Visually inspect the rotor for scoring or grooving on
the braking surface, Figure 13-24. Scoring and grooving
are deep cuts in the rotor surface. They always follow the
rotor’s curve of rotation. If the pads have worn to the rivets
or the metal shoe surface, the rotor will be badly scored.
Sometimes, a rotor will be lightly scored by long usage,
especially in sandy or dusty areas. Do not assume the
inboard rotor surface is good if the outboard surface shows
no damage. Scoring can exist on one or both braking sur-
faces of the same rotor. Machining (turning) the surfaces is
required if the rotor shows any scoring or grooves.
Excessive brake heat can cause heat-checking (tiny
surface cracks) or bluing. There can be a combination of
heat-checking and bluing or just one condition by itself.
These patterns can also form on one or both sides of the
rotor, Figure 13-25. Bluing can sometimes be removed by
machining. If heat-checking is present or bluing cannot be
removed without excessively reducing rotor thickness, the
rotor should be replaced.
Rotors will sometimes crack. Cracks usually develop
at the wheel stud openings, although they can occur at any
spot on the rotor. Replace a cracked rotor no matter how
small the crack.
Checking Rotor Thickness and Runout
To check for rotor thickness (parallelism), you will
need a micrometer, Figure 13-26. Before checking thick-
ness, calibrate the micrometer. Then install the micrometer
on the rotor. If the rotor is grooved, place the point of the
micrometer in the deepest groove. Then read the thickness
on the micrometer and compare it to the specified mini-
mum thickness. The thickness should be more than the
minimum if the rotor will not be turned. If the rotor must be
turned, there should be enough metal remaining to be at or
above the minimum thickness after the turning process.
Turn the rotor about one-quarter turn and repeat the
thickness measurement procedure. Measure at least four
places on the rotor. If the thickness variation between parts
of the rotor is more than about .01” (.254 mm), the rotor
should be turned to prevent brake pulsation.
To check for excessive runout (warping), a dial indi-
cator should be used. Before checking runout, eliminate
any looseness in the rotor and hub assembly. If the rotor is
separate from the hub, install at least three of the wheel
bolts onto the lugs. If possible, the flat (non-tapered) side
of the nuts should contact the rotor. Lightly tighten the nuts
until there is no play between the rotor and hub. If the
rotor is integral with the hub, make sure there is no play in
the wheel bearings. If play is evident, tighten the wheel
bearings until all play is removed. Then proceed with the
runout checking procedure.
Place the dial indicator over the rotor so that the
pointer is contacting the rotor about two-thirds of the way
to the edge of the braking surface. The pointer should be
on a flat spot, not over any grooves. Then tighten the
mounting clamp. A typical dial indicator installation is
shown in Figure 13-27. Then set the micrometer dial to
zero. Turn the rotor slowly and observe the movement of
the dial indicator needle. If the needle indicates runout of
more than .01” (.254 mm), the rotor should be turned or
replaced.
220 Auto Brakes
Caution: Never attempt to machine a
heat-checked or cracked rotor. Replace any
rotor that appears to be heat-checked or
cracked.
Note: Mark the point of maximum runout
for later reference.
Figure 13-24. Scoring or grooving on the braking surface is
caused by dirt, exposed rivets, etc. Rotor machining will be
needed to reestablish the proper brake surface. Some minorr
scoring is considered normal.
Figure 13-25. This illustrates heat-checking and bluing caused
by heavy braking, severe service, etc. A heat-checked rotorr
should be replaced. (EIS)
Rotor
Heat
checking
Braking
surface
Bluing (heat
discoloration)
Groove in
rotor surface