TRUCK SERVICE MANUAL
WHEELS, RIMS, TIRES
The friction surface of brake drums must be smooth,
true and concentric whether for air or hydraulic brakes. Make
certain with a visual check that drums are not barrel shaped,
bellmouthed, scored or eccentric. A barrel shaped drum (Fig.
1) results from overheating. If this barrel shaped condition is
not corrected, the braking surface is reduced and uneven
lining wear results.
Extreme pressure which over a period of time will
create a bellmouthed drum is shown in Fig. 2. 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. 3). 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.
Brake drum scoring never improves but continually
gets worse until both lining and brake drum are useless.
brake drum surface will quickly destroy new lining and make
effective braking impossible.
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 .10 mm (.004").
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 from
end of drum as shown in Fig. 4. Runout must not exceed .38
NOTE: Before assembling drum, hub and
wheel, all parts must be clean and free of
If runout exceeds .38 mm (.015") remove drum from
hub, rotate drum 180° and reinstall on hub. Check runout
again; if runout still exceeds .38mm (.015"), remove drum
from hub and rotate drum 1/4 turn. Reinstall drum on hub and
REFINISHING BRAKE DRUM
On brake drums manufactured' after January 1, 1971,
the maximum diameter to which drum can be worn is
stamped or cast into 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
CTS-2032N Page 1
PRINTED IN UNITED STATES OF AMERICA