It's been a long time since I last made fuel lines, and those were either Aeroquip 471 or automotive hoses. This time I'm using -8 and -6 versatube (3003 soft aluminum) with AN fittings and the 37 degree single flare. It's a fuel injected system, unpressurized from tank through selector valve to pump, then pressurized at about 30 psi for a short run from pump to injectors. The return lines are after a pressure regulator, and are 6-8 psi. The -8 (1/2 inch OD) Versatube is rated at 380 psi, and the -6 (3/8 inch OD) to 520 psi. The tube isn't the issue, it's the quality of the flare and the correct torque on the fittings.

Flaring tools come in a _wide_ range of styles and price points. After two of the lowest grade I moved up one level, and I'm loving it. Below is a photo showing the equipment. After practice, my process is
- cut with a standard cutting tool
- remove burrs with a step bit
- put B nut and sleeve on the tube
- place tube in the correct sized hole of the multi-hole bar, with the end of the tube proud of the surface
- use a course file on the cut end, then check for a clean and perpendicular cut
- level the end of the tube with the top of the bar
- bar goes into the flaring anvil, centering the cone on the tube
- tighten the anvil on the bar; the anvil anvil is not allowed to twist
- add a drop of oil to the cone and spin it until it self-releases
- disassemble and inspect.

The height and inner surface of the flare has to be perfect, with no burrs or scratches. Tool marks on the outer side wall are acceptable. After practice I like the results.