Brake Drum Inspection
Brake drums are constructed of cast iron or aluminum that's been lined with steel or iron. Both of these materials lose their integrity from heat. Drum brake systems convert energy in motion or kinetic energy into heat energy through friction. It is also true that brake shoes reach temperatures over 450° F. As the temperature rises, the braking system loses efficiency. This heat is why so many brake drums have outer fins that transfer heat energy to the atmosphere.
As the brake drum wears, minor pitting and warping will occur. These imperfections can often be removed by resurfacing the inside surface of the drum. Before resurfacing a brake drum locate the maximum diameter. Never cut a drum or rotor out of these specifications. They apply to the drum's inner surface after it has been cut, not before.
If a brake drum is beginning to crack, it cannot be machined and must be discarded. Small cracks lead to big ones, and the crack may be deeper in the cast than it appears. When resurfacing a drum always use a brake drum silencer band to reduce chatter marks caused by vibrations inherent to the process.
Measuring the inside of a brake drum requires a drum gauge or brake drum micrometer. Take measurements in several different spots around the inside surface of the drum (8-10). Drums wear in different places. They lose their original shape; a barrel-shaped drum is rounded in the middle. A bell-shaped drum has excessive wear on the outside edge.