Jeep cherokee 1989 Owner's Manual page 124

2D-18
The piston ring grooves can be cleaned with a
special tool, as shown here,
of the ring grooves (see illustrations).
5 Once the deposits have been removed, clean the pistonlrod
assemblies with solvent and dry them with compressed air (if available).
Make sure the oil return holes in the back sides of the ring grooves
are clear.
6 If the pistons and cylinder walls aren't damaged or worn exces-
sively, and if the engine block is not rebored, new pistons won't be
necessary. Normal piston wear appears as even vertical wear on the
piston thrust surfaces and slight looseness of the top ring in its groove.
New piston rings, however, should always be used when an engine
is rebuilt.
7 Carefully inspect each piston for cracks around the skirt, at the
pin bosses and at the ring lands.
8 Look for scoring and scuffing on the thrust faces of the skirt, holes
in the piston crown and burned areas at the edge of the crown. If the
skirt is scored or scuffed, the engine may have been suffering from
high operating temperatures. The cooling and lubrication systems
should be checked thoroughly. A hole in the piston crown is an indica-
tion that abnormal combustion (preignition) was occurring. Burned
areas at the edge of the piston crown are usually evidence of spark
17.10 Check the ring side clearance with a feeler gauge
The Motor Manual Guy
Chapter 2 Part D General engine overhaul procedures
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. .
. . .
or a section of a broken ring
knock (detonation). If any of the above problems exist, the causes must
be corrected or the damage will occur again. The causes may include
intake air leaks, incorrect fuellair mixture, incorrect ignition timing and
EGR
system malfunctions.
9 Corrosion of the piston, in the form of small pits, indicates that
coolant is leaking into the combustion chamber andlor the crankcase.
Again, the cause must be corrected or the problem may persist in the
rebuilt engine.
1 0 Measure the piston ring side clearance by laying a new piston ring
in each ring groove and slipping a feeler gauge in beside it (see illustra-
tion). Check the clearance at three or four locations around each groove.
Be sure t o use the correct ring for each groove
If the side clearance is greater than specified, new pistons will have
to be used.
1 1 Check the piston-to-bore clearance by measuring the bore (see Sec-
tion 15) and the piston diameter. Make sure the pistons and bores are
correctly matched. Measure the piston across the skirt, at a 90" angle
t o and in line with the piston pin (see illustration). Subtract the piston
diameter from the bore diameter t o obtain the clearance. If it's greater
than specified, the block will have t o be rebored and new pistons and
rings installed (see illustration).
17.1 l a Measure the piston diameter at a
piston pin and in line with
-
they are different.
angle t o the
it