K 3500 Truck 4WD V8-5.7L VIN R (1998)
Brake Drum: Testing and Inspection
Brake Drum Inspection ( Drum Servicing)
CRACKED, SCORED OR GROOVED
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.
OUT-OF-ROUND OR TAPERED
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.
REFINISHING
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, instead, replace the drum.
When you refinish a brake drum, resurface the drum to a dimension not more than 0.76 mm (0.030 inch) 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. Cross-feed for rough cutting should range from 0.15 to 0.25 mm
(0.006 to 0.010 inch) per revolution. Finish cuts should be made at crossfeeds no greater than 0.05 mm (0.002 inch)
per revolution.
NEW REPLACEMENT DRUM REFINISHING
When installing new brake drums, do not refinish the braking surface. These parts are already at the correct level of surface finish.
BALANCE
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.
