How big an issue using different metals?

[glider4]

I've read about the problems with having aluminum heating ducts on the firewall so I went to a stainless duct. But what about the fuel line bulkhead fittings? I'm fuel injected so I don't have a gascolator attaching to my firewall bulkhead fuel and purge return line fittings. I have some non-stainless steel bulkhead fittings I could use but a local A&P recommended I stick with aluminum AN832s since my AN sleve fittings and tubing are aluminum. He thinks the corrosion potential from having steel on aluminum would be a bigger issue. If this is a bigger problem, what about using stainless brake lines on the gear legs if they attach to aluminum AN832 bulkhead fittings? If you want to use stainless or steel anywhere in the fuel and brake line system does every single part need to be the same metal?

Al Thomas
N880AT RV-8A QB
FWF & Finishing

[L.Adamson]

This is my take on this subject. I've been in the HVAC (Heating & Air Cond.) business for 36 years. We're always mixing steel, aluminum, and stainless together without corrosion problems.

When it comes to copper being in contact with these other metals, I take the precautions for dissimilar metals. I have yet to see a problem with copper tubing, such as the 1/8" primer lines, combined with AN fittings.

I also prefer "steel" fittings for making the connection to the carb. where engine movement/vibration is critical.

FWIW--- An air conditioner coil has copper tubing in direct contact with aluminum fins. However, the tubing is swedged in extremely tight, which severely limits moisture between the two metals. This arrangement is somewhat sacrificial, corrosive wise, but will last many, many years. But for our purposes, we don't have this tight of fit.

L.Adamson

[frankh]

Yes I know all about dissimilar metals and if you were pumping salt water it would be a big deal.

But fuel is oily and I flew an airplane for several years with dissimilar metals in the fuel fittings and its not an issue in real life.

And remember your little hydraulics shop in your own town. They have just about every fitting imaginable in zinc plated fittings including bulkhead fittngs....They are called JIC threads (not AN) but they are the same thing.

I really felt the need to change out my bulkhead connector to steel...But sorta forgot and used the aluminium one anyway.

It would take a pretty big fire I think to burn through the fitting...lets hope none of us find out!

Frank

[rzbill]

Originally Posted by frankh

They are called JIC threads (not AN) but they are the same thing.

While true, be careful to get 37 degree and not the 45 degree JIC's. You may find the 45's are more common in non-aviation circles (therefore commonly in stock at local hydraulic shops), so purchasing steel aviation fittings might be the safest bet.

[frankh]

Hydraulic fittings are 37 deg not 45.

45 is found in plumbing but hydraulics is 37 deg.

The two hydraulics shoppes I frequent stock nothing but 37 deg flares. Its easy to take an AN fiting with you and compare to make absolutely sure.

Frank

[rzbill]

My mistake.

The 45's I mentioned are SAE spec, not JIC.

I have seen them mixed inappropriately in industrial situations, so I thought they would both be readily available at a local vendor that supplies parts to maintenance crews, in order to cause the mixup. I bow to your direct experience at the hydraulic shops.

[frankh]

Of a cheapskate...

I like my hydraulics shoppe, the fittings are so cheap that its fun to weld up cutom fittings...I had to weld a 45 to nipple at an included angle of 60 deg to clear the oil filter adaptor...Quite satisfying..

Frank