How to Measure Drum Brakes

by Jody L. Campbell

Brake drums encapsulate brake shoes. The wheel cylinder bores extend hydraulically upon demand and push the shoes outward to contact the interior diameter of the drum. The brake shoe composition is softer than the cast metal drums and wear away more quickly than the drum. The drums do wear, however, and can only get to a certain measurement--called discard--in order to remain effective. An over-sized or thin drum will compromise the effective braking power of the drum brakes and can also be dangerous if the drum disintegrates.

Step 1

Remove the drum from the wheel employing a drum puller if necessary. De-adjusting the shoes may be required using a drum brake spoon and a thin screwdriver to depress the star adjuster retaining clip (if applicable to the specific drum braking system). Some drum brake systems may also employ retaining screws on the hub facing of the drums to hold them in place to the hub. Remove the screws first--in this application--before attempting to remove the drums.

Step 2

Inspect the outer facing of the rotor and locate the stamped maximum diameter or machine-to diameter in the cast. If unable to read due to rust and corrosion or not applicable to the specific drum, a drum and rotor discard specifications chart will be required to learn the machine-to and discard measurements on the specific drum. Each vehicle is different, so finding the year, make and model of the vehicle on the chart will illustrate the necessary measurements. Determine the size of the drum used on the specific car, the amount of run-out allowed, the machine-to specification and the discard specification on the chart.

Step 3

Place the drum face down onto a flat surface so the interior diameter of the drum is facing upward. Inspect the interior interior diameter and hub mating surface of the drum for visual surface cracks. Look for scores or grooves along the surface of the interior diameter. If present, these damaged areas will be where the anvils on extendable arms of the micrometer need to sit.

Step 4

Loosen the lock screw on one of the arms along the shaft of the micrometer. Apply the actual drum size for this measurement. If the vehicle uses a 10.5-inch drum, set one side of the micrometer to the 10-inch mark. Set the other arm to 10-inch and then add four more of the 0.125-inch marks stamped along the shaft for the 10.5-inch drum.

Step 5

Place the micrometer anvils inside the drum--place them in any scores or grooved is applicable--and rock the micrometer back and forth along the inside diameter of the drum. Watch the dial indicator of the drum to detect anomalous wear such as run-out (out-of-roundness). Take several readings and compare the highest measurement. This measurement will have to be added to the 10.5 inch diameter to be compared with the machine-to and discard specifications in order to determine if the drum can be machined or has to be discarded and then replaced.

Step 6

Use a depth gauge to measure the thickness of the shoes. Riveted shoes would require placing the tip of the depth gauge into the rivet hole on the facing of the shoes. Bonded shoes can only be measured along the edges where the shoes bond to the backing plate of the shoe. Take several measurements and apply the lowest point of the shoe as the actual reading. A wide variance of measurements along the same shoe would indicate uneven wear and the shoe should be replaced. Any measurement at or below 2/32 of an inch should require replacement of the shoes.

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