Pontiac firebird 1968 - Specifications

6-66
1968 PONTIAC SERVICE MANUAL
Fig. 6-117 Measuring Cylinder Bore
Fig. 6-118 Measuring Cylinder Gauge
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 meas-
urement found from the largest* If this figure ex-
ceeds .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.
Fig. 6-119 Piston Sizing Point
When boring, 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 taking final cut with a
boring bar, leave .001" on the diameter for finish
honing to give required pistonrto-cylinder clearance
specifications.
NOTE: Honing or boring operation must be done
under close supervision so that specified clear-
ance between pistons, rings, and cylinder bores
is maintained.
By measuring the piston to be installed at the
sizing points (Fig. 6-119) and adding the mean of the
clearance specification, the finish hone cylinder
measurement can be determined. It is important that
both block and piston be measured at normal room
temperature, 60°-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 .010"
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 .0030" to .0036" on 428 engines.
If cylinder bores have been reconditioned, or if
pistons are being replaced, reconditioning of bores