Electronic Brake Drum Caliper / Drum Micrometer

The device typically consists of two long, sturdy arms or beams made from durable materials like stainless steel, connected at a pivot point. These arms extend outward to reach across the inner surface of the brake drum. At the ends of the arms are hardened measurement tips or points that make contact with the drum's interior walls. The key feature of an electronic version is its digital display, which provides precise readings in both imperial (inches) and metric (millimeters) units, often with selectable modes and a zero function for calibration.

To use it, you first ensure the caliper is calibrated—many models allow you to "zero" the display at a closed position or a known reference point. Then, you insert the caliper into the brake drum, positioning the tips so they rest against the inner surface at the widest point of the drum's diameter. Some advanced models, like the one from Chicago Brand, include features such as rim stops or preset depth settings (e.g., at 1" or 1.5" below the drum's edge) to ensure consistent measurement depth, which is critical for accuracy. This helps detect issues like "bell-mouthing" (where the drum widens unevenly toward the edge) or an uneven diameter.

Once positioned, the arms are adjusted—either manually or via a mechanism—until the tips firmly contact the drum's inner walls. The distance between the tips is then calculated electronically using an internal sensor, and the measurement is shown on the digital display. For example, the Chicago Brand Electronic Brake Drum Caliper offers a range of 0-17.25 inches, reads to 0.005" with 0.001" accuracy, and has a reach of 4.25 inches, making it suitable for a variety of drum sizes.

The process is repeated at multiple points inside the drum—typically at different depths or angles—to check for uniformity. A smooth, consistent diameter is essential for even brake shoe contact and safe braking performance. If the measurements exceed the manufacturer's maximum allowable diameter or show significant variation, the drum may need to be machined or replaced.

In essence, the electronic brake drum caliper combines traditional caliper mechanics with modern digital precision, making it faster and easier to get reliable measurements compared to older analog gauges, all while reducing human error in reading the results.