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CHAPTER 5 The Two-Stroke Cycle Engine
Copyright Goodheart-Willcox Co., Inc.
crankpin. The force is applied at the “throw” dis-
tance from the center of the crankshaft, causing
the crankshaft to rotate with torque. Torque is
a force applied at a distance from a centerline.
Counterbalance
The counterbalance is a mass of metal connect-
ing the crankshaft ends to the crankpin. The
configuration of the counterbalance is designed
so that at any point in the crankshaft rota-
tion, the force acting on one side of the crank-
shaft centerline equals the force acting on the
opposite side. The crankshaft is always “bal-
anced.” This balancing reduces engine vibra-
tion. Figure 5-17 shows how the momentum
of the counterweights applies a counterbalanc-
ing force as the crankshaft rotates. Without the
counterweights, force would be applied to only
one side of the crankshaft. This would tend to
cause the crankshaft to wobble, create excessive
vibration, and place large stresses on the bear-
ings supporting the crankshaft.
Left-Hand and Right-Hand Threads
Because a crankshaft is intended to rotate the
same direction all the time, the threads on each
end of a complete crankshaft may be threaded
in opposite directions. One end of the crank-
shaft will have a right-hand thread while the
other end may have a left-hand thread. Stan-
dard fasteners for two-stroke equipment use
right-hand threads. Components, as well, typi-
cally have right-hand threads. However, a tech-
nician needs to be aware that some components
have left-hand threads. Applying excessive force
in an attempt to loosen them in the wrong rota-
tion could seriously damage the equipment.
Bearings
Bearings are used multiple places in two-stroke
engines to support rotating shafts (like the
crankshaft, crankpin, and piston pin) and min-
imize the friction between those shafts and sta-
tionary parts like the crankcase or engine cover.
The two types of bearings are plain and anti-
friction bearings. A plain bearing has no mov-
ing parts; it simply provides a smooth surface
between the shaft and stationary part. Plain
bearings are machined directly into the part
and cannot be replaced. They are used where
there is relatively little movement between the
bearing and shaft or very little load placed on
the bearing, and thus no more than a smooth
surface is needed.
SHOP PRACTICE
Always refer to the shop manual for the specific
engine you are working on to determine the cor-
rect thread direction.
CAUTION
Follow the manufacturer’s recommended pro-
cedure for removing the flywheel and clutch. Do
not put the clutch into a vise to remove the fly-
wheel nut. This process could possibly shear the
flywheel key. On some engines, the flywheel key
is molded into the flywheel. In such cases, the
flywheel must be replaced if the key is damaged.
Also, do not put the flywheel in a vise to remove
the clutch. Always refer to the service manual
for the correct way to remove the clutch and
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Goodheart-Willcox Publisher
Figure 5-17. Pressure from combustion pushes
down on the piston. Force is transmitted to the
crankpin through the connecting rod. The force
then causes the crankshaft to rotate, creating
torque. The momentum of the rotating counter-
weights applies a counterbalancing force to the
other side of the crankshaft.