steel but may also be made of aluminum in smaller
The upper end of the connecting rod is connected to
the piston by the piston pin. The piston pin is locked in
the pin bosses, or it floats in both piston and connecting
rod. The upper hole of the connecting rod has a solid
bearing (bushing) of bronze or similar material. As the
lower end of the connecting rod revolves with the
crankshaft, the upper end is forced to turn back and forth
on the piston pin. Although the movement is slight, the
bushing is necessary because the temperatures and
pressures are high. If the piston pin is semifloating, a
bushing is not needed.
The lower hole in the connecting rod is split, so it
can be clamped around the crankshaft. The bottom part,
or cap, is made of the same type of material as the rod
and is attached by two or more bolts. The surface that
bears on the crankshaft is generally a bearing material in
the form of a split shell, although, in a few cases, it may
be spun or die-cast in the inside of the rod and cap during
manufacture. The two parts of the separate bearing are
positioned in the rod and cap by dowel pins and
projections or by a short brass screw. The shell may be
of Babbitt metal that is die-cast on a backing of bronze
or steel. Split bearings may be of the precision or
The PRECISION type of bearing is accurately
finished to fit the crankpin and does not require further
fitting during installation. It is positioned by
projections on the shell that match relief in the rod and
cap. The projections prevent the bearings from moving
sideways and from rotary motion in the rod and cap.
The SEMIPRECISION type of bearing is fastened
to or die-cast with the rod and cap. Before installation, it
is machined and fitted to the proper inside diameter with
the cap and rod bolted together.
The connecting rod bearings are fed a constant
supply of oil through a hole in the crankshaft journal. A
hole in the upper bearing half feeds a passage in the
connecting rod to provide oil to the piston pin.
Connecting rod numbers are used to assure a proper
location of each connecting rod in the engine. They all
assure that the rod cap is installed on the rod body
correctly. When connecting rod caps are being
manufactured, they are bolted to the connecting rods.
Then the lower end holes are machined in the rods.
Since the holes may not be perfectly centered, rod caps
must NOT be mixed up or turned around. If the cap is
installed without the rod numbers in alignment, the bore
will NOT be perfectly round. Connecting rod caps,
crankshaft, and bearing damage will result.
In addition to the proper fit of the connecting rod
bearings and the proper position of the connecting rod,
the alignment of the rod itself must be considered. That
is to say, the hole for the piston pin and the crankpin
must be precisely parallel. Equipment of suitable
accuracy is available for checking connecting rods (fig.
3-39). EVERY connecting rod should be checked for
proper alignment just before it is installed in the engine.
Misalignment of connecting rods causes many hard to
locate noises in the engine.
As the pistons collectively might be regarded as the
heart of the engine, so the CRANKSHAFT (fig. 3-40)
may be considered its backbone. The crankshaft is the
part of the engine that transforms the reciprocating
motion of the piston to rotary motion. It transmits
power through the flywheel, the clutch, the
transmission, and the differential to drive your vehicle.
Crankshafts are made from forged or cast steel.
Forged steel is the stronger of the two and is used in
commercial and military engines. The cast unit is
primarily used in light- and regular-duty gasoline
engines. After the rough forging or casting is produced,
it becomes a finished product by going through the
Each hole is located and drilled.
Each surface is rough machined
Figure 3-39.Checking connecting rod alignment.