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A flare fitting is a type of connection fitting used on hard lines, such as steel and aluminum brake lines. Commonly comprised of a hard line that has been flared out to mesh against a fitting and a nut, the flare fitting sits snugly against the fitting and is held tightly in place with the nut. The flared end of the hard line matches the angle on the end of the fitting so exact that gaskets or sealers are not required to prevent leaks. Most flare fitting connections are created by home mechanics with a flaring tool. The key to creating a good flare fitting is to not crack the tubing and avoid over-tightening the connection.
One benefit in using a flare fitting is the amount of line pressure the connection can withstand. By eliminating the gasket, the connection is virtually leak-proof. In severe duty connections, a double flare will be used that actually doubles the material in the flared line, making it possible to be tightened even tighter; this squashes the flare into the steel fitting even tighter. The action practically eliminates any possibility of fluid passing the union.
The angle of the flare in a Society of Automotive Engineers (SAE) flare fitting is 45 degrees. All related fittings and connections used with a flared line are also 45-degree angles. While the flare fitting gives a high-pressure rating and is used on critical systems such as brake lines and fuel lines, the system does not provide the same level of strength and cannot be combined with the AN fitting. The Army Navy (AN) fittings use a 37-degree flare and undergo a much more stringent quality control and safety testing procedure. While commonly used on aircraft and very high-performance racing applications, the AN fitting is overkill for most non-military or street-driven applications.
When creating a flare fitting, a flare nut is placed over the hard line before the line is placed into the flaring tool. The hard line is then placed in a size-appropriate collar in the flaring tool, with the end of the line being flush with the tool before it is tightened in place. A tapered mandrel is then positioned on the flaring tool directly centered over the exposed hard line. As the mandrel is tightened, it begins to drive into the tubing, and the flared shape of the mandrel forms the flare. A little lubrication typically prevents any galling or cracking of the tubing as it is stretched into shape against the walls of the flaring tool.