What size wrench for brake lines?
For brake-line work, use a flare-nut wrench sized to the brake line nut. In most cars you’ll encounter 3/8-inch or 7/16-inch across-flats sizes, with metric equivalents around 10 mm or 11 mm. A thin-profile flare-nut wrench helps prevent rounding and damage to fittings.
Brake line wrench basics
Brake-line fittings require a tool that can grip the nut without slipping or marring surrounding parts. A flare-nut wrench concentrates force on the flats of the nut and provides better access in tight spaces, reducing the risk of damaging the line or the fitting during loosening or tightening.
Common sizes you’ll encounter
Here are the sizes you are likely to encounter on brake line fittings, along with what they fit.
- 3/8 inch (across flats) — a common size for many domestic vehicles’ brake line nuts.
- 7/16 inch (across flats) — another frequent imperial size for brake line nuts and hoses.
- 10 mm — metric brake line nuts on many Japanese and European cars.
- 11 mm — metric size found on several European and Asian models.
- 12 mm — less common, but appears on some older or specialty vehicles.
Note: Always verify the exact size by trying a wrench on the nut or measuring across the flats with a caliper; using the wrong size can strip the nut or damage the line.
How to determine the right size for your vehicle
Follow these steps to confirm the correct tool for your brake line fittings.
- Inspect the brake line nut to determine its approximate flat-to-flat width.
- Match the nut to a flare-nut wrench of the same size; if it’s tight or doesn’t seat, try the nearest size up or down carefully.
- When in doubt, consult the vehicle’s service manual or a parts diagram for the specific brake line fitting size for your make/model.
Using the correct size minimizes the risk of rounding the nut, damaging the line, or introducing leaks. A proper flare-nut wrench also allows safer, steadier torque application.
Tips for safe and effective work
Practical advice to protect the brake system while you work.
- Wear eye protection and gloves; brake fluid can be irritating to skin and eyes.
- Apply penetrating oil to stubborn fittings and work slowly; avoid forcing a wrench that doesn’t seat properly.
- Relieve pressure and, if possible, drain brake fluid in a controlled manner before loosening fittings.
In brief: having the right size flare-nut wrench is essential for brake-line work. It helps prevent damage and leaks while allowing safe, controlled tightening or loosening of fittings.
Summary: The size you need depends on the nut size of your brake lines. Typical options include 3/8" and 7/16" across flats (imperial) or 10 mm and 11 mm (metric). Always use a thin-profile flare-nut wrench designed for brake lines, verify the size on the nut, and follow proper safety procedures to protect yourself and the braking system.
What size is a brake line fitting?
Common brake line fitting sizes are determined by the line diameter and thread size, with most passenger cars using 3/16-inch brake lines and larger trucks using 1/4-inch. Common threaded fittings include 3/8"-24 (for 3/16" lines) and 7/16"-24 or 1/2"-20 (for 1/4" lines).
| Common Diameter | Common Thread Sizes |
|---|---|
| 3/16" | 3/8"-24 |
| 1/4" | 7/16"-24, 1/2"-20 |
(this||self).Wufxzb=function(c,e,f,l,k){var d=document.getElementById(c);if(d&&(d.offsetWidth!==0||d.offsetHeight!==0)){c=d.querySelector("div");var g=c.scrollWidth-c.offsetWidth,h=Math.min(e?g:0,g);c.scrollLeft=e&&(l||f)?0:h;var a=d.getElementsByTagName("g-left-button")[0],b=d.getElementsByTagName("g-right-button")[0];a&&b&&(e=RegExp("\\btHT0l\\b"),f=RegExp("\\bpQXcHc\\b"),a.className=a.className.replace(e,""),b.className=b.className.replace(e,""),h===0?a.className="pQXcHc "+a.className:(a.className=
a.className.replace(f,""),k&&c.classList.add("pA30Ne")),h===g?b.className="pQXcHc "+b.className:(b.className=b.className.replace(f,""),k&&c.classList.add("FpCCub")),setTimeout(function(){a.className+=" tHT0l";b.className+=" tHT0l"},50))}};}).call(this);(function(){var id='_pNAlaayFG6ylkdUPrZOKiQM_200';var rtl=false;var gecko=false;var edge=false;var soh=false;
(this||self).Wufxzb(id,rtl,gecko,edge,soh);})();
Common sizes by diameter
- 3/16" (4.75mm): This is the most common size for passenger vehicles and often uses a 3/8"-24 threaded fitting.
- 1/4" (6mm): Typically found in trucks and larger vehicles, this size is used for higher-pressure applications or where a greater volume of fluid is needed. Common thread sizes include 7/16"-24 and 1/2"-20.
Other common fittings
- 1/8" NPTF and 1/4" NPTF: These are also found in automotive braking systems.
- Metric sizes: Some systems use metric threads, such as M10x1.
Important considerations
- Always use the correct fitting for your specific vehicle or application.
- A brake line fitting size gauge can help identify unknown threads.
- Ensure the flare type is correct for your system, as different flare types require different fittings.
What is the most common brake line nut size?
3/16" (4.75mm): This is the most common size used in most passenger vehicles. It is often referred to as the "universal size" for brake lines. 1/4" (6mm): Typically found in trucks and larger vehicles, this size can handle higher pressure and is used in applications where the braking system requires more robust lines.
What type of wrench is used to remove a brake line fitting?
flare nut wrenches
Wrenches (flare nut wrenches, line wrenches)
Flare nut wrenches and line wrenches are specifically designed to grip the hexagonal nuts of brake line fittings without stripping them, ensuring a secure and efficient removal process.
What size wrench for brakes?
If you are purchasing SAE wrenches, start at 1/4” and go up to at least 7/8” or 1”. For metric wrenches, you'll want to go from about 7mm to 22mm.
