RECONDITIONING BRAKE DRUMS AND SHOES
Brake lining in an eccentric or out-of-round drum cannot
make full contact with the drum This will result in rapid or
uneven lining wear and could even cause brakes to seize
or
chatter.
Maximum
allowable
out-of-round
or
eccentricity should be 0.25 mm (0.010 inch).
If the inspection shows that any of the preceding
conditions exist, brake drum should be either turned or
replaced. To assure a balanced braking system, always
install turned or new brake drums in pairs on each axle.
Any time a new brake drum is to be installed on a
vehicle, the runout should be checked as follows. Place
the new brake drum with hub and wheel assembled in
brake drum lathe making certain drum is centered.
Mount Dial Indicator SE-1848 on lathe and check runout
about 12.7 mm (0.5 inch) in from edge of drum (on
braking surface) as shown in Figure 6. Runout must not
exceed 0.25 mm (0.010 inch).
Before assembling drum, hub and wheel, all parts must
be clean and free of foreign matter.
Figure 6 - Checking Drum Runout on Brake
Drum Lathe
REFINISHING BRAKE DRUMS
On brake drums manufactured after January 1,1971, the
maximum diameter to which drum can be worn is
stamped or cast into the drum Drum should be discarded
if worn beyond this limit.
Minor scores on brake drum can be removed with fine
emery cloth or steel wool, but always clean emery or
steel wool particles from the drum after this operation.
More heavily damaged or out-of-round drums should be
ground or turned on brake drum lathe.
If depth of scoring, bellmouth or barrel shaping exceeds
0.13 mm (0.005 inch), measured with micrometer across
part of all of brake surface, drum should be refinished.
Reboring limits (see drum) must not be exceeded and no
heat checks, cracks or bluing should be evident. Use a
micrometer also to check for an out-ofround drum. Make
check by measuring drum brake surface diameter at
various
points
450apart
around
circumference.
Eccentricity (out-of-round) should not exceed 0.25 mm
(6.010 Inch) on diameter
Remember that each time brake drums are turned, less
metal remains to absorb the heat developed by braking
action. Brake drums containing less metal will operate at
a higher temperature As a result, brake fade, slow
recovery and erratic wear will be more noticeable. Also,
extremely high temperatures shorten lining life and
cause heat checks and cracks (Figure 7) to form on
inner surface of drums These conditions will become
progressively worse until drums fail.
Figure 7 - Cracked Drum
REMACHINING DRUMS
Brake drums that are otherwise in good condition can be
turned in a lathe However, It must be remembered that
the recommended remachining or rebore limit for brake
drums with a diameter over 355 mm (14 inches) may not
be increased more than 2 03 mm (0.080 inches)
diameter (total cut) and discard at 3 05 mm (O 120
inches) over normal diameter
CTS-4082B - Page 4
