Small Engine Parts
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Small Engine Parts Four-Cycle. By Katie Murray, UGA Agriculture Education Edited by Georgia Agriculture Education Curriculum Office May 2006. 1. Crankcase. Engine Block. 2. Cylinder Block. 1. Crankcase. Crankcase Cover. Cylinder Block. 1. Cylinder Bore. Head gasket goes here.

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Small Engine Parts


By Katie Murray, UGA Agriculture Education

Edited by Georgia Agriculture Education Curriculum Office

May 2006

1. Crankcase

Engine block
Engine Block

2. Cylinder Block

1. Crankcase

Crankcase Cover

Cylinder block
Cylinder Block

1. Cylinder Bore

Head gasket goes here

3. Cooling Fins

2. Valves

Cooling Fins-Cooling fins keep heat away from the cylinder block by increasing the surface area of the cylinder block and contacting the already existing cooler air for cooling efficiency.

Spark Plug


Throw-measurement from the center of the crankshaft to the center of the crankpin journal. Determines the stroke of an engine. Throw = ½ stroke.


Crankshaft-converts the linear motion of the piston into rotary motion.

Counterweights-balances the forces of the reciprocating piston and reduces the load on crankshaft bearing journals.

Crankgear-interlocks with the cam gear to turn the crankshaft


Crankpin Journal-attaches the connecting rod to the crankshaft

PTO Journal

Magneto Journal

Bearing Journal




Piston Head

Piston Pin Bore

Piston rings
Piston Rings

1. Compression Ring-seals the combustion chamber from any leakage during the combustion process.

2. Wiper Ring- used to further seal the combustion chamber and to wipe the cylinder wall clean of excess oil.

3. Oil Ring-Wipes excess oil from the cylinder wall during piston movement. Openings in the ring returns the excess oil to the engine block.

Ring Grooves

Ring Lands

Crankpin journal bearing surface

Connecting Rod-transfers motion from the piston to the crankshaft and functions as a lever arm

Piston Pin

Crankpin Journal Bearing Surface


Piston pin Bearing Surface

Rod Cap

Valve Tappets crankshaft and functions as a lever arm

Valve Tappets-Rides on the camshaft and pushes the bottom of the valve stem to open the valve

Cam shaft includes cam gear and cam lobes driven by the crankgear
Cam Shaft- includes cam gear and cam lobes; driven by the crankgear

1. Cam Gear-portion of the camshaft that interlocks with the crankgear.

2. Cam Lobes- egg-shaped protrusion on the camshaft that moves a tappet to open a valve; controls the lift and duration of the opening and closing of the valves

Valve retainer

Valve Spring crankgear

Valve Retainer

Valve Head

Valve Stem

Intake Valve- allows the air-fuel mixture to flow into the cylinder

Exhaust Valve- allows exhaust valves to flow out of the cylinder

Flywheel crankgear

Flywheel Nut

Magneto crankgear

Spark Plug Wire



Air filter
Air Filter crankgear

Carburetor crankgear

Carburetor- provides the proper mixture of air and fuel to the intake valve

Intake manifold
Intake Manifold crankgear

Muffler crankgear

Muffler Guard

Breather Assembly crankgear

Breather Cover

Shields crankgear

Cylinder Shield

Dipstick crankgear

Starter housing
Starter Housing crankgear

Gas tank
Gas Tank crankgear