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Section 4 Engine Service
Installing Insert-Type Rod Bearings
The diameter across the parting surfaces of
insert bearing halves is slightly larger than the
diameter across the curve machined into the rod
and rod cap. This condition is called bearing spread.
The correct amount of bearing spread gives tight
insert-to-bore contact around the entire bearing
and provides support and alignment. It also helps
to carry heat away through the rod and bearing cap
and holds the bearing in place during assembly.
To seat the insert, the ends must be forced
down and snapped into place. Never press down
in the center of the insert to seat it in the rod bore.
When precision inserts are snapped into the rod
bore, the ends will protrude slightly above the
parting surface. See Figure 19-15A. This built-in
design feature is called bearing crush. Generally,
bearing crush varies from .001″ to .002″.
When the rod cap is installed and drawn into
place, the insert ends meet fi rst and force the insert
halves tightly against the rod bore. This provides
fi rm support for the insert. The forced fi t makes
the insert round and, through close metal-to-metal
contact, allows heat to be carried away through the
rod. Figure 19-15B shows how radial pressure is
exerted against the rod bore.
Figure 19-13.
A ring compressor is used to squeeze the ring ends
together while the piston is pushed into the cylinder.
Figure 19-14.
The ring end gap allows the ring to expand and contract so they can stay in constant contact with the cylinder wall, even
when the cylinder becomes tapered due to wear. The taper of the cylinder in this drawing is extremely exaggerated
for illustrative purposes. A—At the top of piston travel, the rings expand outward. B—At the bottom of piston travel, the
rings are forced into their grooves.
Large end gap
Tapered cylinder
Tapered cylinder Small end gap
Engine block
A B
Engine block
cylinder wall, even if the cylinder is slightly
tapered. See Figure 19-14.
If the rod has integrated bearings, the saddle
should be guided into place on the crankshaft
journal as the piston is pushed into the cylinder. If
the rod uses bearing inserts, they must be installed
before the saddle is brought into contact with the
crankshaft journal.