92
Two-Stroke Engines
Copyright Goodheart-Willcox Co., Inc.
Connecting Rod
The connecting rod receives the force from the
piston and transmits it to the crankshaft, caus-
ing the crankshaft to rotate. The piston pin end
of the connecting rod connects to the piston, and
the crankshaft end connects to the crankshaft
journal. A journal, also called a bearing journal,
is a surface on a shaft that rotates within a bear-
ing. When the engine is running, the piston end
of the connecting rod swivels on the piston pin.
A caged needle bearing separates the connect-
ing rod from the piston pin and reduces fric-
tion. The crankshaft end of the connecting rod
rotates in a circle and also uses a needle bearing
to reduce friction.
Crankshaft
A complete crankshaft consists of two ends—a
flywheel end and a PTO end, Figure 5-16. The
flywheel end is typically identified by a taper
which is used to mount and drive the flywheel.
This end of the crankshaft is also called the
magneto end (or mag end) because the flywheel
houses the magnets used in creating the igni-
tion spark. The power take-off (PTO) is the end
of the crankshaft where crankshaft rotation is
transmitted to the rest of the equipment. Power
can be transmitted through a clutch, pulley, or
direct connection to the rest of the equipment.
Crankpin Bearing
The crankpin is the offset portion of the crank-
shaft. The crankshaft end of the connecting rod
is connected to the crankpin. The needle bear-
ing between the crankpin and the connecting
rod is called the crankpin bearing. It prevents
direct contact between the crankpin and con-
necting rod, minimizing friction as the parts
move. The crankpin is also referred to as the
crankpin journal because it rotates within the
bearing.
On most of the two-stroke engines, the con-
necting rod is permanently assembled onto the
crankshaft at the factory. The crankshaft end
of the connecting rod receives a needle bear-
ing and is positioned on the crankpin. The two
ends of the crankshaft are then pressed onto the
crankpin using a large force, forming a com-
plete crankshaft.
The crankpin needle bearing on the complete
crankshaft is considered non-serviceable, which
means it cannot be replaced. If the bearing goes
bad, the crankshaft and connecting rod assem-
bly is replaced as a unit.
The crankpin offset is called the throw. See
Figure 5-15. The throw is the distance from the
centerline of the crankshaft to the centerline of
the crankpin. As the piston pushes down on the
connecting rod, the force is transmitted to the
Piston pin
needle bearing
Crankpin
PTO
Counterweights
Connecting rod
Crankpin
needle bearing
Flywheel taper
Throw Th Th Th Th Th Th Th Th Th Th Th Th Th Th Th T Th Th Th Th Th Th Th Th Th T hr Th Th Th Th h Th h h Th Th h Th Th h T T T Th h T Th T T h h T h h T h T h h ro ro ro ro ro ro ro ro ro ro ro o ro ro ro ro ro ro o ro ro ro r ro ro ro o ro ro o o r ro o o o o o o o r o r o o o o o w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w
Goodheart-Willcox Publisher
Figure 5-16. The complete crankshaft on most two-stroke engines consists of the crankshaft with the
connecting rod permanently connected to it.