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Section
7-Flywheel, Cronkshaft,
and Main Bearings
25
SCRATCHES
DIRT
IMe'Eo()Eo
INTO BEARING MATERIAL
SCRATCHED
BY DIRT
LACK
OF OIL
IMPROPER
SEATING
RADIUS RIDE
fATIGUE FAILURE
6491
Fig. 33-Bearing Failures-Typical
result
in crankshaft
failure,
while too large
a
radius will
result in bearing
failure.
CAUTION:
Never
grind journals or
crankpin.
in
e%ce ••
of
O.030·inch unde"ize.
After
grinding, chamfer
the
oil
holes,
then polish the
pin or journal
with No.
320 grit polishing cloth and
engine oil.
Crocus
cloth may
also be used as
a
polishing
agent.
c. Main Bearings.
The
insert-type
main bearings are
select
fit They
are
available
for service in
standard and undersizes
for
use
on
journals that
have
been reground.
The
installation
of
new
bearings must be
closely
checked
to maintain the
proper
clearance between the
journals
and bearing
surfaces.
(1)
INSPECTION.
Check
the bearings for
any
dam·
age or excessive
clearance.
Examples
of
bearing
failures
and their
causes
are illustrated
in fig.
33.
(2) FITTING
MAIN BEARINGS
- PLASTIGAGE
METHOD
.
The following
procedl!re
applies to fitting
main
bearings w
ith
the
engine
either installed on a
workstand
or in the
truck.
If the bearing
fits
are
to
be
checked with the
engine
in
the truck,
support
the
weight
of
the
crankshaft with
.a
small jack
positioned
to hold
the crankshaft upward
against
the
block half
of
the main
bearings. Place the
jack
to
bear
against the crankshaft
counterweight
adjoin-
ing the
bearing which
is being checked for clearance.
The shaft can
also be
supported
by a thin rubber pad
between
the cap
insert and
the journal of two bearings
that
are
not
being
checked. Tighten
these bearing
cap
bolts
just
enough
to
hold the
crankshaft up against the
upper
bearings.
NOTE:
It i.
nece .. ary to IUpport the weight of the
crankshaft.
when
checking
main bearing clearances
to
prevent the weight of the
crank.haft
from com·
pressing the Plmtigage, thereby providing
an
erro·
neous reading.
Place a piece
of
Plastigage, the full width of the
bearing, on the bearing surface (or on the crankshaft
journal if the engine is inverted) about Y.·inch off
center. Install the cap and tighten
the
bolts
to
specifications.
CAUTION: Do not turn the crank.haft while the
Plastigage
i. in place.
Remove the cap, then check the width of the Plasti·
gage at
the
widest
point with' the Plastigage scale
(fig. 34).
If
the clearance
is excessive, try
another
selective fit
bearing
to
bring
the clearance
within
the
desired limit.
.002"
CLEARANCE
Flattened Plastic
GRADUATED
CONTAINER
1282
Fig. 34-Measuring Plastigage
- Typical
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