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Mixing Steel and Aluminum AN fittings?

rv8ch

Well Known Member
Patron
Hi,

I would like to use steel fittings FWF wherever I can, particularly the bulkhead fitting for the fuel line. However, the hose that comes with the FWF kit has an aluminum "nut" on it. Anyone know if this will cause issues?

Thanks,
Mickey
 
It's not a problem. Steel fittings, such as the one for a constant speed prop oil line will thread into the engine's aluminum casing.

L.Adamson --- RV6A
 
I was taught not to mix steel and aluminum fittings. They will thread together just fine. The problem is getting them apart after years of dissimilar metal corrosion. I would suggest either using an aluminum bulkhead fitting, or changing the female end of the line to steel to match the bulkhead fitting.

Threading steel fittings in to aluminum castings is different. Normally those are a non-flared pipe thread fitting, and are coated with pipe dope before installation. The dope not only acts to seal the threads, but also inhibits corrosion. In contrast, flared AN fittings are threaded together dry.
 
I was taught not to mix steel and aluminum fittings. They will thread together just fine. The problem is getting them apart after years of dissimilar metal corrosion. I would suggest either using an aluminum bulkhead fitting, or changing the female end of the line to steel to match the bulkhead fitting.

With over 40 years in the HVAC business, I just haven't found sever cases of dissimilar corrosion between aluminum & steel. We'll always protect the copper from steel, but not aluminum.

What's interesting, is your typical air conditioner coil. It will be copper tubing pressed into aluminum plates...........and with a lot of moisture present. So what's the secret? It's the fact that the copper is expanded, and pressed so tightly against the aluminum.

So............let's get a few more opinions, perhaps.

L.Adamson --- RV6A
 
IMHO, all fuel and oil fittings FWF should be steel. You did the right thing swapping out the aluminum bulkhead fitting with steel so don't use the Vans hose with an aluminum fitting,,,,send it back for credit or sell it to someone who doesn't care and replace the hose(s) with some nice aeroquip hoses with fire sleeve and steel fittings. If you want to use that hose though,,, just use some EZ Turn (fuel lube) http://www.aircraftspruce.com/catalog/cspages/ezturnlube.php on the fitting when you assemble,,,,,it will be fine. DO NOT USE PIPE DOPE or TEFLON SEALANT!
 
Titeseal

After a leaky oil cooler fitting, I was recently taught that Fuel Lube is not the best sealant to use for the oil system because of the high pressure it is under. A better choice is Titeseal, available from aircraft spruce. This stuff has a much different consitency than fuel lube.

A question I posted years ago about putting steel nuts onto aluminum fittings was that of what torque value to use. I believe the concencus was to use the smaller (aluminum) torque value, but I've never seen authoritative data on this. I guess the authoritative sources would say you shouldn't be mixing the two.
 
Isn't the oil cooler itself aluminum?

Seems to me that at *some* point in most of the systems we're talking about here (oil, fuel), you'll have steel-to-aluminum in some fashion or another. E.g., fuel...There's a mix of steel and aluminum parts and fittings. The fuel servo is aluminum, the fittings are what, brass, I think? (Not in the shop at the moment). Steel fittings on steel-braided hose to the pump, which has steel fittings, but what's the pump made of? From there, steel fittings-steel-braid-steel fitting at the firewall, but then Al fittings and AL tube from there, right?

So what's the *general* principle of when it's okay to mate a steel fitting to an Al fitting?
 
no lube on AN fittings?

After a leaky oil cooler fitting, I was recently taught that Fuel Lube is not the best sealant to use for the oil system because of the high pressure it is under. A better choice is Titeseal, available from aircraft spruce. This stuff has a much different consitency than fuel lube.
Hi Phil - I thought that you weren't supposed to use sealant on AN fittings... that the seal came from the fitting tightly holding the flare in the tube against the matching shape on the mating fitting.

Are you putting the Titeseal on the threads, or on the flare? Or am I way off base here?
 
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I believe he was referring to the pipe threads requiring sealant, not the flared end. No sealant on the flared end (but a dab of engine oil does not hurt).
 
Steel fittings

I used steel fittings in the oil cooler as advised by several sources. I used Van's oil cooler hoses, which have steel flares. The system leaked at both the inlet and outlet of the cooler, at the B-nut. I tried tightening the nut, but it was already tight and the leak continued. I tried "Dell" washers, a soft aluminum washer meant to seal the flare. No joy, and a pain to install correctly. I changed the steel fittings on the oil cooler to aluminum. The softer material allowed the flares to mate better and seal.
 
My experience / Examples on Certified Aircraft.

Back in the day when I used to remove and install Aircraft Hardware for a living instead of selling it; Here is what I experienced on Type Certificated Aircraft concerning Hydraulic/Plumbing/AN-MS Fittings.

Oil coolers got steel fittings with black cad II finish. Most of them were male pipe going into the oil cooler body. The other end of the fitting was a 37 degree flare.

If the fitting was larger than -6, it would get an aluminum B nut on the hose, if it was -6 it would have a steel nut with steel center that it pivoted on, The hose fitting outer shells were almost always aluminum. These are like the ms24587 fitting assemblies or equivalent. IE Aeroquip 491-? or Stratoflex 300-? A or S

Hope this helps:D
 
Take it easy on the torque - don't strip those threads

I was directly involved with a serious incident, wheels-up landing after a hydraulic line failed and the blow-down bottle did not drop the gear.

Problem? steel to aluminum fittings.

A mechanic over torqued the steel fitting from the hydraulic line to the aluminum bulkhead fitting. The result was a weakened aluminum fitting that failed. The 1,800 psi hydraulic pressure "popped" that line right off the fitting and when that happened, the nitrogen "emergency" blow down bottles just went out the blown out line. Of course this was a 35 year old commercial airplane, but the point is the line was over torqued.

AC 43.13-1B Chapter 9 table 9-2 shows the torque data for the fittings.

Hopefully you choose steel only if all your components are steel.

Here's the reason. You're "torquing" the mating surfaces of the flared fittings. If you've ever flared aluminum tubing, you know how malleable "soft" aluminum actually is. When you "torque" the fitting, you're crushing the soft aluminum mating surface against the cold steel.

Take it easy on the torque - don't strip those threads.



I read a lot more than most A&P, IAs
 
Dave, the aluminum fitting was intact, and the steel nut failed? Or did the flare, or MS sleeve fail on the steel line? The aluminum fitting didnt break and a structurally superior steel rigid tube failed?
Not to challenge or disagree with you because stranger things have happened, but usually its the other way around. Aluminum nut on an aluminum rigid tube fails after overtorque. The tube fails behind the flare at the merge point of the tube and the start of the flare. The overtorque squeezes the sleeve against the merge point and creates a stressed area. Dont see it alot on steel tubes, but so see it with damaged flares due to overtorque.

If you have to overtorque a joint to get it to seal, take it apart and check the mating surfaces. We have run some of these lines and fittings to 7500 psi in our hydrostatic pressure tester with tightening with 2 stubby wrenches.

Tom
 
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