Line Wrench
A wrench with a flared, open ring end designed to grip hydraulic tube fittings on five sides to prevent stripping.
A line wrench (also known as a flare nut wrench or tubic wrench) is a specialized hand tool designed to work on hexagonal fittings attached to hydraulic tubes, such as brake lines, fuel lines, power steering lines, and clutch lines. These fittings are typically made of soft metals like brass or mild steel and are highly prone to rounding off if turned with a standard open-end wrench.
The line wrench features a thick, open ring-end with a slot just wide enough to slide over the metal hydraulic tube. Once on the tube, the wrench slides down onto the fitting, grabbing five of the six flat sides of the hex nut. This distributes the turning force much more evenly than an open-end wrench (which only contacts two sides), allowing the mechanic to loosen tight or rusted fittings without stripping the hex corners.
Line wrenches are essential tools for brake repairs and line replacements. Using a standard wrench on a rusted brake line fitting almost always results in rounded corners, requiring the line to be cut and re-flared. When working on hydraulic lines, technicians also apply penetrating oil to help break loose the threads before turning them with the line wrench.