I make all my own lines, I can't do a good double flare every time! I have both bubble & double flares on my VWAC trike (German used a bubble). I always use 3/16th line to match factor flow. I bend up my lines, and then cut it where it needs to, and then take them to Bumper to bumper machine shop a couple of blocks from me, and they flare all my lines as long as I buy it there. They are the same price as any where else I've looked.
Most people can flare a double flare without a problem. I can't every time, and I don't like to keep cutting it off, and trying again.
Pat

I don't see where he said anything about copper, or fuel lines?