Red Lead Test; Adjustments Affecting Tooth Contact - Pontiac FIREBIRD 1972 Service Manual

NON TYPE C DIFFERENTIAL
differential to prevent possibility of subsequent fail-
ure.
GEAR TOOTH NOMENCLATURE
The side of the ring gear tooth which curves outward, or
is convex, is referred to as the drive side. The concave side
is the coast side. The end of the tooth nearest center of ring
gear is referred to as the toe end. The end of the tooth
farthest away from center is the heel end. Toe end of tooth
is smaller than heel end. It is very important that tooth
contact be tested before the differential carrier assembly is
disassembled and before it is installed. Allowable varia-
tions in the carrier or pinion rear bearing may cause the
pinion to be too far away from, or close to, the ring gear.
Thus, the tooth contact must be tested and corrected, if
necessary, or the gears may be noisy.
RED LEAD TEST
1. Mix a small amount of gear marking compound, part
no. 1051 196 or powered red lead (available from
paint manufacturers and suppliers) with a drop of
engine oil. Apply this mixture sparingly to all ring
gear teeth, using a medium stiff brush. When prop-
erly used, the area of pinion tooth contact will be
visible when hand load is applied.
2. Tighten bearing cap bolts to 70 Ib. ft. torque, tapping
heads of bolt intermittently while tightening, to en-
sure proper seating of caps and sufficient tightness.
3. Insert crank (Fig. 4A-1) in companion flange and,
while turning, apply pressure to back side of ring gear
by hand (a leather glove can be used). A test made
without loading the gears will not give a satisfactory
pattern. Turn companion flange so that ring gear
rotates one full revolution, then reverse rotation so
that ring gear rotates one revolution in opposite di-
rection. Excessive turning of ring gear may indicate
good tooth pattern because one or two teeth are mak-
ing proper contact.
NOTE: The crank in Fig. 4A-1 may be easily
made as follows:
a. Weld a 3/8" heavy duty flat washer to a piece of
1/4" diameter rod, approximately 6" long and
form as shown.
b. Tap door knob for 3/8" bolt and attach knob to
crank as shown. Leave bolt loose enough to per-
mit knob to turn.
4. Closely inspect tooth pattern on ring gear to deter-
mine whether pressure lines are apparent. If observa-
tion reveals pressure lines are present (dark narrow
band at edge of pattern), examine for pressure line
position on drive and coast sides of ring gear. If lines
on drive side are too deep and those on coast side are
too high (near heel and toe respectively), additional
DIA. ROD APPROX. 6 LONG.
r J / R
I.
D. H. R WASHER
A
WELD SECURELY
I
Fig. 4A-1 Differential Cranking Tool
shims to bring pinion gear out (providing a more
centrally located tooth pattern on ring gear) will re-
sult in a noisy tooth contact. This occurs because the
pressure line has been moved deeper into the tooth on
the drive side and farther out on the coast side. It
does not follow that the ring gear and pinion are not
good or should be destroyed. It only means they will
not operate quietly in the carrier in which they are
presently installed. These same parts may operate
quietly in another carrier when tooth pattern is
checked.
Removing backlash moves ring gear into pinion gear, driv-
ing the pinion deeper into the ring gear. Whenever pres-
sure lines are noted, as explained above, install another
ring gear and pinion set.
5. Observe pattern on ring gear teeth and compare with
Fig. 4A-2.
EFFECTS OF INCREASING LOAD ON TOOTH
CONTACT PATTERN
When "load" on ring and pinion gear is increased, such
as when car is accelerated from standstill or from normal
drive, the tooth contact will tend to spread out and under
very heavy load will extend from near toe to near heel. The
entire contact also tends to shift toward heel under in-
creasingly heavier loads and will become somewhat
broader with respect to tops and bottoms of teeth. The
patterns obtained by red lead tests, dependent upon degree
of "loading"; approximate a normal light load. For this
reason, they will extend only about halfway (Fig. 4A-2A).
The important thing to note is that the contact pattern is
centrally located up and down on the face of the ring gear.
ADJUSTMENTS AFFECTING TOOTH CONTACT
Two adjustments can be made which will effect tooth
contact pattern: backlash and position of drive pinion in