TRUCK SERVICE MANUAL
INSPECTION OF DRUMS
The friction surface of brake drums must be smooth,
true and concentric. Make certain with a visual check that
drums are not barrel shaped, bellmouthed, scored or
Hard or chill spots, Fig. 1, in brake drum may
produce pedal pulsation and roughness or brake surge. If
these effects are present, drum should be replaced.
Fig. 1 Hard or Chill Spotted Drum
A barrel shaped drum (Fig. 2) results from
overheating. If this barrel shaped condition is not corrected,
the braking surface is reduced and uneven lining wear results.
Fig. 2 Barrel Shaped Drum
Extreme pressure which over a period of time will
create a bellmouthed drum as shown in Fig. 3. Brake linings
on a bellmouthed brake drum will make contact only on the
inner surface of the drum. In addition to cutting the braking
surface to a minimum, it will also cause uneven and rapid
Scored drums are the result of worn linings to the
point where the drum-to-shoe contact is made or an
accumulation of small steel particles imbed themselves in the
brake lining (Fig. 4). The steel particles form a tough scale
which is sometimes harder than the drum. As a result, deep
grooves are formed in friction surface of drum.
Fig. 3 Bellmouthed Drum
Fig. 4 Scored Drum
Brake drum scoring never improves but continually
gets worse until both lining and brake drum are useless.
Attempting to reline brakes without turning scored brake drum
surface will quickly destroy new lining and make effective
Brake lining in an eccentric or out-of round drum
cannot make full contact with the drum resulting in rapid or
uneven lining wear and could even cause brakes to seize or
chatter. Maximum allowable out-of-round or eccentricity
should be .25 mm (.010 in.)
If 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 lathe
making certain drum is centered. Mount Dial Indicator SE-
1848 on lathe and check runout about 12.7 mm (.5 in.) in
from edge of drum as shown in Fig. 5. Runout must not
exceed .25 mm (.010 in.).
Before assembling drum, hub and wheel, all parts
must be clean and free of foreign matter.
CTS-2779 Page 3
PRINTED IN UNITED STATES OF AMERICA