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prkaye

Well Known Member
Around the time that the outside temperature dropped in the fall, I started noticing higher oil consumption. I also noticed tonnes of oil on the belly of the plane, so I assumed it was coming out the breather. But I couldn't understand what would have caused the change in the amount of oil being spat out.
Today after a flight I took the cowls off to drain the oil, and had a careful look around. I noticed oil around the bottom fitting of the oil cooler. And there was a pool on the inside of the bottom cowl more or less below this fitting. So I took the oil cooler off and brought it home to inspect it, but I don't see any cracks.
It appears oil started working through the pipe thread on the AN fitting that goes into the bottom of the oil cooler. I'm wondering whether the drop in outside air temperature could have caused the fitting to deform from thermal contraction, causing it too loosen up enough for a leak to start.
Is this a reasonable theory?
 
possibly, but--

Phil---its "possible" for thermal contraction to cause a small leak, but you are saying there was lots of oil on the cowl. First, are they Aluminum AN fittings, or steel fittings? Also---were the hose ends tight on the fittings? IF they are AN fittings, just as a precaution, I would check for fatigue cracks at the threads--especially if the fittings were tight in the cooler. You may get alittle seepage with NPT threads, but not "alot" of oil without a loose or cracked fitting. If you have any doubt, change the fittings in the cooler.
MY opinion-
Tom
 
used fuel lube... is there something better?

it's an aluminum fitting. The oil did appear to be concentrated around the pipe thread as opposed to where the hose connects.

I removed the fitting, cleaned the threads and inspected it visually. There is no obvious cracking or gauling of the threads, just a few slight scuffs where the blue coating has rubbed off the edge of the threads.

I forgot to bring hte plans home with me, and i want to order a new fitting. Can anyone tell me what the size of that fitting is for the oil cooler?
 
fittings

Some recommend steel fittings. Pitts factory uses aluminum, but if you don't use the right sealant it is easy to destroy the fitting and the cooler. I have always used Titeseal, but found fairly recently that Titeseal components will separate is extremly hot weather and then will not function properly. This requires mixing the Titeseal throughly with a thin screwdriver or something similar, it will again work properly. Titeseal was the standard for a very experienced mechanic I worked for years ago and I have never used anything else.
 
I would guess that the increased oil pressure caused by cold temperatures pushed the fuel lube out of the spiral leak path formed by the NPT threads ...fuel lube is not the first choice for NPT connections.

I use Titeseal to seal NPT threads on a hose pressure tester that routinely reaches 8,000 psi. but any commercial pipe thread sealant should work.

NPTpath.jpg


When using NPT there is a passageway (shown with red mark in picture above) from the inside to the outside that exists regardless of how tight one screws-in the fitting. The sealant must plug this passageway and be strong enough to resist oil pressure.
 
Thanks John, that is very informative! Two things I did not realize:
- oil pressure is higher in cold temps
- fuel lube is not the best choice here (I had come to the belief that fuel lube was the magical substance to use on *all* NTP thread fittings).

My plan is to get steel fittings and titeseal (and possibly a better oil cooler).

Questions
1) how much titeseal should i put on the NTP threads?
2) for an -8 size steel fitting into an alum oil cooler does the "finger tight, then 3 full turns" rule apply?
 
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What I meant to say is that oil pressure is higher with cold (thicker) oil, not that it is higher in colder temps.

Use sparingly; only enough thread dope (Titeseal) to fill the engaged male threads.


"3 full turns" - right-on

ANSI/ASME B1.20.1-1983 states that the nominal engagement for 3/8 wrench tight is 4.32 for "hand tight engagement" +3 for 'wrench makeup" =7.32 total turns.

At 18 threads per inch, that is .407" engagement.

This is for threads that meet the specification. Since you hardly ever find quality NPT threads, here are my instructions:

Steps to installing a NPT fitting into a port:
1. Fitting should rotate 2-3 turns until finger tight. This checks for three things:
a. it is easy to cross-thread a NPT so the finger-tight makes sure no crossed threads.
b. Some poor quality fittings have threadforms that are out of spec. If the fitting goes past 4 1/2 turns or less than 2 then something is wrong.
c. If the fitting screws all the way to the end of the threads then you don't have a NPT port and you are using the incorrect fitting.
2. Wrench make-up is 2-3 turns from finger tight.

Fitting should be snug.
 
We used Permatex aircraft gasket sealer (the gooey brown stuff) on the threads of aluminum AN fittings. Not a drop of leakage in almost 150 hours on the RV-8. The -8's hangar-mate, an RV-4 was done the same way, and not a drop of oil leakage from its cooler fittings in over 750 hours and 6 years either.
 
Thanks again John. And your comment about the channel needing to be plugged with a sealant was bang-on, according to what is written on the Wikipedia article about NPT:

"The taper on NPT threads allows them to form a seal when torqued as the flanksof the threads compress against each other, as opposed to parallel/straight thread fittings or compression fittings in which the threads merely hold the pieces together and do not provide the seal. However a clearance remains between the crests and roots of the threads, resulting in a leakage around this spiral. This means that NPT fittings must be made leak free with the aid of thread seal tape or a thread sealant compound. (The use of tape or sealant will also help to limit corrosion on the threads, which otherwise can make future disassembly nearly impossible.)"

Of course, I know that the seal tape is a no-no for us. I've ordered some Titeseal Medium compound, as per your recommendation.
 
sealer

Hi guys, I was wondering, would teflon tape be good for these type fittings? It can take the tempature and does act like a lubricant when installing to prevent galling of aluminum fittings.

bird
 
bird said:
Hi guys, I was wondering, would teflon tape be good for these type fittings? It can take the tempature and does act like a lubricant when installing to prevent galling of aluminum fittings.
Click to expand...

No....

A combination of teflon tape & a pipe thread sealer work great...........but teflon tape can get into the system, which means into the oil passages, which plugs things. Once you take a teflon tape sealed part.....apart, small pieces can get left inside the fittings.

L.Adamson --- RV6A
 
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