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6A5-22
1977 PONTIAC SERVICE MANUAL
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 pistons from standard size, up to ,030" oversize,
weigh exactly the same.
PISTON
Fit and Replace
Pistons should be fitted in the bores by actually
measuring the fit (measure O.D. of piston at sizing point and
I.D. of cylinder bore). See engine specifications for
clearances. 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.
Using a cylinder checking gage, measure the cylinder
bore at right angles to piston pin to find the smallest diameter.
Record the diameter of each bore.
C A UT I 0 N
:
When measuring cylinder bores and
pistons, it is very important that the block and
pistons be at room temperature. If anv or all o f the
parts ore hotter or colder than nornjal room
temperature, improper fitting will result.
Measure the piston skirt perpendicular to the piston pin
boss (piston pin removed) and at the sizing point indicated
in Fig. 6A5-47.
properly mated, mark the pistons with the cylinder numbers
they fit so they will not become mixed.
PISTON RINGS
Install
Two compression rings and one 3-piece oil control ring,
all above the piston pin, are used on pistons. The top
compression rings are barrel faced and also have either a step
or a chamfer on the inside diameter of the bottom side. The
lower compression ring had a step also.
Always install compression rings with the side marked
with letters "GM" toward the top of the piston.
New rings are serviced for the standard size pistons, and
for .01OW, ,020" and .030" oversize pistons. When selecting
rings be sure they match the size of the piston on which they
are to be installed, i.e. standard rings for standard pistons,
.010" oversize rings for .010" oversize pistons, etc. Ring gap
and side clearance should be checked while installing rings
as follows:
I. Check pistons to see that ring grooves and oil return
holes have been properly cleaned.
2. Place ring down at the bottom of the ring traveled part
of the cylinder bore in which it will be used. Square ring in
bore by pushing it into position with head of piston.
3. Measure gap between ends of ring with feeler gage
(Fig. 6A5-48).
Fig.
6 A 5 - 4 7
Measuring Piston
-
Typical
Fig.
6 A 5 - 4 8
Checking Ring
Gap -
Typical
Make sure the micrometer is in full contact.
As the ist tons are measured thev should be marked for
size identification and the measurements recorded.
If there is excessive clearance between a cylinder bore
and the piston which was installed in that bore, a new piston
should be used.
New pistons are serviced in standard size and oversize.
See parts listings
Since these are nominal or basic sizes, it is important that
new pistons be measured to ensure proper fit. All new pistons
are serviced with selectively fitted piston pins.
After all measurements have been made, match the new
pistons with the cylinders where they will fit with proper
clearance. Honing of cylinder bore may be necessary to effect
a proper fit. (See Note in Honing and Boring Section.) When
Incorrect ring gap indicates that wrong size rings are
being used. If rings are selected according to the size of the
bore, they should have the proper gap. It should not be
necessary to alter ring gap by filing.
4. Install rings on piston, using suitable ring installing
tool to prevent breakage or fracture of rings, or damage to
pistons.
NOTE: Stagger ring end locations equially around
pistons.
5. Measure side clearance of rings in ring groove (Fig.
6A5-49) as each ring is installed.
If side clearance is excessive, piston should be replaced.
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