Pontiac tempest 1968 Service Manual page 285

6-68
1968 P O N T I A C SERVICE MANUAL
N O T C M
OIL S
HC
.MSHAFT
F i g . 6-121 Piston and Rod Assembly
4 . Carefully examine piston for rough or scored
bearing surfaces; cracks in s k i r t or head; cracked,
broken, or worn ring lands; scored, galled, or worn
piston bosses. Damaged or faulty pistons should be
replaced.
5. Inspect bearing shells to see that they are
serviceable. F i t of bearings should be checked when
engine is being assembled.
6. Inspect cylinder bores for out-of-round or ex­
cessive taper. See CYLINDER BORES-INSPECT.
PISTON RING CHECK AND
INSTALL ON PISTON
Two compression rings and one 3-piece o i l con­
t r o l ring, a l l above the piston pin, are used on p i s ­
tons for both standard and premium fuel engines.
The compression rings are taper faced and also have
either a step or chamfer on the inside diameter of
the bottom side. The top compression ring is mo­
lybdenum filled, which results i n the center section
of ring sealing edge appearing porous or grainy. The
P I S T O N
B A S E
S U P P O R T
T O START
REMOVAL
350
C U . I N .
J6901-11
4 0 0
C U . IN.
J6901-1 J6901-1
4 2 8
C U . I N .
J6901-11
- J 6901-3
- J 6901-5
- J
6901
Fig. 6-122 Installing Piston Pin
lower compression ring varies depending upon the
engine. See specifications Pg. 6-
Regardless of engine type, always install com­
pression rings with the stamped markings toward the
top of piston.
New rings are serviced for the standard size p i s ­
tons, and for .005", .010", .020", and .030" oversize
pistons. When selecting rings be sure they match
the size of the piston on which they are to be i n ­
stalled, 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 as follows:
1. Check pistons to see that ring grooves and o i l
return holes have been properly cleaned.
2. Place ring down at the bottom of the ring t r a v ­
eled part of the cylinder bore in which i t w i l l be
used. Square ring in bore by pushing i t into position
with head of piston.
3. Measure gap between ends of ring with feeler
gauge (Fig. 6-123). Gaps should be as follows:
Compression Ring
Oil Ring
.010"-.030"
.015"-.055"
Incorrect ring gap indicates that wrong size rings
are being used. If rings are selected according to
the size of the bore (standard .005" oversize, etc.),
they should have the proper gap. It should not be
necessary to alter ring gap by f i l i n g .
4 . Install rings on piston, using J 22563 or J 7117
to prevent breakage or fracture of rings, or damage
to pistons.