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8-14
CRANKSHAFf
Dress minor imperfections with an
oil stone.
If
the journals are severely
marred or exceed the wear limit,
they should be refinished to size for
the next undersize
bearing.
Refinishing Journals. Refinish the
journal to give the proper clearance
with the next undersize bearing.
If
FIG. 18
-Valve Seat Width
the journal will not clean up to
give the proper clearance with the
maximum undersize bearing avail-
able, replace the crankshaft.
Always reproduce the same jour-
nal shoulder radius that existed
originally. Too small a radius will
result in fatigue failure of the crank-
shaft. Too large a radius will result
in bearing failure due to radius ride
of the
bearing.
After refinishing
the
journals,
chamfer the oil holes, then polish
the journal with a No. 320 grit
polishing cloth and engine oil. Cro-
cus cloth may be used also as a
polishing agent.
PISTONS, PINS AND RIMGS
Fitting
Pistons.
Pistons
are
available for service in standard
sizes and the oversizes shown in
Table 2.
The standard-size
pistons
are
color coded red or blue on the dome.
Refer to the specifications for the
standard-size
piston
dimensions.
Piston pins are provided with new
pistons.
Follow the procedures in Section
3 to measure the piston O.D. and
cylinder bore
I.D.
The dimensions
should
be
within specifications, and
the piston to bore clearance (bore
I.D.
minus piston O.D.) must be
within the specified
limits.
If the clearance is greater than the
maximum limit, recheck calculations
to be sure that the proper size piston
GROUP 8-
ENGINE
has been selected, check for a daDl-
aged piston; then,
try
a new piston.
If
the clearance
is
less than the
minimum limit, recheck calculations
before trying another piston.
If
none
can be fitted, refinish the cylinder for
the next size pistor;1.
When a piston bas been fitted,
mark it for assembly in the cyUn-
der to which it was fitted.
If
the taper, out-of-round and
piston to cylinder bore clearance
conditions of the cylinder bore are
within specified limits, new piston
rings will give satisfactory
service.
TABLE 2-0versize Service Piston
PISTON
ENGINE
OVERSIZE
(inches)
200
0.003, 0.020,
250
0.030, 0.040,
302
0.003, 0.020,
351
0.030 0.040
FIG.
19 -Piston Ring Gap
If
the new rings are to be installed
in a used cylinder that has not been
refinished, remove the cylinder wall
glaze. Be sure to clean the cylinder
bore thoroughly, following the pro-
cedure in Section 3 page 8-19.
To Fit a Piston:
1. Calculate
the~size
piston
to
be
used by taking a cylinder bore.
check.
Follow the procedures out-
lined in Section 3 page 8 .. 19
.
2. Select the proper size piston
to provide the desired clearance
(refer to the specifications). Measure
the piston diameter in line with the
centerline of the piston pin and at
90° to the piston pin axis.
3. Make sure the piston and cyl-
inder block are at room tempera-
ture
{70°F).
After any refinishing
operation, aUow the cyUnder bore
to cool and make sure the piston
and bore :ae clean and
dry
before
the piston fit is checked.
Fitting Piston Rings
1. Select the proper ring set for
the size piston to be used.
l. Position the ring in the cylinder
bore in which it is going to be used.
3; Push the ring down into the
bore area where normal ring wear is
not
encountered.
4. Use the head of a piston to
position the ring in the bore so that
the ring is square with the cylinder
wall. Use caution to avoid damage
FIG.
20 -Ring Side Clecwance
to the ring
or
cyUnder bore.
5. Measure the gap between the
ends of the ring with a feeler gauge
(Fig. 19).
if
the ring gap
is less
or
greater than the specified limits,
try another ring set.
6. Check the ring side clearance
of the compression rings with a
feeler cauge inserted between the ring
and its lower land
(Fig. 20).
The
gauge should slide freely around the
entire ring circumference without
binding.
Any wear that occurs will
form a step at the inner portion of
the lower land.
If
the lower lands
have high steps, the piston should
be
replaced.
Fitting Piston Pins.
Install the piston pin, following
the procedure under Piston Assem-
bly page 8
·35).
Piston piD bores must not be reamed
with
hand-driven
reamers.
Use
motor-driven reamers, but do not
exceed the cutting
speed
(rpm)
recommended by the reamer manu-
facturer.
If
a reamer is used, set the ream-
er to the size of the pin bore; then
expand the reamer slightly and trial
ream the pin bore. Take a light cut.
Use a pilot sleeve of the nearest
size to maintain alignment of the
bores.
Check the hole
size,
using the
new piston pin.
If
the bore is small,
expand the reamer slightly anrt make
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