Yes, a single flare will work but they have more of a tendency to split at the tube end where the metal is stretched or thinned out and the double flare fittings are designed to be used with the extra thickness. In most cases the flare nuts will bottom out before the single layer of tubing is compresses. With a double flare the edge of the bell is supported by the metal that is being folded in at the end of the tube. The end of the tube remains the same size and actually gets compressed a tad smaller than it was thus eliminating the splitting. Another advantage is that the double flare complies to the shape of the seat better when it is made up (the first time).

A mistake that a lot of people make when forming a double flare is to compress it too much. This is difficult to control with most of the flaring tools unless there is a stop built into the tool. You want a little give in the fold so that it will conform to the seat when you tightened it down. Something to remember when using SS lines is that the flare is already somewhat harder because of the forming operation than a mild steel flare. It’s much harder than the brass seat so once you compress the folds in the double flare in all probability the portion that is now yielding is the cone in the brass fitting.

As distasteful as some might find this, these types of joints (metal-to-metal) are really designed to be a one-time shot. This is why there are so many complaints of leaks after the joint is broken and then you try to reseal it. If they were made to take apart and reassemble then they would use metal seats with soft rubber or plastic seals. I might mention that there are metal-to-metal joints that are made for reassembly but they use very soft generally expensive materials for the seal. One exception to this is the banjo fittings that use a soft copper seal that can be annealed and reused several times.

I’ll try to give you an analogy I once heard on the subject of which is harder; it’s sort of like asking, "Which tastes better, Chinese food or Japanese food?" It’s just too general of a subject to be answered directly. Stainless is not necessarily harder or softer than steel but it does get tougher once you start to form it. Austenitic stainless steel, usually type 304 is what is generally used for SS brake lines and it tends to work harden more than mild steel. This is where the problem comes in with forming the double flare. If you take the same size samples of SS and mild steel and bend them it takes more force to bend the SS especially as it begins to work harden. Mild steel and SS generally start out in the same Brinell scale at around 120-140 HB but the SS will test much higher after moderate cold working.
Denny Graham
Sandwich, IL

Denny G
Sandwich, IL