Leaky Red Cube fuel flow transducer

[brogers]

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

[BillL]

Nicely done and well documented, Bill, Bravo!

I was just talking with a -9 builder a couple of weeks ago and he (painfully) found there was a batch of the red cubes that had the taper treaded too deep and the fittings bottomed. It was a defect, but this is better than the original design!

I despise tapered thread fittings as they always point the wrong way.

Now what will you use the tooling for?

[brogers]

Originally Posted by BillL

Now what will you use the tooling for?

No plans yet... but you can bet I will use it on the next 1/4" NPT fitting that gives me trouble.

Bill Rogers
RV-7

[Rupester]

Major kudos to Bill Rogers for his creativity and persistence, but no way should any of us have to machine (remanufacture?) a $200 sending unit to assure it doesn't leak

[Snowflake]

Originally Posted by Rupester

Major kudos to Bill Rogers for his creativity and persistence, but no way should any of us have to machine (remanufacture?) a $200 sending unit to assure it doesn't leak

That's pretty much what I was thinking too. I'd return it and let the manufacturer do it. I'd want them to pay for shipping, too.

[Walt]

I've installed a number of these and never had one leak, guess I'm just lucky

[brogers]

Agreed that if it leaks from the start, send it back.

This sender has 360 hour on it and had leaked twice before on fairly regular intervals. I have been able to get it to seal temporarily (sealant and max torque) but it only seems to last just over 100 hours before leaking again. The threads appear to be cut slightly too deep causing the fitting to bottom out, but with 360 hours on it, I didn't figure they would take it back (although that assumption may be wrong). Not being a big fan of NPT fittings anyway, I opted for the more permanent fix above. Definitely not for everyone, just one way to do it.

Bill Rogers

[N427EF]

It's a shame that so many of those pretty red cubes have such flawed threads or more precisely inadequate thread depth.
Many of us just don't want to send things back and wait for replacements or deal with warranty hassles and shipping charges when broken parts can be fixed.

I fixed mine with pro seal, semi permanent nipple installation and no chance of a leak.

[TS Flightlines]

Actually I wondered why they didnt make them with O'ring seals. But--I guess its all in the preparation. We've had several RV's and a Rocket plumbed and no issues.
Tom