Buick LeSabre 1965 Service Manual page 65

ENGINE
BEARINGS 2 - 4 3
not out-of-round or tapered, it
usually is possible to remedy the
condition by honing the bore to
fit a standard service piston,
since standard service pistons
are of high limit diameters. If
cylinder bore is very rough or
deeply scored, however, it may
be necessary to rebore the cyl-
inder and fit an oversize piston
in order to insure satisfactory
results.
If cylinder bore is tapered .005"
or more, or is out-of-round .003"
or more, it is advisable to rebore
for the smallest possible oversize
p i s t o n s and rings. With this
amount of bore wear, some piston
wear has usually taken place so
that the total clearance in the
ring travel will be sufficient to
produce noisy piston operation.
c. Inspection of Pistons,
Rings and Pins
Clean carbon from piston sur-
faces and under side of piston
heads. Clean carbon from ring
grooves with suitable tool and
remove any gum or v a r n i s h
from piston skirts with suitable
solvent.
Carefully examine p i s t o n s for
rough or scored bearing surfaces,
cracks in skirt or head, cracked
or broken ring lands, chipping or
uneven wear which would cause
rings to seat i m p r o p e r l y or
have excessive clearance in ring
g r o o v e s . Damaged or faulty
pistons should be replaced.
The pistons are cam ground,
which means that the diameter at
a right angle to piston pin is
greater than the diameter parallel
to piston pin. When a piston is
checked for size it must be meas-
ured with a micrometer applied
to skirt at points exactly 90 de-
grees to piston pin. See Figure
2-44. Measurements should be
made at top and bottom ends of
skirt; the diameter at top end
will normally be very slightly
less than at bottom end after a
MEASURE
AT TOP &
BOTTOM
Figure 2-44—Measuring Piston with
Micrometer
piston has been in service in an
engine.
Inspect bearing surfaces of piston
pins and check for wear by meas-
uring worn and unworn surfaces
with micrometers. Rough or worn
pins should be replaced. Test fit
of piston pins in piston bosses.
Sometimes pins w i l l be found
tight due to gum or varnish de-
posits. This may be corrected
by removing the deposit with a
suitable solvent.
If piston bosses are worn out of
round or oversize, the piston and
pin assembly must be replaced.
Oversize pins are not practical
with the pressed pin and rod as-
semblies. Piston pins must fit
pistons with an easy finger push
fit at 70°F. (.00005" - .0001").
Examine all piston r i n g s for
scores, chips, or cracks, and for
tension as compared with new
rings. P l a c e all compression
rings in cylinder bores at lower
end of ring travel and check gaps,
which are normally .010" to.020".
If gaps are excessive it indicates
that rings have worn considerably
and should be replaced.
d. Reboring Cylinders and
Fitting New Pistons
If one or more cylinder bores
are rough, scored, or worn beyond
limits prescribed under Inspec-
tion of Cylinder Bores (subpar. b),
it will be necessary to smooth or
true up such bores to fit new
pistons.
If relatively few bores require
correction it will not be neces-
sary to rebore all cylinders to
the same oversize in order to
maintain engine balance, since all
over - size service pistons are
held to the s a m e weights as
standard size pistons. If condi-
tions justify replacement of all
pistons, however, all new pistons
should be the same nominal size.
Standard size service pistons are
high limit or maximum diameter;
therefore, they can usually be
used with a slight amount of
honing to correct slight scoring
or excessive clearances in en-
gines having relatively low mile-
age. Service pistons are also
furnished in .001", .005", .010",
020" and .030" oversizes. All
service p i s t o n s are diamond
bored and selectively fitted with
pistons pins; pistons are not fur-
nished without pins.
No attempt should be made to cut
down oversize pistons to fit cyl-
inder bores as this will destroy
the surface treatment and affect
the weight. The smallest possible
oversize service pistons should
be used and the cylinder bores
should be honed to size for proper
clearances.
Before the honing or reboring
operation is started, measure all
new pistons with micrometer con-
tacting at points exactly 90 de-
grees to piston pin (Figure 2-44)
then select the smallest piston
for the first fitting. The slight
variation usually found between
pistons in a set may provide for
correction in case the first piston
is fitted too free.
If wear at top of cylinder does
not exceed .005" on the diameter
or exceed .003" out of rqund,
honing is recommended for truing
the bore. If wear or out of round
exceeds these limits, the bore
should be trued up with a boring
bar of the fly cutter type, then
finish honed.
When r e b o r i n g cylinders, all
crankshaft bearing caps must be
in place and tightened to proper