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Chapter 19 Engine Reassembly and Break-In
Precision inserts must be kept in matched
pairs. Never mismatch bearing inserts. Always use
the exact size needed. For most engines, standard
sizes and various undersizes are available. Bear-
ings cannot be made larger or smaller in the shop.
Checking Rod Bearing Clearance
Bearing clearance is the space between the
inner bearing surface and the crankshaft rod
journal. When checking bearing clearance, use a
special compressible plastic material called Plas-
tigage. This material is color coded and selected
according to the recommended clearance range. It
comes in a thin, round strand, which is stored in
a paper package.
To use Plastigage, select the correct color for the
specified clearance. Cut a piece of plastic equal to
the width of the bearing and lay it across the bear-
ing surface, Figure 19-16A. Next install the cap and
tighten it to the proper specification. Be careful to
install the rod cap in the proper orientation.
Torque the bearing cap in place. Then, remove
the cap and compare the compressed width of the
plastic with the scale printed on the Plastigage
package. See Figure 19-16B. The number within
the graduation on the package (envelope) indi-
cates the bearing clearance in thousandths of an
inch or in millimeters depending on which side of
the package is used. Taper is indicated where one
end of the Plastigage is flattened wider than the
other end. Measure each end of the flattened Plas-
tigage and the difference between reading is the
approximate amount of taper. Excessive amount of
taper indicates that a new or reground crankshaft
is required. In effect, the wider the plastic, the less
clearance there is.
If bearing clearance is too great, undersize
inserts will have to be used. If the crank journal is
worn, it will require grinding to clean it up. After
grinding the journal, recheck the clearance with a
Plastigage, and select the proper undersize inserts.
Installing the Connecting Rod Cap
After measuring the bearing clearance, scrape
the Plastigage from cap bearing surface with a
fingernail. If the clearance and taper were within
specifications, lubricate the connecting rod bearing
surfaces and the crankshaft journal with assembly
lube. Next, install the rod cap and tighten the bolts
to the proper torque specification. See Figure 19-17.
When the engine is manufactured, the rod
cap is bolted into position on the rod. Then, the
assembly is bored to an exact size. It is important,
therefore, that the rod cap is always put back in
its original position. If the cap is turned 180°, the
upper and lower halves will be offset. This error
in assembly will eventually result in bearing and
shaft failure. See Figure 19-18.
Figure 19-15.
The effect of bearing crush. A—Rod and cap separated.
B—Rod and cap drawn together, creating radial
pressure on the inserts. (Sunnen Products Co.)
Radial
pressure
A B
Bearing
inserts
Crush
height
Rod
cap
Figure 19-16.
Plastigage is used to check bearing clearance. A—A strip
of Plastigage is placed across the bearing surface of
the rod cap. B—The cap is tightened to specification
and then removed. The width of the flattened strip is
compared to the scale on the package.
Plastigage
A
B