88
Two-Stroke Engines
Copyright Goodheart-Willcox Co., Inc.
an outer race, inner race, balls, and a ball sep-
arator cage. The balls and races are hardened
on the surface, precision ground, and polished
to reduce friction. The separator cage keeps the
balls spaced evenly between the two races. The
outer race is held stationary in the crankcase
and the crankshaft end fits through the inner
race. As the crankshaft rotates, it is supported
by the bearings and very little heat is gener-
ated between the rotating crankshaft and the
crankcase.
The crankcase must seal the engine to keep
the air-fuel charge inside and unwanted air out-
side the engine. The term maverick air is used r
to describe unwanted air that enters the engine.
Air that enters through the carburetor picks up
the correct amount of fuel at the correct ratio
for the engine to run correctly. Maverick air that
enters the engine will change the ratio of the
air-fuel mixture. The crankcase must be sealed
around the crankshaft ends, where the crank-
case halves are joined, and where the cylinder is
bolted onto the crankcase.
The crankshaft seals are round inserts
pressed into the crankcase around the crank-
shaft ends to hold in the air-fuel charge when it is
compressed in the crankcase and keep out mav-
erick air when the crankcase is under a vacuum.
They have a pliable lip that seals against the
crankshaft surface, even when the crankshaft is
rotating. Figure 5-10 shows a crankshaft seal
before and after it is installed. The crankshaft
seals must hold in one crankcase pressure pulse
and one vacuum pulse with each revolution of
the crankshaft.
The crankcase itself is typically assembled as
multiple parts called crankcase halves. When
two crankcase halves are bolted together, the
joining surfaces must be sealed. This is done
with a gasket or a sealant. A gasket is a fiber
material formed, cut, or molded to fit between
two surfaces to be joined. Sealant is a com-
pound applied to one of the two mating surfaces
before they are joined to prevent them from
leaking. The engine manufacturer determines
how the crankcase should be sealed. The cylin-
der is bolted to the top of the crankcase. The
mating surface between the cylinder and crank-
case must also be sealed for the engine to oper-
ate properly.
The way the crankcase halves and cylinder
are mated varies from engine to engine. In the
engine shown in Figure 5-11A, two crankcase
halves are joined, the cylinder is bolted down,
and then the engine is inserted into the engine
housing. In the engine shown in Figure 5-11B,
the cylinder and the upper part of the crankcase
are cast together. The lower half of the crank-
case, the engine pan, is bolted to the cylinder.
Figure 5-11C shows an engine design in which
the cylinder is bolted to the top of the crank-
case and a separate cover is bolted onto the back
Outer race Separator cage Balls
Crankcase half Inner race
Goodheart-Willcox Publisher
Figure 5-9. The balls within the ball bearing allow
the inner race to rotate while the outer race
remains stationary. The balls between the races
reduce friction so much that almost no heat is
produced in the bearing.
A B
Seal Flexible lip
Goodheart-Willcox Publisher
Figure 5-10. Crankshaft seal. A—The flexible
inner lip of this uninstalled seal is clearly visible.
B—When the seal is installed, the flexible inner
lip seals against the crankshaft while allowing the
crankshaft to rotate.