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What is a flare nut spanner?

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Flare nut spanners have an opening so they can fit onto fasteners at the end of pipes. A flare nut spanner (also known as a ‘brake spanner’ or ‘crow’s-foot spanner’) is a simple, single-piece tool. It has two heads that are similar to ring spanner heads but have an opening cut into them.
Flare nut spanner has an opening cut into a ring profile with a fifteen 15 degree angle to the shaft to increase reach. The centre of the opening is often at a 15 degree angle to the shaft, which means you can maximise the turning arc of the spanner by flipping it over.
Flare nut brake, corw's foot spanners are perfect to use on vehicle break pipes. Flare nut spanners can fit fasteners that are inaccessible to ring spanners, while retaining the stability provided by a ring head. They are designed for use on fasteners that are attached to tubes, for example, on the end of vehicle brake pipes or plumbing pipes.
Pass pipe through opening and then slide spanner head over fastener nut. The tube fits through the opening of the spanner and the spanner can then be slipped over the fastener’s head as normal.
Thick jaws maximise contact with fastener so theyare less likely to damage the nut fastener. Plumbing fasteners are often made from soft metals, such as copper, so flare nut spanners have thick jaws to maximise the contact the head has with the fastener head, preventing damage to the fastener.
Flare nut spanner with a flex flexible head which increases the reach of the spanner. Some models incorporate a flexible head style (see What is a flex-head spanner?) or ratchet mechanisms (see What is a ratchet spanner?).
Flare nut spanner size and fastener sizes are mostly in metric and range from 7 to 41mm. Flare nut spanners are available to fit hexagonal fasteners with head sizes of 7mm to 41mm and the length of the spanner increases with the head size from 140mm (5½”) to 350mm (1′ 1¾”). The width of the opening increases with the size of the spanner head too and will be given in the product description. Imperial flare nut spanners are hard to find and cover more common sizes between 7/16″ and 7/8″.
Flare nut spanners can be in 6 and 12 point profiles. Flare nut spanners can have a 6-point or 12-point profile, like ring spanners. They are still called 6-point or 12-point even though there is a section cut out which reduces the actual number of points. Unlike 12-point ring spanners, 12-point flare nut spanners are not suitable for use on square fastener heads.
Flare nut spanner with an improved profile which focusses force on the sides of the fastener instead of the corners. The shape of the profile varies slightly between manufacturers and some have designs that are less likely to damage the fastener because they make better contact with the fastener sides. One such design you may come across is referred to as AS (anti-slip). These designs use curved peaks and troughs instead of flat edges. The curves put more pressure on the flat sides of the fastener head and less pressure on the corners (the weakest points).