Fuel Fittings - Thread Sealant

Discussion in 'Maintenance Bay' started by Rob58, Aug 9, 2017.

  1. Capt. Geoffrey Thorpe

    Capt. Geoffrey Thorpe Touchdown! Greaser!

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  2. Stewartb

    Stewartb En-Route

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    Mostly flare fittings. Makes sense. Odd fuel system. Worthy of replacement! Where are the drops for sumping the low points? I skimmed so maybe I overlooked that? Did you flow test it before and after?
     
  3. mondtster

    mondtster Pattern Altitude

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    Just because they’re a “pro” doesn’t necessarily make them right or good at fuel system repairs. Their talents may lie elsewhere. ;)

    Some of the comments leave me wondering a bit as well.
     
  4. 3393RP

    3393RP Pattern Altitude

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    Some of the responses above allude to an important, yet basic fact about NPT threads. The threads are tapered, and if properly cut, the joint becomes an interference seal when the proper torque is applied. No clearance remains between the peaks and roots of the threads because of the taper.

    Pipe thread sealants perform an important function. They lubricate the joint, which allows the joint to be tightened further than dry threads would.

    This is the primary function of pipe sealants. They don't act as a gap displacement filler or adhesive. They simply provide lubrication, and as the joint is tightened, the thread taper reduces the spiral gap between the peaks and valleys of the threads to an interference fit.

    Of course, thread sealant compounds do provide a mechanical barrier to the passage of fluids and gases, albeit on a microscopic level. However, in most cases, this is a superfluous action, incidental to the lubricating function. The thread taper is the primary means of making a leak tight joint.
     
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  5. Dan Thomas

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    All true, but in many cases you don't dare tighten that fitting as much as it would take to seal it without sealant; you'd crack the pipe boss. The bowl drains on the Marvel Schebler/Precision Airmotive carbs are a prime example of that. The 1/8" pipe plug will easily crack that boss.

    Or you can get galling of the threads; any time an aluminum fitting goes into an aluminum body, there's going to be trouble without some anti-seize of some sort. The galling can happen on installation, and certainly on removal. Think about the oil cooler line fittings in some oil coolers; some OEMs used aluminum fittings instead of the steel they should have used, and I have had unlubricated/unsealed threads tear out of the cooler when trying to remove the fittings.

    And when installing elbows, orientation is important, and most of the time the fitting isn't going to end up pointing where you need it, so you have to back it off or tighten it further, depending on what the assembly can tolerate. Sealant, particularly a locking sealant, is handy there.

    I wish pipe threads had gone away a long time ago. Larger airplanes have been using O-ring sealed straight threads for ages (MS series), which eliminate the need for goopy sealants and the residue they leave, and allow orientiation of elbows with no trouble at all. They seal first time, every time, and disassembly is easy. You'll find them on Dukes or Weldon fuel boost pumps, for instance, but otherwise we're still finding pipe threads.

    [​IMG]
     
  6. Capt. Geoffrey Thorpe

    Capt. Geoffrey Thorpe Touchdown! Greaser!

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    Sumps are in the corners of the tanks and the gascolator on the firewall. There were no drains in the original setup where the fuel lines ran down under the doors and then back up - one of the reasons for rework.
    Didn't bother to flow check since all of the fittings have a (significantly) larger ID and the total run was shorter. So, there was no reason to believe that the flow would be reduced.
     
  7. Timbeck2

    Timbeck2 En-Route

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    I've got two in my cart. Question, will fuel spill out all over the place when I take the primer apart to change out those O rings?
     
  8. JAWS

    JAWS Line Up and Wait

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    Nice video. The one thing that caught my eye was the troublelight. I hate them with a passion - burned my hands more than once. They get too hot, are hard to get in the right place and the bulb is exposed, which is a shatter hazard. I prefer the sealed fluorescent sticks - they are light and can be taped into the needed position. Also the LED style.
     
  9. kgruber

    kgruber En-Route

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    No. Maybe just a couple of drops.
     
  10. 1RTK1

    1RTK1 Line Up and Wait

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    I usually get about 1/2 oz.
     
  11. 3393RP

    3393RP Pattern Altitude

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    Your points are well founded. I didn't intend my post to be taken as stating thread sealers and lubricants aren't needed.
     
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  12. Dan Thomas

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    I much prefer, and have used for years, the LED headlights. The light is always where you're looking, no burn or ignition hazards, and the batteries last a long time. My favorite is the Eveready:

    [​IMG]

    That's not me. I'm much older and more handsome...:)
     
  13. slacktide

    slacktide Line Up and Wait

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    This is not correct. NPT threads have, by design of the threadform tolerancing, a spiral leak path that requires a sealant to be leak-free. The standard for NPT threads is ASME B1.20.1. Quoting directly from the specification:

    1.4 Sealing (NPT and NPSC Only)

    1.4.1 Mating Threads. Mating threads should
    always contact on the thread flanks. The design tolerances
    are such that mating crests and roots may clear,
    contact, or interfere (see Fig. 1). This joint may not necessarily
    seal, unless a sealant is used.

    1.4.2 Sealant. Where pressure-tight, leak-free joints
    are required, it is intended that threads conforming to
    this Standard be made up wrench-tight with a sealant.
    To prevent galling during installation, the sealant may
    have lubricating properties.

    There is a different tapered threadform, NPTF Dryseal, that guarantees a leak-free joint. This threadform is under a different specification, ASME B1.20.3.
     
  14. bnt83

    bnt83 Final Approach

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    I've got about three of those, absolutely the best money I've ever spent on lighting for maintenance.
     
  15. G-force

    G-force Pre-Flight

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    Correct, like I posted a week ago but people can't be bothered with facts lol. NPTF actually has two classes of thread form precision, class 1 still needs sealant, class 2 it truly "dry seal." See: http://greensladeandcompany.com/wp-...hreads/Threads - NPTF Taper Thread Gaging.pdf The problem is there are many tapered thread forms with various classes of fit, however most people just use "NPT" to cover them all when they should be more specific.

     
  16. 3393RP

    3393RP Pattern Altitude

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    Well, once again stuff I've been told is accurate turns into pie on my face. :oops:
     
  17. Dan Johnson

    Dan Johnson Filing Flight Plan

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    IMG_1445.JPG "I wonder how many guys who replied actually have built or installed an aircraft fuel system? I know a couple of the pro mechanics have and some of their replies have me scratching my head."

    Okay so here is a shot of my 20 year old can!
     
  18. Tom-D

    Tom-D Taxi to Parking

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    I wonder how many folks here know the difference between " (mil-Spec-Grease Valve plug) " and fuel lube?

    https://www.sae.org/standards/content/amsg6032a/
     
  19. Let'sgoflying!

    Let'sgoflying! Touchdown! Greaser!

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    Continental's 31 Jul 2017 M-0 Standard Practice Maintenance Manual Table 3-4 Sealants p11/40

    657042 Loctite 565 Adhesive Sealant
    Use on all pipe threads to oil coolers and other oil sources All models (use sparingly on male threads only)

    Loctite 592 Teflon PS/T Pipe Sealant
    Use on all pipe threads except as noted elsewhere
    All pressure relief valve housing threads Permold 2 studs engine mount 1-3-5
    side bottom All models, where applicable
    All threaded fasteners installed in a stud hole through to an oil source
    Apply before installing threaded fastener

    And below that in Table 3-5 in the Lubricant section:
    646943 Loctite 76732 Anti-Seize Lubricant Fuel injector nozzles (at cylinder head)
     
  20. Dan Thomas

    Dan Thomas En-Route

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    No difference. Exactly the same MIL spec as Fuel Lube and EZ Turn.
     
  21. bnt83

    bnt83 Final Approach

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  22. Let'sgoflying!

    Let'sgoflying! Touchdown! Greaser!

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    Did you fail to add the above? (I added it in bold for you) Glad to help out!
     
  23. Tom-D

    Tom-D Taxi to Parking

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  24. Tom-D

    Tom-D Taxi to Parking

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    Not true. The mil-spec stuff is a heavy wax, unlike the greasy stuff you can buy in the tube. Does the same thing, but different.
     
  25. Dan Thomas

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  26. Tom-D

    Tom-D Taxi to Parking

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    Probably just like Bondrite and Alodine 1200, same mil-spec, different stuff. does the same thing.
     
  27. Dan Thomas

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    It does stiffen with age. I've seen it. The tech sheet pdf I posted gives the specs for the AMS-G-6032: consistency is soft, NLGI number is 3, and so on. It all starts out as the sticky grease. We had an old can of it that was far stiffer, more like hard honey. Probably 30 years old. There are some solvents in it that gradually vaporize.
     
  28. Tom-D

    Tom-D Taxi to Parking

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    I am old enough to remember opening new cans of the "Grease Valve Plug" when I was in the NAVY. The stuff in the can was a lot stiffer than any thing we get in a tube now days.