What Is a Line Wrench? | Flare Nut Wrench Explained

For the full breakdown, see our best Brake Line Wrench guide.

Knowing what is a line wrench and when to use it can save you from rounding a soft brake or fuel fitting. Unlike a standard open-end wrench that contacts only two sides of a nut, a line wrench wraps around five of six sides for a much more secure grip. Professional mechanics and DIYers alike keep one in their box specifically for brake work, transmission lines, and hydraulic systems where a stripped nut means replacing the entire line.

What Exactly Is a Line Wrench?

A line wrench—also called a flare nut wrench, flare-nut wrench, or tube wrench—is an open-end wrench with a slot cut into its head. That slot lets the wrench slip over a tube and engage the fitting from more sides than a conventional open-end wrench can. The design follows the DIN 3118 standard for open-end wrenches, which specifies recessed corners that distribute turning force evenly across the nut rather than concentrating it on the corners.

Standard open-end wrenches grab just two flats of a hex nut and tend to slip or round the corners, especially on softer fittings like brass or aluminum. A line wrench, by contrast, contacts roughly five of six sides, spreading the load and dramatically reducing the chance of damage. That difference matters most on brake lines, fuel lines, and hydraulic systems where a rounded nut can turn a five-minute job into a major ordeal. The tool is also frequently called a brake wrench in automotive contexts because it is so commonly used on brake line fittings.

What Sizes and Types Are Available?

Line wrenches come in both SAE and metric sizes to match common fitting dimensions. Six-point and twelve-point versions are available, and the wrench may be flat or offset for clearance in tight spaces.

Metric sets are also widely available for import vehicles, with common sizes ranging from 10 mm up to 22 mm. When buying a set, look for forged steel construction and a clean, precise opening that matches the fitting without play.

How to Use a Line Wrench Correctly

Using a line wrench is straightforward, but technique matters. Start by cleaning the fitting with a wire brush so the wrench seats fully—dirt and debris prevent full engagement. Slide the wrench over the tube and onto the nut, making sure it contacts as many flats as possible. Apply steady, controlled pressure in small increments; jerking the tool can twist or snap the line.

If you are shopping for one, our picks for the best brake line wrenches cover what to look for. The most common mistake is grabbing a standard open-end wrench instead of a line wrench—that two-flat contact is exactly what rounds soft fittings. The other frequent error is using a near-fit size: if the wrench rocks on the nut, it is the wrong size. Line wrenches are available as six-point or twelve-point versions; six-point provides a more positive lock, while twelve-point offers more positioning flexibility in tight spots.

For heavily corroded fittings, penetrating oil and patience are your allies. A line wrench gives you the best mechanical advantage, but it cannot fix a seized nut alone—work the fitting back and forth rather than cranking in one direction. Warming the surrounding area with a heat gun (never a torch near fuel lines) can help break the bond.