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31 ENGINE
Under normal operating temperatures, expansion
of the piston bosses forces the piston to assume a
circular form.
The pistons are also tapered, measuring ap-
proximately .0007" to .0012" larger at the bottom
of the piston skirt than at the thrust face.
Cam grinding makes careful fitting of the
piston in the cylinder bore necessary. A .002"
feeler gauge 1/2" wide, extending the full length
of piston travel, is inserted between the cylinder
bore and the thrust face of the piston directly
opposite the valve mechanism. The feeler gauge
should be moved by a pull of from 3 to 4 pounds.
Use Tool J-888 to measure the pull. A variation
of .001" will change the pull on the feeler gauge
only a few pounds. Use of the scale will eliminate
guesswork.
NOTE: Check the piston fit in the cylinder bore
when both the cylinder block and piston are at
room temperature (70° F). Always be certain
that the ridge at the top of the cylinder has been
removed before attempting to fit pistons.
CYLINDER BORING
Before fitting pistons, examine the cylinder
walls for scratches, scores and wear. Cylinders
should be checked for taper and out of round with
a Cylinder Checking Gauge KMO-913. It is rec-
ommended that the cylinder bores be recondi-
tioned when the taper exceeds .020" and out of
round is greater than .005"
Honing will remove cylinder wall scratches
and scores up to .005" metal depth. An experi-
enced operator can remove metal up to .010" to
.015" depth. Reboring is recomm e n d e d only
where cylinders are so badly scored, worn, ta-
pered or out of round that honing will not provide
a satisfactory finish.
After boring operations have been completed,
the cylinder walls should be polished with crocus
cloth dipped in kerosene. If the cylinder walls
have been either honed or rebored, they should be
thoroughly washed with soap and water, using a
brush to remove all traces of grit, chips and abra-
sive materials. Otherwise, extremely rapid wear
of new parts will result.
PISTON PINS
Piston pins are of the full floating design. The
pin rotates in the connecting rod bushing with
sufficient movement in the piston to equalize wear.
The piston pin hole is diamond bored for close
fitting of the pin.
The piston pin and connecting rod bushing
should be replaced if necessary. Select the proper
size pin to fit the piston and ream the connecting
rod bushing to size.
To remove the piston pin, remove the piston
pin lock rings. Heat the piston and connecting rod
assembly to 200°F. in water o r in an electric
furnace. (Never heat-the piston with a blow torch
as this may distort the piston.) Push the piston pin
out with hand pressure, using Tool J-2948 Piston
Pin Remover. If the pin cannot be removed by
hand pressure, tap lightly on the tool with a ham-
mer while holding the piston in one hand. Do not
hold the piston solidly during this operation as
distortion and misalignment could result.
Piston pins should be a tight hand push fit in
the piston bores with the pistons heated t o 200° F.
P i s t on pins are available for service in the
oversizes shown and are identified with a color
code by a spot of paint on the end of each pin.
Following is a chart show in g pin sizes, color code
and bushing diameters after reaming:
PISTON
SIZE PIN DIAMETER COLOR DIAMETER
Standard
.7499
.001"
.7509
.002"
.7519
.005"
.7549
.010"
.7599
.015"
.7649
.020"
.7699
If the pin selected cannot be installed with
hand pressure, enlarge the hole with an expansion
reamer or a hone. Take very light cuts until the pin
can be fitted to the piston as outlined above.
CONNECTING ROD BUSHINGS
Connecting rod bushings are steel back bronze
burnished in place in the connecting rod bore and
BUSHING
None
.7502
Yellow
.7512
Orange
.7522
White
.7552
Blue
.7602
Green
.7652
Brown
.7702
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