Jeep 1996 Grand Cherokee Service Manual page 955

9 - 18 4.0L ENGINE
DESCRIPTION AND OPERATION (Continued)
OVERSIZE AND UNDERSIZE COMPONENT CODES
Some engines may be built with oversize or under-
size components such as:
• Oversize cylinder bores.
• Oversize camshaft bearing bores.
• Undersize crankshaft main bearing journals.
• Undersize connecting rod journals.
These engines are identified by a letter code (Fig.
3) stamped on a boss between the ignition coil and
the distributor (Fig. 4).
Fig. 3 Oversize and Undersize Component Codes
SERVICE PROCEDURES
VALVE TIMING
Disconnect the spark plug wires and remove the
spark plugs.
Remove the engine cylinder head cover.
Remove the capscrews, bridge and pivot assembly,
and rocker arms from above the No.1 cylinder.
Alternately loosen each capscrew, one turn at a
time, to avoid damaging the bridge.
Rotate the crankshaft until the No.6 piston is at
top dead center (TDC) on the compression stroke.
Rotate the crankshaft counterclockwise (viewed
from the front of the engine) 90°.
Install a dial indicator on the end of the No.1 cyl-
inder intake valve push rod. Use rubber tubing to
secure the indicator stem on the push rod.
Fig. 4 Oversize and Undersize Component Code
Set the dial indicator pointer at zero.
Rotate the crankshaft clockwise (viewed from the
front of the engine) until the dial indicator pointer
indicates 0.305 mm (0.012 inch) travel distance (lift).
The timing notch index on the vibration damper
should be aligned with the TDC mark on the timing
degree scale.
If the timing notch is more than 13 mm (1/2 inch)
away from the TDC mark in either direction, the
valve timing is incorrect.
If the valve timing is incorrect, the cause may be a
broken camshaft pin. It is not necessary to replace
the camshaft because of pin failure. A spring pin is
available for service replacement.
PISTON FITTING
BORE GAGE METHOD
(1) To correctly select the proper size piston, a cyl-
inder bore gauge, capable of reading in 0.003 mm (
.0001 in.) INCREMENTS is required. If a bore gauge
is not available, do not use an inside micrometer.
(2) Measure the inside diameter of the cylinder
bore at a point 49.5 mm (1-15/16 inches) below top of
bore. Start perpendicular (across or at 90 degrees) to
the axis of the crankshaft at point A and then take
an additional bore reading 90 degrees to that at point
B (Fig. 5).
(3) The coated pistons will be serviced with the
piston pin and connecting rod pre-assembled. The
coated piston connecting rod assembly can be
used to service previous built engines and
Location
ZJ