How to Measure a Brake Line Diameter

by C. Taylor
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Knowing the diameter of brake lines can be important if you need to replace the brake lines or nuts. The size of lines the manufacturer installed was determined by the hydraulic system used. The line diameter also determines the nut sizes. If you need to flare the end of the tubing for installation of nuts, you have to use a flare tip designed for that diameter. There is little room for error. These are all reasons knowing the line diameter is useful. You an use a precision caliper to measure the diameter, but you'll want to get a good one, as the cheap ones may have too large a margin of error.

Step 1

Locate the brake lines by consulting your car's owners manual or a shop manual. The lines will run from the master cylinder, located at the top of the engine to the brakes on the wheels. The exact location of the master cylinder obviously varies among automobile makes and models. The vehicle owner's manual will reveal the precise location.

Step 2

Clamp the caliper over an unobstructed area of the brake line. Rotate the dial on the caliper until the caliper jaws clamp snugly onto the brake line. Make sure they are not too tight, but there should not be any slack or gaps either.

Step 3

Read the measurement on the caliper. Consult the caliper's manual to make sure you properly interpret the readin. The dial measurement will be in either inches, millimeters or possibly both.

Step 4

Convert the reading, if necessary. You can convert inches into millimeters by multiplying the measurement by 25.4. You can convert millimeters into inches by dividing by 25.4. Manufacturers may use metric or standard measurements when describing their brake lines. If you arm yourself with both measurements, you will be well prepared when you purchase new lines, nuts or flaring kits.

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