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140 ENGINE
6A1-21
Cleaning end Inspection
Wash connecting rods in cleaning solvent and dry with
compressed air.
Check for twisted or bent rods and inspect for nicks or
cracks. Replace connecting rods that are damaged.
Clean varnish from piston skirts and pins with a cleaning
solvent.
DO NOT WIRE BRUSH ANY PART OF THE
PISTON. Clean the ring grooves with a groove cleaner and
make sure oil ring holes and slots are clean.
Inspect the piston for cracked ring lands, skirts or pin
bosses, wavy or worn ring lands, scuffed or damaged skirts,
eroded areas at top of the piston. Replace pistons that are
damaged or show signs of excessive wear.
Inspect the grooves for nicks or burrs that might cause
the rings to hang up.
The piston pin clearance is designed to maintain
adequate clearance under all engine operating conditions.
Because of this, the piston and piston pin are a matched set
and not serviced separately.
Inspect piston pin bores and piston pins for wear. Piston
pin bores and piston pins must be free of varnish or scuffing
when being measured. The piston pin should be measured
with a micrometer and the piston pin bore should be
measured with a dial gage or an inside micrometer. If
clearance is in excess of the .OOln wear limit, the piston and
piston pin assembly should be replaced.
Piston Selection
1. Check used piston to cylinder bore clearance as
follows:
a. Measure the cylinder bore diameter with a telescope
gage (2 1/2" from top of cylinder bore) (Fig. 6A1-56).
b. Measure the piston diameter (at skirt across centerline
of piston pin, Fig. 6A1-57).
c. Subtract piston diameter to determine piston-to-bore
clearance.
d. Compare piston-to-bore clearance obtained with those
clearances recommended in specifications section and
determine if piston-to-bore clearance is acceptable range.
2. If used piston is not acceptable, check piston size (Fig.
6A1-58) And determine if a new piston can be selected to fit
cylinder bore.
Assembly
1. Lubricate piston pin holes in piston and connecting
rod to facilitate installation of pin.
2. Place connecting rod in piston and hold in place with
piston pin guide and piston pin. Place assembly on fixture and
support assembly.
3. Using piston pin installer, J 24086-7, press the piston
pin into the piston and connecting rod (Fig. 6A1-60).
NOTE: The piston pin installer is a variable insertion
length tool designed to be applicable to all GM piston
assemblies. The insertion length is varied by rotating the
hub on the shaft much like adjusting a micrometer. An
alpha-numeric scale is used to determine the desired
length for a given piston pin assembly. Use Pin Guide
J 24086-7, Installer G-8 with pin size .927.
Fig.
6A1-56
Measuring Cylinder Bore
Fig.
6A1-57
Measuring Piston Diameter
1
PISTONS AVAILABLE
4490
Fig.
6A1-58
Piston Size Chart
CAUTION: After instalJer hub bottoms on
support assembJyt do not exceed dCKX)psipressure,
as th~s could cause structural damage to the tool.
4. Remove piston and connecting rod assembly from tool
and check piston for freedom of movement on piston pin.
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