GMC 1976 ZEO 6083 Maintenance Manual page 258

If only one disc requires turning, the disc on the
opposite wheel should be sanded with 60 or 80 grit
emery cloth to give braking surfaces a non-direc-
tional surface.
A discard dimension 1 .170" is stamped on all
production installed brake disc's (See figure 54). This
is the allowable wear dimension and NOT the allow-
able turning dimension. There must be .015" left for
wear after turning disc's .
BRAKE DRUMS
INSPECTING AND RECONDITIONING
BRAKE DRUMS
Whenever brake drums are removed, they should
be thoroughly cleaned and inspected for cracks,
scores, deep grooves and out-of-round . Any of these
conditions must be corrected since they can impair
the efficiency of brake operation and cause prema-
ture failure of other parts.
CRACKED, SCORED, OR GROOVED
DRUM
NOTE:
A cracked drum is unsafe for further ser-
vice and must be replaced . Do not attempt to
weld a cracked drum .
Smooth up any slight scores by polishing with
fine emery cloth. Heavy or extensive scoring will
cause excessive brake lining wear, and it will proba
bly be necessary to rebore in order to true up the
braking surface.
If the brake linings are slightly worn and the
drum is grooved, the drum should be turned just
enough to remove grooves. The ridges in the lining
should be lightly removed with a lining grinder.
If brake linings are to be replaced, a grooved
drum should be turned for use with oversize linings.
A grooved drum, if used with new lining, will not
only wear the lining, but will make it difficult, if not
impossible to obtain efficient brake performance.
OUT-OF-ROUND OR TAPERED DRUM
An out-of-round drum makes accurate brake
shoe adjustment impossible and is likely to cause
excessive wear of other parts of 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 braking
surface of a brake drum exceeds the factory specifica-
tion limits in taper (and/or) being out-of-round, the
Figure 55-Discard Dimension (Drum)
drum should be turned to true up the braking sur-
face .
Drum out-of-round can be measured with a dial
indicator and extension rod. Out-of-round measure-
ments exceeding .006", (total indicator reading) re-
quire turning or replacement of drum .
TURNING DRUMS
REPLACING DRUMS
BRAKES 5- 3 9
If irregularities in the braking surface of the
drum cannot be removed with emery cloth or out-of-
round exceeds .006" (total indicator reading), the
drum can be turned to .060" greater than the original
inside diameter. If a drum has smooth score marks
.010" to .020", it is serviceable without turning.
If a drum is turned to a diameter less than .030"
standard replacement linings may be used . Over
.030" oversize linings should be used .
A discard dimension
11 .090" (figure 55) is
stamped on all production installed brake drums.
This is the allowable wear dimension and NOT the
allowable turning dimension. There must be .030"
left for wear after turning drums. The maximum
turning diameter is 11 .060" .
Whenever new drums are to be installed, the
braking surface of the drum must be thoroughly
cleaned with lacquer thinner to remove the rust-
proof coating.