How to Check & See How Many Millimeters of Brakes Are Left

Modern vehicles are so heavily engineered that many components have very fine tolerances. Brakes are one of these, and knowing exactly how to measure the pads, shoes, drums and rotors correctly and compare them the brake specifications is integral to knowing the number of millimeters left on these components. Once you understand the process, you can schedule you next brake job with some accuracy and plan accordingly.

Getting To the Brakes

Checking the exact number of millimeters left on your vehicles disc brake pads cannot be performed by simply eyeballing the pads from the outside; you have to remove the pads, if you are checking disc brakes, or remove the drums, if checking drum brakes. On the majority disc brake systems, you need to remove the two bolts securing the caliper, remove the caliper and hang it from a nearby suspension component with a bungee strap or mechanics wire. With the caliper removed, the pads on most systems slide out of the caliper bracket. But in some cases, if the caliper holds the pads, you need to pry them from the caliper. On drum brakes, remove any hardware securing the drums bolts, locking washer, etc then just pull the drum from the hub with a slight wiggling motion. If the drum is seized, give it a few taps with a hammer to break the rust free.

Knowing the Specs

Every brake pad, shoe and rotor has a minimum thickness specification and drums have a maximum diameter specification, which the manufacturer defines. This is the zero point for these components; never run pads, shoes, drums or rotors until there is no friction material left to use. After this zero point, there are three main problems. First is that brake pads and shoes also act as insulators protecting the sensitive brake hardware from the extreme heat cause by braking. When the friction level falls below the minimum thickness specification, it does not dissipate enough heat and can allow enough heat transfer to boil the fluid, which can create havoc in the brake system. Second is that now the caliper or wheel cylinder pistons must press excessively far to push the friction material against the drum or rotor, potentially resulting in the wheel cylinder or caliper failing. Third is that a rotor or drum that is too thin can crack much easier than one that has the correct thickness. Typically, the manufacturer will list these minimum specifications in the vehicles owners manual. If they are not in there, call the dealer or a local parts store to get these important specifications.

Checking Pads and Shoes

Open a micrometer about an inch and notice that a small rod comes out from the bottom -- this is the micrometers depth gauge rod. Close the micrometer and calibrate it by hitting the Zero button to ensure an accurate measurement. Open the caliper again and position the depth gauge rod so it contacts the pad or shoes metal backing plate. Push the head of the micrometer downward until the base contacts the top of the friction material. Read the millimeter measurement on the micrometer. Repeat this measuring process in four to five areas around the pad to find the lowest measurement. Subtract the minimum thickness specification from the lowest reading -- this is how many useful millimeters are left on the brake pads.

Checking Rotors

Instead of measuring with the depth gauge rod, measure the rotor with the jaws of the micrometer. Close the jaws around the disc part of the rotor the part the pads contact and hold firm pressure on the micrometer. Read the millimeter measurement. Repeat the measurement process in four to five areas around the rotor to find the thinnest section. Subtract the minimum thickness specification of the rotor, not the machine-to specification, from the thinnest measurement of the rotors thickness. The resulting number is how many millimeters your vehicles rotor has left.

Checking the Drums

Look on the backside of the micrometer and notice there is a pick-like section. Open the micrometer slowly and notice the pick separates into two halves, each with a flat section to measure the overall diameter of the drum. Position the pick-like section into the center of the drum and open it until the two sections touch each side of the inside of the drum the part that the pads contact. Read the micrometers millimeter measurement. Repeat the measurement process in four or five areas around the drum to find the largest measurement. Now -- this is the tricky part -- the drum gets wider as it wears out, so subtract the widest measurement you made from the maximum diameter specification, and the result is how many millimeters are left on the drum.