The precision type bearing is accurately finished to
crankshaft. Connecting rods are drop forged from a
fit the journal and does not require further fitting during
steel alloy capable of withstanding heavy loads without
installation. It is positioned by projections on the shell,
bending or twisting. Holes at the upper and lower ends
which match reliefs in the rod and cap. The projections
are machined to permit accurate fittings of bearings.
prevent the bearings from moving sideways and from
These holes must be parallel.
rotary motion in the rod and cap.
The upper end of the connecting rod is connected to
The semiprecision type bearing is usually fastened
the piston by the piston pin. If the piston pin is locked in
to or die-cast with the rod and cap. Prior to installation,
the piston pin bosses, or if it floats in both piston and
it is machined and fitted to the proper inside diameter
connecting rod, the upper hole of the connecting rod
with cap and rod bolted together.
will have a solid bearing (bushing) of bronze or similar
material. As the lower end of the connecting rod
To maintain good engine balance, connecting rods
revolves with the crankshaft, the upper end is forced to
are carefully matched in sets. When it becomes
turn back and forth on the piston pin. Although this
necessary to remove the connecting rods, make sure
movement is slight, the bushing is necessary because
they are marked so they can be replaced in the same
the temperatures and the pressures are high. If the
cylinder from which they were removed. Most rods are
piston pin is semifloating, a bushing is not needed.
marked at the factory, but if they are not, use a number
punch to mark them in such a way that you will not get
The lower hole in the connecting rod is split to
permit it to be clamped around the crankshaft. The
bottom part, or cap, is made of the same material as the
In addition to the proper fit of the connecting rod
rod and is attached by two or more bolts. The surface
bearings and the proper position of the connecting rod,
that bears on the crankshaft is generally a bearing
the alignment of the rod itself must be considered. That
material in the form of a separate split shell, although,
is to say, the hole for the piston pin and the crankpin
in a few cases, it may be spun or die-cast in the inside of
must be precisely parallel. Equipment of suitable
the rod and cap during manufacture. The two parts of
accuracy is available for checking connecting rods.
the separate bearing are positioned in the rod and cap by
(See fig. 3-32.) Every connecting rod should be
dowel pins, projections, or short brass screws. The shell
checked for proper alignment just before it is installed
may be of babbitt metal that is die-cast on a backing of
in the engine. Misalignment of connecting rods will
bronze or steel. Split bearings may be of the precision
cause many hard-to-locate noises in the engine.
or semiprecision type.
Figure 3-32.--Checking connecting rod alignment.