Fit And Replace Piston - Pontiac Grand Prix Service Manual

V-8 ENGINE 6-67
comparable, at top, middle and bottom of bore.
Measure cylinder bore parallel and at right angles to
the centerline of engine to determine out-of-round.
Variation in measure from top to bottom of cylinder
indicates taper in cylinder.
Fig. 6-117 illustrates area in cylinder where normal
wear occurs. Cylinder bore can be measured by setting
cylinder gauge dial at zero in cylinder at the point of
desired measurement. Lock dial indicator at zero
before removing from cylinder, and measure across the
gauge contact points with outside micrometer with
gauge at the same zero setting when removed from
cylinder (Figs. 6- 1 18 and 6- 1 19).
Take several measurements parallel and at right
angles to the crankshaft, between 1/2" and 4" from
the top of the cylinder. Subtract the smallest measure-
ment found from the largest. If this figure exceeds
.0006", a piston cannot be fitted properly, and the
cylinder must be honed. New rings and a new
oversized piston must then be fitted.
Fine vertical scratches made by ring ends will not
cause excessive oil consumption, therefore, honing to
remove is unnecessary.
HONING OR BORING
If a piston in excess of .005" oversize is to be installed,
the cylinder should be bored, rather than honed, to
effect a true bore.
To eliminate the possibility of honing taper into the
cylinder, full strokes of the hone should be made in
addition to checking measurement at top, middle and
bottom of bore repeatedly.
Always be sure the crankshaft is out of way of boring
cutter when boring each cylinder. Crankshaft bearings
and other internal parts must be covered or taped to
protect them during boring or honing operation. When
7-
SIZING POINT
taking final cut with a boring bar, leave .001" on the
diameter for finish honing to give required piston-to
cylinder clearance specifications.
NOTE:
Honing or boring operation mut be done under
close supervision so that specifed clearance between
pistons, rings, and cylinder bores is maintained.
By measuring the piston to be installed at the sizing
points (Fig. 6-120) and adding the mean of the
clearance specification, the finish hone cylinder mea-
surement can be determined. It is important that both
block and piston be measured at normal room
temperature, 600
-
90°F.
After final honing and before the piston is checked for
fit, each cylinder bore must be thoroughly cleaned.
Use soapy water solution and wipe dry to remove all
traces of,abrasive. If all traces of abrasive are not
removed, rapid wear of new rings and piston will
result.
Intermixing different size pistons has no effect on
engine balance as all Pontiac pistons from standard
size up to .030" oversize weigh exactly the same,
Pontiac does not recommend boring beyond .OIOt'
during warranty period so that if necessary, engine can
be serviced at high mileage without cylinder block
replacement.
FIT AND REPLACE PISTON
Pistons should be fitted in the bores by actually
measuring the fit. Clearance between the piston and
the cylinder bore should be .0025" to .0031" on
standard engines; and .003OU to .0036" on 428
engines.
If cylinder bores have been reconditioned, or if pistons
are being replaced, reconditioning of bores and fitting
of pistons should be closely coordinated. If bore has
been honed, it should be washed thoroughly with hot
soapy water and a stiff bristle brush.
Fig.
6-120 Piston Sizing Point
Fig.
6-121
Measuring Piston