:
I had to shorten the reamer portion of the port tool as not to change the hole depth in the sender. The tool comes longer than is necessary for the fitting I am using so this was fine.
Modified tool:
Prior to machining the new hole, you need to prevent debris from getting into the sensing unit. I did this with some electrical tape cut with a hole punch.
I then machined to the appropriate depth using my drill press and a secure clamping setup. The existing NPT was sufficient for a pilot.
Then the threads were cut for the fitting. A bottoming tap set is required to tap deep enough.
Then I test fit the fitting. Note that the fitting is the same strait fitting used in the lycoming engine fuel pump. -6 oring to -6 flare.
Then I also alodined the exposed aluminum being careful not to get any Alodine in the sensor.
Then final assembly with a little fuel lube on the orings.
Then a pressure test to verify the seal.
This was a surprisingly easy modification and should provide a consistent seal for the life of the component. Hope this helps others who may be having similar chronic leaking of the fuel flow sender.
Bill Rogers
RV-7
Noting that the fittings were bottoming out in the hole, my first attempt was to chamfer the bottom of the NPT fitting and use an o-ring in the bottom of the hole to seal it. This worked, but I do not recommend this as a long term solution as the fit at the bottom of the hole is not well controlled and I did cut a few o-rings getting it seated properly.
Modified NPT fitting and o-ring.
Not being fully comfortable with the "make shift" o-ring option, I decided it was time to put a real o-ring fitting in so I don't have to deal with the leak again. I bought the -6 o-ring port cutting tool and a 9/16-18 bottoming tap set.
Tools used:
I had to shorten the reamer portion of the port tool as not to change the hole depth in the sender. The tool comes longer than is necessary for the fitting I am using so this was fine.
Modified tool:
Prior to machining the new hole, you need to prevent debris from getting into the sensing unit. I did this with some electrical tape cut with a hole punch.
I then machined to the appropriate depth using my drill press and a secure clamping setup. The existing NPT was sufficient for a pilot.
Then the threads were cut for the fitting. A bottoming tap set is required to tap deep enough.
Then I test fit the fitting. Note that the fitting is the same strait fitting used in the lycoming engine fuel pump. -6 oring to -6 flare.
Then I also alodined the exposed aluminum being careful not to get any Alodine in the sensor.
Then final assembly with a little fuel lube on the orings.
Then a pressure test to verify the seal.
This was a surprisingly easy modification and should provide a consistent seal for the life of the component. Hope this helps others who may be having similar chronic leaking of the fuel flow sender.
Bill Rogers
RV-7
