Drum Brake System: Testing and Inspection

CRACKED, SCORED, OR GROOVED
A cracked drum is unsafe for further service, and must be replaced. Never attempt to weld a cracked drum. Smooth any light scores. Heavy or extensive scoring will cause excessive brake lining wear and it may be necessary to refinish the drum's braking surface.

For refinishing procedures, refer to Brake Drum Refinishing.

OUT-OF-ROUND OR TAPERED
A drum that is found to be out-of-round or tapered can be refinished to correct the problem, if it is not to severe. Refer to Brake Drum Refinishing.

TAPER AND RAGOUT CHECKS
An out-of-round or improperly tapered drum makes accurate brake shoe adjustment impossible and is likely to cause excessive wear of other parts of the brake mechanism due to its 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 for out-of-round and/or taper, refinish the drum to true up the braking surface. If the drum cannot be brought within specifications by refinishing, it must be replaced.

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

Runout is a low spot in the Roundness of a drum. If the runout exceeds maximum specification, the drum must be replaced as improper stopping or pulsating brakes could result.
^ Out-of-Round (maximum): 0.01 mm (0.0004 inch)
^ Taper (maximum): 0.1 mm (0.004 inch)
^ Roundness (maximum): 0.04 mm (0.0016 inch)

SURFACE FINISH
Whenever brake drums are removed, they should be thoroughly cleaned and inspected for cracks, scores, deep grooves, out-of-round and taper. Some minor surface wear is normal and will not affect brake operation. However, inside diameter should never exceed the maximum specification. See Inside Diameter Check for more information.

A cracked drum is unsafe for further use and must be replaced. Never attempt to weld a cracked drum. Smooth up any slight scores. Heavy or extensive scoring will cause excessive brake lining wear, and it may be necessary to resurface the drum braking surface.

If brake linings are to be replaced, always refinish a grooved drum. A grooved drum, if used with new lining, will wear the lining and make proper brake performance difficult to obtain.

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