AN Fuel Fitting torque?

Plenty of posts about this on the vans forum, but I haven’t seen anything here.

There are reports of torquing them to spec and still having leaks.

I have read finger tight, then 1 or 1 1/2 flats.

One post recommends the flat method, but repeating it 3 times.

Another post says to use a sacrificial fitting for the first tightening, and then swap it out for a permanent one.

Another says on the loose end to start, and tighten until they don’t leak.

What’s the Bearhawk builder’s consensus on this?

I didn't stress too much about it. There are so many rabbit hole opportunities that I have to choose carefully which ones to fall into. I have had fittings that leaked, most often because I forgot to tighten them. Sometimes because I didn't make a good flare. Excepting those cases, it just hasn't been a big problem. Sometimes a fitting seeps a little and I snug it just a little more and that fixes it.

Tight but not too tight . Jared is right. the vast majority of AN fittings work fine the first time within a pretty wide range. If not, you can try tightening them a bit. If that fails,you can swap it out.

One annoying thing is avgas is dyed blue. Which does a wonderful job of blending in with blue or black AN fittings. I had 1 leak on a blue union. It leaked so slow it didn't drip - it evaporated and leaked at about the same rate. Show up barely as blue goo in the AN fitting joint.

I use the "flats" method.

jaredyates said:

I didn't stress too much about it. There are so many rabbit hole opportunities that I have to choose carefully which ones to fall into. I have had fittings that leaked, most often because I forgot to tighten them. Sometimes because I didn't make a good flare. Excepting those cases, it just hasn't been a big problem. Sometimes a fitting seeps a little and I snug it just a little more and that fixes it.

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It's physics, math, and engineering. Machine it, draft it, build it, test it, break it. Every time something new gets built, the entire world advances." Mike Patey

I've jumped down this hole. What Jared says is real good gouge. Ive never had one leak that was tightened down. its a real good design.

You will learn a lot if you flare a short piece of aluminum tubing, then tighten it up of a fitting at various torques or flats past finger tight. 3000 soft aluminum will smash down and its easy to over torque it. This deformation will be hidden until it is disassembled. It must loose strength. The flared end will flatten and enlarge between the nut and male flare so much that the nut may not easily come off the tube. Tighten it this far to see how it behaves. Its a good shop education. Kind of like Mike Patey. I think of this exercise I did every time I tighten a flare fitting now.

Thanks for the input. I think I’ll do as suggested and make a few test pieces to see how they perform.

A good flare depends on a nice and pretty, square, deburred end of the tubing. I would focus attention to that stage especially.

Tubing "flares" and AN fittings have a pretty short number of "matings". Matings being fully torqued up. This also applies to Aeroquip type AN fittings. But tubing flares are probably lower in matings.

jaredyates said:

A good flare depends on a nice and pretty, square, deburred end of the tubing. I would focus attention to that stage especially.

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A new tubing cutter blade helps a lot with that. I found that initially, I was making better flares, and the quality dropped off after making a few. I replaced the blade, and was surprised at how better the cuts got. I didn't notice the gradual dulling of the blade. I ordered several replacements, and treat them as consumables.

The whole reason I brough up this discussion is that I am getting close to finishing my wings. I have typically been torquing fasteners as I go, but I have left the fuel fittings loose.

My biggest concern (not that I am losing sleep over it), is the fuel line that runs from the aux pump to the wing root area. That would be a huge challenge to replace.

Build up a test rig with a party balloon on one end and a cap on the other. There might be more if you are testing the line and the whole tank, but you get the idea. The key is using low psi, hence the party balloon and not a pressure guage and shop air.

I had to really tighten mine up to stop leaks. I made them tight.

You can lean on the AN fittings pretty firmly and they hold up great. Clearly you are dealing with aluminium parts, so discretion is required. I have never damaged one myself, but I am not a strong guy. My point is, they are large fittings and can handle being tightened up quite hard, as tight as you can reasonably get them with a standard length spanner for that size. The softer aluminium fuel tube sort-of deforms into the perfect fit / seal, based on the ones I opened up afterwards to inspect. For sure they haven't given any trouble for over a decade since.

The sacrificial fittings idea sounds poorly conceived. The assembly is often a finished part and you can't get the sleeves or nuts off.

Do not start loose, that's just terrible advice. If they fret they'll damage themselves and then leak, I had this happen with my oil return lines - search up the post here. If you leave them loose, they could become looser at any stage in future. That's just dangerous. Plus, if that fuel die gets in your fabric with a slow leak inside the aircraft, you'll have a gross brown stain there forever until you recover the area - it eventually bleeds right through once it gets into the fabric, years after the fuel leak is resolved.

Start with the specified torque value. If you aren't sure, go tighter and do so with caution, but my experience is the fittings can handle a strong pull on the correct sized spanner.