Brake Drum Inspection Drum Inspecting

Any time you remove the brake drums, thoroughly clean and inspect them for cracks, scores, deep grooves and out-of-round.

Slight scoring can be cleaned up with a fine emery cloth. Heavy or extensive scoring causes lining wear. The drum braking surface will need machining to remove these scores.

If the drum is grooved, and the linings are slightly worn, the drum should not be machined. Instead, polish the drum braking surface with a fine emery cloth. Eliminating all of the drum grooves and ridges on the lining would require removing too much metal and lining material. The grooves and ridges match and satisfactory service can be obtained by leaving them alone.

Measure the inside diameter of the brake drum at two or more places around the circumference of the braking surface. The measurements must be made at the same distance in from the edge of the drum. Compare the results with the wear specifications in Unit Repair at the end of this section.

Measuring a drum for taper involves taking measurements at the inner and outer edges of the machined surface at two or more places around the drum. These measurements should be equal.

Brake Drum Inspection Drum Servicing

A cracked drum is unsafe for further service and must be replaced. Do not attempt to weld a cracked drum.

Smooth any slight scores. Heavy or extensive scoring will cause excessive lining wear, and it may be necessary to resurface the drum braking surface.

If the linings are slightly worn (but still reusable) and the drum is grooved, polish the drum with a fine emery cloth, but do not refinish. Eliminating all grooves in all drum and smoothing the ridges on the lining would require removing too much metal and lining. If left alone, the grooves and ridges match and satisfactory service can be obtained.

If you are replacing the brake linings, always refinish a grooved drum. Using a grooved drum with new linings will wear the linings and make proper brake performance difficult to obtain.

An out-of-round or tapered drum prevents accurate brake shoe adjustment and is likely to cause excessive wear of other brake parts due to the eccentric action. An out-of-round drum can also cause severe and irregular tire tread wear as well as a pulsating brake pedal. When the drum exceeds the specification limits in taper and/or out-of-round, refinish the drum to true up the braking surface. Out-of-round and taper can be accurately measured with an inside micrometer and extension rods.

When measuring a drum for out-of-round and taper, take measurements at the open and closed edges of the machined surface and at right angles to each other.

If you determine a drum needs refinishing, remove only enough metal to obtain a true, smooth braking surface. If a drum does not clean up when refinished to the maximum refinish diameter, replace the drum. These guidelines are shown in Specifications at the end of this section. Removal of more metal will affect the heat dissipation and can cause drum distortion.

All brake drums have a maximum diameter cast into them. This diameter is the maximum wear diameter and not a refinish diameter. Do not refinish a brake drum that will not meet the Specifications at the end of this section. Instead, replace the drum.

When you refinish a brake drum, resurface the drum to a dimension not more than 0.76 mm (0.030 in) less than the discard diameter. The refinish diameter is the maximum diameter the drum can be refinished to and still allow safe braking action. If you exceed this diameter, the brake drum will wear beyond the discard diameter during normal brake use.

Always use sharp cutting tools or bits. Dull or worn tools leave a poor surface finish that will affect the initial braking performance. Always use vibration dampening attachments when you refinish braking surfaces. These attachments eliminate tool chatter so you can obtain a better surface finish.

The best speed for refinishing braking surfaces is a spindle speed of 150 RPM. Crossfeed for rough cutting should range from 0.15 to 0.25 mm (0.006 to 0.010 in) per revolution. Finish cuts should be made at crossfeeds no greater than 0.05 mm (0.002 in) per revolution.

When installing new brake drums, do not refinish the braking surface. These parts are already at the correct level of surface finish.

During manufacturing, weights are used to balance the brake drums. Do not remove these weights.

After you refinish brake drums, or when maintaining wheel balance is difficult, check the drums for balance. They can be checked on most off-vehicle balancers. If found to be out of balance, replace the drum.