1. piston/connecting rod assembly
engine block
cylinder
crankshaft
cylinder head l
Pictured here are the main parts of a single cylinder four-stroke engine. On the left is the engine block it is the main body that all the parts are assembled to. At the top center is the cylinder head assembly. This is where the engine valves are located and the manifolds where the carburetor and exhaust attach. At the top right is the crankshaft, which is fastened in the block and rotates in the block on bearings. The reciprocating (up and down) motion of the piston and connecting rod assembly is changed into rotary motion by the crankshaft. Just below the crankshaft in the picture is the camshaft with the lobes or “cams” protruding from the shaft. As it rotates, it causes the valves to open and close with assistance of the valve springs. In the lower center of the picture is the piston and connecting rod assembly. The piston goes up and down in the cylinder, and is attached to the small end of the connecting rod with the piston pin sometimes called the wrist pin. The larger lower end of the connecting rod clamps around the crankshaft.
The operating cycle this type of engine works on is the four stroke cycle. The four stroke cycle has a sequence of four strokes: intake, compression, power and exhaust are the name and order in which they take place. You will learn more about this later in the course.
camshaft
piston/connecting rod assembly

2. Crankpin
crankpin centerline
crank centerline
Throw
Main bearing journal
Main bearing journal
Shown here is the crankshaft, with its offset crankpin where the connecting rod joins to. The name of the finished part on the crankshaft that the connecting rod joins to is the crankpin. The name of the finished part of the crankshaft that the main bearings support the crank in the cylinder block are called main bearing journals. Notice both main bearing journals are on the same centerline. As the crankshaft rotates on this centerline, the crankpin rotation causes the connecting rod and piston to go up and down in the cylinder. In actual fact it is the piston and connecting rod that cause the crankshaft to rotate. This is where that power created in the motion of the piston is converted to rotary motion in the crankshaft. Note the distance between the crankshaft centre line and the crankpin centre line is known as the crank throw. The throw equals ½ the stroke.

3. connecting rod bolts and oil
compression rings
oil control rings
piston pin
connecting rod
piston
Here the piston, ring set, connecting rod and cap, piston pin and circlip, connecting rod bolts and oil slinger tab are shown. Together, these parts are known as the piston and rod assembly. The piston is attached to the connecting rod by the piston pin, also known as a wrist pin. It is held in place by circlips or is a shrink fit. The piston can pivot about this pin, allowing the large bottom end of the connecting rod to travel at an angle. This is important as it is fastened to the crankpin and must rotate to compensate for connecting rod angle as the crank rotates. The connecting rod cap can be removed, allowing the separation of these parts for service and removal. Notice the three thin rings at the left. These are called piston rings. The compression rings are located at the top of the piston and prevent the gases under pressure above the piston from escaping past the piston which would reduce compression and power. The oil control ring consists of three parts and their job is to prevent excessive oil from remaining on the cylinder wall, leading to excessive oil consumption.
piston pin circlip
connecting rod bolts and oil
slinger
connecting rod cap

4. piston/connecting rod assembly
flywheel
crankshaft timing gear
crankshaft
Here we have the crankshaft attached to the connecting rod and piston. The flywheel is also in position. Notice the large counterweights opposite the piston/connecting rod assembly. These are to balance the offset mass of the piston/connecting rod assembly as it spins, in order to balance crankshaft and the running of the engine. Also shown is the small crankshaft timing gear, which turns the camshaft is shown at the right side of the shaft.
Crankshaft
counterweight

5. camshaft gear
camshaft
camshaft lobe
Here the camshaft is shown with its two lobes. Driven by the small crankshaft gear, the camshaft gear is exactly twice the crankshaft gear size, making it spin at half the crankshaft speed e.g., if the crankshaft turns 1 revolution (360 degrees) the camshaft will rotate ½ revolution (180 degrees). The lobes are egg shaped and have valve lifters that sit on top of them. As the camshaft rotates the lifters are raised or lowered by lobe of the cam. The lifters cause push rods to act on the rocker arms that push the valves open. When the cam rotates to lower part of its lobe the valve spring forces the valve shut tight against its seat.

6. crankshaft
piston
piston rings
The piston is shown resting in the cylinder. It must be installed with a ring compressor and plenty of lubricant. Notice the piston rings are larger than the cylinder bore. When squeezed with the ring compressor, they go into the piston ring grooves that are cut into the piston, allowing the assembly to slide right into the cylinder.
camshaft gear
engine block

7. crankshaft gear
camshaft gear
crankshaft
This picture shows the relationship of the camshaft with its large gear and the crankshaft with its small gear. It is important that these two gears be timed to each other as the timing of the valve opening is relative to the position of the crankshaft. Both gears have some form of timing marks on them. Always check manufactures specifications for timing.
camshaft

8. spark plug location
exhaust valve
cooling fins
intake valve
The cylinder head fits atop the engine block and contains the combustion chamber where the air-fuel mixture is ignited. The spark plug is put into the cylinder head as well as the intake and exhaust valves and their sub assemblies. The intake valve is normally larger than the exhaust valve. The intake valve allows the fresh air-fuel mixture into the combustion chamber and the exhaust valve allows the exit of the spent, or burnt gases after combustion. The cylinder head is sealed to the block using a cylinder head gasket. This is a thin composite gasket that can withstand the great heat and pressure that engines develop. The carburetor and muffler are attached to the cylinder head on some small engines. The fins on the aluminum head are to help cool the engine.
combustion chamber
head gasket

9. valve spring
engine block
This view shows where the cylinder head fits onto the block, one of the valve springs and one rocker arm. The lower portion of the cylinder block assembly is known as the crankcase. This is the area that the crankshaft is located.
crankcase
rocker arm
cylinder head

10. bore for crankshaft mounting
cooling fins
magnet
flywheel
The flywheel is a heavy wheel fastened to the end of the crankshaft. It is uses interia to keep the momentum of the crankshaft going between power strokes and smooth out the power pulses, inherent in four cycle engines. Small engines also mount magnets in the flywheel in order to provide a magnetic field for the ignition system (magneto ignition). The ignition system provides the spark that ignites the air-fuel mixture in the engine at the end of the compression stroke. Notice the black fins which help blow air across the cylinder and cylinder head to cool the engine, hence the name ”air-cooled” engine.
bore for crankshaft mounting