WARNING: 1/8-NPT thread weight limit.

Builders:

THIS IS A WARNING ABOUT SOMETHING THAT COULD KILL YOU.

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Please feel free to forward this to other aviation groups. It is already standard practice and knowledge, it is not a new concept, but evidently one that needs additional publication. I saw a builder photograph today, that initially I thought was a joke. When I realized it wasn’t, I understood that this warning had to be written, because what I had previously considered common sense, evidently isn’t common to everyone.

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SUBJECT: The smallest common pipe thread in aircraft is 1/8″-NPT (National Pipe Thread). It appears many places on aircraft. The thread is small, and it has a low tolerance for heavy items being screwed into it, particularly if these items are cantilever on aircraft engines. Things that weigh too much, are prone to breaking off the 1/8″-NPT fittings. This is much more common on aircraft that automobiles, because aircraft are a much more vibration prone environment. If the line or fitting contains pressure oil, breaking it off will result it a total loss of oil in minutes, a possible engine fire, a certain engine stoppage, AND A FATAL ACCIDENT. It is the 100% responsibility of the builder to make sure his aircraft does not have excessively heavy items supported on 1/8″-NPT threads. This is not the responsibility of the engine builder, nor the kit manufacturer, nor the tech councilor, nor the DAR, nor the FAA. In this country we have the liberty to build planes of our own construction. With this comes the absolute, 100% responsibility for the outcome. If someone chooses to ignore this, neither they, nor their next of kin can claim someone else was even partially to blame.

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Above: AN fittings, L-R: 1/2″-NPT , 3/8″-NPT , 1/4″-NPT , and 1/8″-NPT. Quarter, .22LR, Standard 13/16″ sparkplug and 6″ rule are for scale, use what you are familiar with. The Aircraft Spruce catalog has always contained full scale drawings of these threads.

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Above, THE PICTURE ABOVE IS NOT AIRWORTHY.  It is a simulation of the photo I saw.  The original was sent to my friend, and when he saw how angry I got looking at it, he decided that he shouldn’t reveal to me who was planning on flying a plane with such a rig. He correctly understood that I would use the persons name in this story if I knew it. The sending unit they were using was even larger than the one pictured. This isn’t an installation, it is a suicide attempt. The brass elbow and nipple above all have 1/8″-NPT threads. Almost all brass fittings on the market today are made in the communist Peoples Republic of  China, and are very prone to failure. If this installation would have lasted an hour 20 years ago, it will likely break in minutes today. If this fitting breaks, all the oil in the engine will be overboard in 2-3 minutes, the engine will seize about 1-4 minutes later, and if the plane didn’t catch on fire from having a gallon of hot oil sprayed on the exhaust, it will then have a forced landing, where the pilot, passenger, and people on the ground could be killed. This is 100% the fault of the person who made screwed an overweight fitting in an 1/8″-NPT hole.

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Above, Just to be clear, THE PICTURE ABOVE IS NOT AIRWORTHY EITHER. The giant sender is a ridiculous idea on an aircraft, period. If a “Glass Cockpit” company suggests  an oil pressure sender that weighs 6 ounces, get a new company run by people who know planes. If someone is using stuff like this mated to an I-Pad and thinking that is “Aircraft Quality” I respectfully ask them to use someone else’s products to commit suicide with. The unit above is vastly too heavy to be supported by a 1/8″-NPT thread on a running aircraft engine. If someone wants to debate this, let them give their full name and address, so they can get sued when the inevitable happens.

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Above: If a heavy sender must be used, the correct way of doing it is to move the sender to the firewall, and run it off an aircraft hose. This hose must be installed in accordance with the manufacturers hose installation hand book. If you look at well built examples of Van’s Aircraft, it is very common to see installations like the one above, and the company sells small manifolds to complete the firewall installation on sending units.

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Above: There are several places on a Corvair with 1/8″-NPT threads, like the 5th bearing housing. BUT NOTE THE HOSE ATTACHED IS PROPERLY SUPPORTED WITH AN AIRCRAFT CLAMP, so it is a cantilever installation, nor is the fitting subjected to loads imposed by the hose.

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Above, Another 1/8″-NPT thread on a Corvair are the oil pressure ports in the gold oil filter housing. Again, note the hoses attached to the fittings are properly supported with pass-through grommets in the baffling and Adel clamps. The smaller braided line runs to the firewall, to take the weight off the 1/8″-NPT threads. Lycomings and Continentals are done this way also. There is nothing wrong with smaller threads, just as long as they are not overloaded by fools putting in excessively heavy fittings and senders.

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Above: THIS IS THE WEIGHT LIMIT ON A 1/8″-NPT THREAD: The sending unit on the left is a common oil temperature sending unit. It is being balanced on the see-saw made out of a 6″ ruler by three U.S. Quarters. If the fitting being screwed into the thread MATCHED OR EXCEEDS THE WEIGHT OF THREE U.S. QUARTERS, IT IS TOO HEAVY.  This is my personal limit. Again, if anyone is bold enough to say something heavier will work, please include your full name and address, and your net worth, because these are the kinds of things the lawyers will want to know later.

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-wewjr.

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xxxxxxxxxxxxxxxxx

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* If there is anyone who finds my tone in this angry, or paranoid, they should send me their name and number, and later when I can speak about the ridiculous things an attorney could try to blame on someone , I will call and share with them why I write this way, and then they will have a chance to apologize to me for their assumptions.

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About William Wynne
I have been continuously building, testing and flying Corvair engines since 1989. Information, parts and components that we developed and tested are now flying on several hundred Corvair powered aircraft. I earned a Bachelor of Science in Professional Aeronautics and an A&P license from Embry-Riddle Aeronautical University, and have a proven 20 year track record of effectively teaching homebuilders how to create and fly their own Corvair powered planes. Much of this is chronicled at www.FlyCorvair.com and in more than 50 magazine articles.

12 Responses to WARNING: 1/8-NPT thread weight limit.

  1. Sarah Ashmore says:

    Vans Aircraft makes a simple part,VA-168, that can serve as a remote mounting block for three different pressure senders on the firewall. All you need is a “-3″ line from the source to the mounting block so there will be no weight on those little 1/8” NPT fittings.This does the dual function then of protecting the fittings on the engine and isolating the transducers from engine vibration. I have one in my aircraft projects parts inventory that I plan to use for Oil, Fuel and Manifold Pressure and at $19 it is a good option.

  2. L. Harris says:

    If I recall correctly, it was about 1982. Albuquerque Balloon Fiesta. Professional balloon owner had a similar setup with a fitting and a propane line that needed repair. Unable to get an aircraft quality part quickly, he bought a brass fitting at the nearest Ace Hardware and installed it. He took a bunch of people up in the balloon. The part failed on landing spraying pressurized liquid propane into the gondola. Intense fire ensued sending the balloon back into the air. 6 or 8 people died. Some burned to death, some fell to their deaths. Heed this advice.

  3. Chrissi Bush says:

    A brass elbow coming off the carb on a friend’s Long-Ez snapped, he put it down on a farm road. He did not build the plane, afterwards he went through it with a fine comb for wonky parts.

  4. Dave Hoehn says:

    William,
    This is some valuable info that you have provided.. I originally had my sender cantilevered off of a brass fitting similar to your picture. The eyeball engineering of it made me uncomfortable, but I had no feel for what was too much. I changed things around to support the sender with an adel clamp mounted off of the cooling baffling, but I’m not completely sold on that either. I like Sarah’s use of the Vans manifold, and will probably use that once I have a firewall to work with. Question: I currently have the nason switch that you sell mounted in the top oil pressure hole. As I recall, it is fairly light, but I don’t recall if it was ‘3 quarters’ light. Should it be moved off of the oil filter housing too?

    • Sarah Ashmore says:

      That Vans manifold has three ports for each system so you could attach the Nason switch there along with the oil pressure sender, the third one is the pressure line. In my own planned installation all the extension lines are “-3″ sized and I have a flow restrictor installed at the engine in case there is ever a leak down stream. The flow restrictor allows pressure through but has a very small hole in it to minimize flow, a good safety feature for remote mounted pressure senders but it needs to be first in line. Check Wag Aero (and Wicks) for part number A-123-003 Restrictor Fitting at $9.95 each. It is a brass cylinder with male 1/8″NPT at one end and female 1/8″ NPT at the other, there is a 0.040” orifice inside to limit any flow through it. Acft Spruce has 05-11908 at $33.95 which is a modified AN824-3 (45 deg elbow, NPT to AN flared) and I think Van’s carries it as well. If you are going to do the right thing and remote mount pressure senders then add the restrictor and get that bit of extra safety.

      • Dave Hoehn says:

        Sarah,
        Roger. I had already started thinking along those lines. Thanks for the price and part number info.

        William,
        A related question: With or without a flow restrictor, there is no oil flow thru the line to the pressure manifold, and the line and manifold represents a substantial air pocket. While the pressure everywhere within a small closed vessel is the same regardless of the shape of the vessel and the mix of fluids in it, the mechanics of the pressure sender may require actual contact with the oil (an unbled brake line comes to mind). Should the remote line and the manifold be pre-filled with oil prior to installation?

      • Sarah Ashmore says:

        As an engineer who has worked in Flight Test I would say that Pressure is Pressure be it exerted by a liquid or gas. In fact if you look at the catalog for UMA, a large manufacturer of instruments, their pressure transducer section makes no mention of what they are going to measure, just the range of values they can detect.

  5. Fred says:

    A little arrogant are we?

    • Fred, the general rule for my blog is your last name is required for a comment. Maybe you missed the last paragraph I wrote. In the last 28 years in aviation I learned what I know from any source I had, I didn’t label people and then decide if they had something to teach.

  6. David Lynch says:

    Extremely helpful thanks!

    Sent from my iPhone

  7. collen ryan says:

    As an engineer who has worked in Flight Test I would say that Pressure is Pressure be it exerted by a liquid or gas
    not a mechanic so am not quite sure how these sending units work. But as a HVAC mechanic im reminded of an expansion tank on a closed hot water heating system. The liquid [water]can not really be compressed mechanically though it will expanding under temp this expansion could burst the metal system and must be calculated. A gas filled (air) expansion tank of the calculated size is added for the water to expand into by compressing the air. I suppose the sending unit has such a gas filled chamber as well as a sensor to measure the pressure into the chamber.

    WW
    you know I think maybe a pass is in order for the 1/4″ No doubt you know what your are talking about but i would say in the non aircraft world its not uncommon to screw heavier things than that sending unit which i guess weighs less than an oz into 1/4 npt, im thinking of some guages we use in hvac or even sending units in cars. i wouldn’t have made up that contraption with so many fittings because yes its vulnerable being so cantilevered but also because its way to many places to leak and i would always use a reducing fitting rather than even two fittings.And though in HVAC we have rules about proper support i have to say simple screwing the sender into the gold fitting would have seemed more elegant than a bunch of hoses and remote location.
    My only point is maybe some things really are not so common sense. And dont forget not everyone doing this has even a mechanical; background in an unrelated field. reading a couple three dozen builders blogs last month i was astounded how many people built RV 10 that seemed to me had never so much as hung a shelf before.

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