TRUCK SERVICE MANUAL
WHEELS, RIMS, TIRES
To recondition a brake drum in a lathe (Fig. 6),
the drum must be mounted so that it is centered. Use
proper size cone to provide accurate centering. Turn
drum, taking only light cuts and remove just enough
material to clean up drum. Then grind the finished
surface if grinder is available or use emery cloth on a
straight piece of wood and polish the drum friction
emery or steel wool particles from drum after this
operation. More heavily damaged or out-of-round drums
NOTE: Brake drums that are otherwise
should be ground or turned on brake drum lathe.
in good condition can be turned in a
lathe. However, it must be remembered
If depth of scoring, bellmouth or barrel shaping
that recommended rebore limit for brake
drums over 35.6 cm (14") in diameter
across part or all of brake surface, drum should be
must not be increased more than
refinished. Reboring limits (see drum) must not be
2.03mm (.080") diameter (total cut) and
exceeded and no heat checks, cracks or bluing is
discarded at 3.05 mm (.120") over
round drum. Make check by measuring drum brake
Brake drums should be cleaned thoroughly with
surface diameter at various points, 450 apart around
a steam cleaner or hot water. Do not use a solvent
circumference. Eccentricity (out-of-round) should not
which leaves an oily residue. If inspection shows the
exceed .38 mm (.015") on diameter.
drums may be used without remachining, rub friction
For older brake drums which do not show a
surface with fine emery cloth or sandpaper to remove
maximum diameter the drum must be discarded when
any foreign deposits. If drum has been reconditioned,
diameter is 3.05mm (.120") over original diameter.
clean friction surface with fine emery cloth or sandpaper
Remember that each time brake drums are
and wash. Next examine very carefully to see that no
turned, less metal remains to absorb the heat developed
metal chips remain in drum.
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 (Fig. 5) form on inner
surface of drums. These conditions will become
progressively worse until finally drums fail.