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Two-Stroke Engines
Copyright Goodheart-Willcox Co., Inc.
Spark Plug Hole
The spark plug hole is a threaded hole in which
the spark plug is installed. Engine manufactur-
ers select the spark plug for an engine based on
the engine’s performance requirements. The size
of the engine, the displacement of the engine, and
even how a typical operator will run the engine
will determine which spark plug should be used
in the engine. Standard spark plug thread diam-
eters could be 10 mm, 12 mm, 14 mm, or 18 mm.
The spark plug hole will be threaded for the plug
that the manufacturer selected for the engine.
Some two-stroke engines must use a specific
spark plug in order for the engine to run cor-
rectly and last a long time. Even though other
spark plugs may fit into the spark plug hole, only
the manufacturer’s recommended spark plug
should be used.
Transfer Ports
The transfer ports are designed to provide a pas-
sage to move the air-fuel charge from the crank-
case to the area in the cylinder above the top of
the piston. The size and number of the transfer
ports are engineered for the displacement and
power requirements of the engine. The configu-
ration of the walls and exit points of the trans-
fer ports are engineered to help the engine meet
EPA emissions standards.
Common to all two-stroke engines are the
cooling fins, the cylinder bore, the exhaust port,
the spark plug hole, and the transfer ports.
Depending on the engine design, the cylinder
could contain any of the following components:
• Intake port.
• Impulse port.
• Decompression valve.
• Stratified scavenging port.
Intake Port
The intake port will be found on the cylinder
of a piston ported engine. Even though the car-
buretor is mounted on the cylinder, the air-fuel
charge is delivered to the crankcase before being
transferred to the area above the piston.
Impulse port
Intake port
Transfer port
Exhaust port
Cylinder bore
Cooling fins
Spark plug hole
Goodheart-Willcox Publisher
Figure 5-6. The basic parts of a two-stroke cylinder include the cooling fins, the exhaust port, the spark
plug hole, and the transfer ports.