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Pipe thread sealant for fuel fitting

Plummit

Well Known Member
I'm going to replace one of the flare fittings on the fuel tank of my -10. It's NPT on one side and flare on the other. I have all of the sealants ever recommended on VAF and a host of others :eek:. What is the current recommended sealant for the pipe threads? Some have said fuel lube, others Locktite 567 etc.

-Marc
 
Flare fittings = no sealant. NPT = Loctite or Permatex high temp thread sealant.

This is something that's easy to overthink. There are a lot of completely adequate thread sealants for this application.
 
NOT RTV

This is something that's easy to overthink. There are a lot of completely adequate thread sealants for this application.

It is unfortunately also easy to underthink it. I believe there was a case of an RV-10 (?) that was lost due to the NPT fuel fittings being sealed with RTV. RTV is generally not fuel-safe, which lead to a gob of it being liberated, lodging in the fuel flow transducer and stopping the engine.
 
567

Loctite 565 -- per Van's manual...

I stand corrected. However, I will keep using 567 for the high temp spec.
"The*Loctite*product list has another pipe thread sealer with PTFE that they refer to as "high temperature" sealant. This is*Loctite 567*which is similar in specifications to*565*except for higher temperature and, of course, higher cost."
 
Flare fittings = no sealant. NPT = Loctite or Permatex high temp thread sealant.

This is something that's easy to overthink. There are a lot of completely adequate thread sealants for this application.

Did you perhaps mean "Permatex High Performance Thread Sealant", which is equivalent to Loctite 565? Loctite 567 is a higher temp version of Loctite 565 as Larry has said.
 
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25 or so years ago when I was building my RV-6, I purchased "Bakerseal" from Aircraft Spruce. I still have a LOT left and use it on all NPT fittings.

Not my images but found them on the web.

Screen Shot 2021-05-28 at 7.20.15 PM.png

Screen Shot 2021-05-28 at 7.20.55 PM.png
 
Did you perhaps mean "Permatex High Performance Thread Sealant", which is equivalent to Loctite 565? Loctite 567 is a higher temp version of Loctite 565 as Larry has said.

Sure, ok. Although I confess that i have all three and have always used them interchangeably on automotive applications, and now airplanes….I recently replaced the boost pump, sump and tank drains, and pretty soon I have to address the recall on my Kavlico fuel and oil pressure sensors.

(‘Scuse me… I should say “service advisory” rather than “recall”…)
 
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Ditto on the Bakerseal; 39 years and nary a leak on any fuel, oil or air fitting on the EZ.
 
It is unfortunately also easy to underthink it. I believe there was a case of an RV-10 (?) that was lost due to the NPT fuel fittings being sealed with RTV. RTV is generally not fuel-safe, which lead to a gob of it being liberated, lodging in the fuel flow transducer and stopping the engine.


Thread sealer (pipe dope) should never be put on the threads inside a fitting, for this very reason. It should ONLY be put on the outside threads of a NPT fitting. And never on a flare fitting.
 
Caution with Perm #2

Did you perhaps mean "Permatex High Performance Thread Sealant", which is equivalent to Loctite 565? Loctite 567 is a higher temp version of Loctite 565 as Larry has said.

Loctite 567
Permatex #2
Not sure about temp.

I’ve used it for years in automotive applications but I could not stop the leaks where I used it on my -12. So I disassembled all the connections, cleaned them really well and reassembled with Loctite 657. No problems since.
 
Not only does 567 have a higher temp rating, but it also has a higher resistance to ethanol, so if you ever think you might be running fuels with some ethanol, you should consider this.

An important difference between the Loctite 567 and Permatex Aviation 2 or 3 is that the permatex can be dissolved and cleaned with alcohol (ethanol), whereas Loctite 567 will need either "Loctite Solvent", MEK or comparable type solvent.
 
Pro seal

I used Pro Seal on pipe threads in the fuel system of both of my Rvs.
No need to explain pro seal but it has very low tensile strength and if you ever need to open any of those fittings, its quite easy.
Obviously nothing on flare fittings.
2 airplanes, over 1000 hours of flying and zero leaks.
 
I've use PTFE Paste sealant on NPT threads for many years, in automotive, industrial and aviation applications. NO ISSUES in 45+ years (stupid me telling my age).
DO NOT UNDER ANY CIRCUMSTANCES USE RTV AS A THREAD SEALANT!.
Teflon tape---inspite of many old timers, (older than me) is not recommended.

There are many excellent products on the market, but I've used PTFE paste almost exclusively.

Tom
 
What about our harshest application, that I can think of at least?


Recommended product for high temp, fuel applications e.g. the 1/8" NPT for fuel injectors?

Being exposed to fuel, the hottest part of the cylinder, and combustion byproducts, I can't think of a tougher application. Let me know. Thanks.
 
What about our harshest application, that I can think of at least?


Recommended product for high temp, fuel applications e.g. the 1/8" NPT for fuel injectors?

Being exposed to fuel, the hottest part of the cylinder, and combustion byproducts, I can't think of a tougher application. Let me know. Thanks.

I believe Locktite 567 works here. 567 is the higher temp rated stuff.
 
I came across this thread researching the latest in thread sealants.

Loctite recommends sealant on flares (unless I’m misinterpreting the attached), and that’s what I’ve been doing for years.

https://www.new-line.com/nldocs/resources/Loctite_Do_It_Right.pdf

You are exceptionally fortunate -- if you apply sealant (of any kind) to the flare surfaces it will "squish-out" when the B-nut is tightened.

From there, the residual may break off into the fuel stream and plug something important (e.g. fuel distribution spider, injector, etc.).

IIRC there was a fellow in an RV-10 who died from similar practices -- although it was PTFE tape on the AN/NPT fittings; same net effect.
 
25 or so years ago when I was building my RV-6, I purchased "Bakerseal" from Aircraft Spruce. I still have a LOT left and use it on all NPT fittings.

Not my images but found them on the web.

View attachment 12023

View attachment 12024

My 22+ year old bottle of Bakerseal. Excellent sealant for pipe threads and good from freezing to 600*F. Every once in a while I need to mix it up with a stiff “mixer” (flat screw driver) because the heavier components can settle after a couple years or so. This bottle is still more than 1/2 full.

4F09D30D-51A9-4332-8FDC-0B08F1831C56.jpg

FEC7F4F5-3914-4745-9DE4-31002EA66902.jpg
 
The term "pipe thread sealant" would only apply to a joint under very low working pressure. Pipe threads are designed to "self seal" due to their design. They are tapered about 3/4" per foot, so the larger diameter keeps compressing into the smaller diameter to finally form a seal.

The pipe thread sealant that you are discussing here IS REALLY a pipe thread Lubricant designed to reduce friction and allow the threads to make a 100% seal, metal to metal.

All male fittings need to be protected from damage before installation. Inspect before you apply your lubricant. A lubricant with PTFE will provide the needed lubrication for leak free joint.

I have used this product on 1000's of connections in my business with very low failures that were resolved by replacing a defective fitting.

PTFE.JPG
Can be found at any hardware store.... 4oz. for about $6.00 enough for over 30 RV projects....

Do not fill the threads. Apply a thin coat with an acid brush with the bristles cut short. Avoid the first 2 threads.

NOTE... before you decide, please read this.... https://vansairforce.net/community/showpost.php?p=1669381&postcount=34
 
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I have mostly used Permatex 2 till recently. I was removing Andair fuel filter from the fuel pump for cleaning yesterday which had the Permatex and had a real hard time removing it. I used Loctite 567 for installation this time hoping it will not harden as Permatex and makes the removal easier next year easier.
 
I used permatex 2 on the brake fittings, but by the time I got to the oil lines I had talked to our DOM at work. He mentioned that they use Loctite 567 for pretty much everything and have never had a problem with it.

Our flying club fleet flies a ton, so if anything was going to leak, it wouldn't take long to show up. Based on that, I switched over. Not flying yet, so take that for what it's worth.
 
What about our harshest application, that I can think of at least?


Recommended product for high temp, fuel applications e.g. the 1/8" NPT for fuel injectors?

Being exposed to fuel, the hottest part of the cylinder, and combustion byproducts, I can't think of a tougher application. Let me know. Thanks.

If your 1/8" npt connection to the fuel injectors is exposed to combustion byproducts, you've got bigger problems than what sealant you're using.
 
If your 1/8" npt connection to the fuel injectors is exposed to combustion byproducts, you've got bigger problems than what sealant you're using.

Yep. Brain farted that one. Had been looking CF turbine stuff earlier stuff earlier. Anyway. Looks like 567 is th pe way to go. Appears to have been used by Titan on mist everything except the spark plugs.
 
Loctite 567

Loctite 567. Really good stuff. No more sticky fingers. I will never return to tight seal or fuel lube.
 

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You are exceptionally fortunate -- if you apply sealant (of any kind) to the flare surfaces it will "squish-out" when the B-nut is tightened.

From there, the residual may break off into the fuel stream and plug something important (e.g. fuel distribution spider, injector, etc.).

IIRC there was a fellow in an RV-10 who died from similar practices -- although it was PTFE tape on the AN/NPT fittings; same net effect.

I wonder why Henkel recommends that then. Their research and documentation seem to be world class.

The “damaged” part hasn’t been applicable in my case as I try to avoid using those….
 
I wonder why Henkel recommends that then. Their research and documentation seem to be world class.

The “damaged” part hasn’t been applicable in my case as I try to avoid using those….

The way I read their use document every instance of applying their product to a tapered sealing surface (flare, ferrule, union etc...) they stipulate damaged faying surfaces. But as you mention in aircraft we don't tolerate damaged sealing surfaces except in the case of emergency (think Flight of the Phoenix).

When I inspect a new aircraft I always assume the builder or past mechanics didn't know what they are doing, as a rule of thumb. However, if I see sealant or Teflon tape poking out of flare fittings I go into emergency inspection mode with solid proof that somebody truly didn't.

A good friend who owns a local aircraft maintenance shop would fire a mechanic who applied any type of sealant substance to a flare fitting. They are supposed to be dry which is part of how they are designed to work. And if it was Teflon tape he would throw them in a pit with a bottle of lotion.
 
The term "pipe thread sealant" would only apply to a joint under very low working pressure. Pipe threads are designed to "self seal" due to their design. They are tapered about 3/4" per foot, so the larger diameter keeps compressing into the smaller diameter to finally form a seal.
It is true that there is a type of tapered pipe thread that will entirely seal on its own by deformation of the material, but standard NPT threads like those on AN or MS aircraft fittings will not do that. Even if you torque it extremely tight, there is still a probability of a micro leek along the spiral path of the threads.
That is why we use thread sealant.
 
I wonder why Henkel recommends that then. Their research and documentation seem to be world class.

The “damaged” part hasn’t been applicable in my case as I try to avoid using those….

They likely said damaged, because it is not a good practice but if you have a situation where you have to solve a leak without being able to replace the line that it is a method that works. It’s just not a good practice, so it shouldn’t be done as a standard process
 
It is true that there is a type of tapered pipe thread that will entirely seal on its own by deformation of the material, but standard NPT threads like those on AN or MS aircraft fittings will not do that. Even if you torque it extremely tight, there is still a probability of a micro leek along the spiral path of the threads.
That is why we use thread sealant.

Most all of my exposure to pipe threads is with SCH 80, 3/4" to 4" pipe used to contain and transfer Liquefied petroleum gas (propane in liquid form). The connections must be 100% leak free with a max working pressure of 250 psi.

Fittings were purchased in bulk, and the pipe was threaded on site. I doubt that the threads were to any higher standards than the fittings we use on our RV's.

The only leak on my RV was a Matco master cyl. After disassembly I found the fm threads were not drilled and tapped deep enough and the fitting bottomed out. I had to shorten the fitting by 3/32".

This is such a small issue, just use what Van's suggest... Loctite 565.
 
I came across this thread researching the latest in thread sealants.

Loctite recommends sealant on flares (unless I’m misinterpreting the attached), and that’s what I’ve been doing for years.

https://www.new-line.com/nldocs/resources/Loctite_Do_It_Right.pdf

I’m still waiting for a clarification reply from Loctite on the above, but on a plastic airplane forum, reportedly McCauley is recommending the product below to seal a not so well designed prop governor plug.

I noticed that it directly states it can be applied on the flare surfaces-“ LOCTITE 5452TM can be used as a sealant on mating surfaces of flare style fittings to seal scratches and surface imperfections.”

https://tds.henkel.com/tds5/Studio/...=REAC&language=EN&plant=WERCS&authorization=2


FYI and obviously I’m not condoning using imperfect flares.
 
Does anybody know if this Permatex product is equivalent to (as good as) Loctite 567?
Permatex 17358 High Temperature 592 Thread Sealant, 50ml
 
Does anybody know if this Permatex product is equivalent to (as good as) Loctite 567?
Permatex 17358 High Temperature 592 Thread Sealant, 50ml

I have used the Permatex 592 on 3 different planes over the last 30 years and it performs as expected.
 
Loctite 565 - Not So Good - Sheared Fitting

I completed the fuel system in my 2013 RV-12 Kit in March of 2023 and used Loctite 565 on the NPT threads between the aluminum AN816-6-2D fittings and the Facet fuel pump.

On April 25, 2023 Van's issued Service Letter 00078 regarding vapor lock in RV-12's and based on a video talk that Rian Johnson gave after a vapor lock incident in Oregon I decided to replace the never-run Facet 40105 pump with a higher pressure Facet 40135 pump in accordance with Van's Notification Letter 18-07-12 before finalizing my flight control hookups.

Disappointed that I snapped one of the AN fuel fittings trying to remove it from the original pump for reuse. Loctite 565 did not have the "controlled strength" that a good thread sealer should.
 
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