A flare wrench is like an open end wrench with 5-6 contact sides and/or 4-5 contact corners. As it’s name implies, the wrench is intended for use on flared fittings. A normal open end wrench makes contact on only two sides, making the flare wrench ideal for better contact in certain situations. In many cases they can be used for the same applications, though both wrenches have specific uses. There are also flare wrenches for 12 point nuts, though rare.
Most mechanics refer to flare wrenches as special purpose tools. It’s primary use is for flare based nuts attached to piping such as brake lines, fuel lines, transmission coolant lines, and other similar lines. Even mechanical based oil pressure and water temperature gauges use flare nuts for their lines, which should be installed using a flare wrench.
Not all flare wrenches are created equal. Doing research on a number of tool based forums you will find brands have a tendency to spread, meaning under heavy usage the opening of the flare wrench spreads apart beyond an acceptable amount. Another important factor is how many corners the flare wrench comes in contact with the flare nut. The S-K flare wrench in the photo above covers 5 corners of the nut, with all sides of the flare nut getting contact on the nut’s sides. This combination of edges and corners is ideal. See the diagram below to see the difference in flare wrenches.
Budget priced flare wrenches tend to use the 4 corner pattern, were-as most quality flare wrenches use the 5 corner pattern.
Depending on your budget, Allen brand (made in USA), KD (made in USA), GearWrench (made over seas), S-K (made in USA) and Williams (by Snap-on) (made in USA) brands have good reviews.
Craftsman no longer offers a raised panel flare wrench (except on their web site, most likely until stock is depleted) and has remarked their Craftsman Professional Flare Wrenches as just Craftsman Flare wrenches. The older made in USA Craftsman Professional flare wrenches were made by S-K. The new line of Craftsman flare wrenches are now made over seas.