Basic Engine
Specifications for standard size piston dimensions.
Always reproduce the same journal shoulder radius that
Measure the cylinder bore and select the piston to assure
existed originally. Too small a radius will result in fatigue
the proper clearance. When the bore diameter is in the lower
failure of the crankshaft. Too large a radius will result in
one-third of the specified range, a red piston should be used.
bearing failure due to radius ride of the bearing.
After
When the bore diameter is in the middle one-third a blue
refinishing the journals, chamfer the oil holes. then polish the
piston should be used. When the bore diameter is in the
journal with a No. 320 grit polishing cloth and engine oil.
upper one-third, the 0.0025 O.S. piston should be used.
Crocus cloth may also be used as a polishing agent.
Measure the piston diameter to ensure that the specified
FITTING MAIN OR CONNECTING ROD
clearance is obtained. It may be necessary periodically to use
BEARINGS WITH PLASTIGAGE
another piston (red or blue) that is either slightly larger or
1. Clean crankshaft journals. Inspect journals and thrust
smaller to achieve the specified clearance. If none can be
faces (thrust bearing) for nicks, burrs or bearing pickup
fitted, refinish the cylinder to provide the proper clearance
that would cause premature bearing wear When replacing
standard bearings with new bearings, it is good practice to
for the piston. When a piston has been fitted, mark it for
fit the bearing to minimum specified clearance. If the
assembly in the cylinder to which it was fitted. If the
desired clearance cannot be obtained with a standard
taper, out-of-round and piston to cylinder bore clearance
bearing. try a 0.002 inch undersize in combination with a
conditions of the cylinder bore are within specified limits,
standard bearing to obtain the proper clearance.
new piston rings will give satisfactory service. If new
2. If fitting a main bearing, position a jack under counter-
rings are to be installed in a used cylinder that has not
weight adjoining bearing which is being checked. Support
been refinished, remove the cylinder wall glaze (refer to
crankshaft with jack so its weight will not compress
Cylinder Block, Refinishing Cylinder Walls). Be sure to
Plastigage and provide an erroneous reading.
clean the cylinder bore thoroughly.
3. Place a piece of Plastigage on bearing surface across full
1. Calculate the size piston to be used by taking a cylinder
width of bearing cap and about 1/4 inch off center ( Figure
bore check.
Follow the procedures outlined under
27).
Cleaning and Inspection.
4. Install cap and torque bolts to specifications. Do not turn
2. Select the proper size piston to provide the desired
crankshaft while Plastigage is in place.
clearance (refer to the specifications). The piston should
5. Remove cap. Using Plastigage scale, check width of
be measured 21/4 inches below the dome and at 900 to
Plastigage at widest point to get minimum clearance.
the piston pin bore.
Check at narrowest point to get maximum clearance.
3. Make sure the piston and cylinder block are at room
Difference between readings is taper of journals.
temperature (70 degrees F.). After any refinishing
6. If clearance exceeds specified limits. on the connecting
operation allow the cylinder bore to cool, and make
rod bearings, try a 0.002 inch undersize bearing in
sure the piston and bore are clean and dry before the
combination with the standard bearings.
Bearing
piston fit is checked.
clearance must be within specified limits. If O 002
FITTING PISTON RINGS
undersize main bearings are used on more than one
1. Select the proper ring set for the size cylinder bore.
journal. be sure they are all installed in cylinder block side
2. Position the ring in the cylinder bore in which it is going to
of bearing. If standard and 0.002 inch undersize bearings
be used.
do not bring clearance within desired limits, refinish
3. Push the ring down into the bore area where normal ring
crankshaft journal, then install undersize bearings.
wear is not encountered.
7. After bearing has been fitted. remove Plastigage and
4. Use the head of a piston to position the ring in the bore so
apply light coat of engine oil to journal and bearings.
that the ring is square with the cylinder wall. Use caution
Install bearing cap. Torque cap bolts to specifications.
to avoid damage to the ring or cylinder bore.
8. Repeat procedure for remaining bearings that require
5. Measure the gap between the ends of the ring with a
replacement.
feeler gauge (Figure 28). 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 gauge inserted between the ring and its lower
land (Figure 29). 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
The piston pins are selected to give the correct fit in the
piston pin bore and bushing in the connecting rod. Pistons are
only supplied in service complete with the piston pin, to ensure
the correct fit. The piston pins should not be interchanged.
VALVE ROCKER ARM
FIG. 27 Installing and Measuring Plastigage
If the pad at the valve end of the rocker arm has a
PISTONS, PINS AND RINGS FITTING PISTONS
grooved radius, replace the rocker arm. Do not attempt to
Pistons are available for service in standard sizes and the
true this surface by grinding.
oversizes shown in the parts book.
The standard size pistons are color coded red or blue, or
have .0025 O.S. stamped on the dome. Refer to the
1-14
