1. Small Engine Parts Four-Cycle
By Katie Murray, UGA Agriculture Education
Edited by Georgia Agriculture Education Curriculum Office
May 2006
1. Crankcase
Revised May 2007

2. Engine Block
2. Cylinder Block
1. Crankcase
Crankcase Cover

3. Cylinder Block 1. Cylinder Bore 3. Cooling Fins 2. Valves
Head gasket goes here
3. Cooling Fins
2. Valves

4. Spark Plug
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.

5. Crankshaft
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

6. Crankshaft Magneto Journal PTO Journal Bearing Journal
Crankpin Journal-attaches the connecting rod to the crankshaft
Magneto Journal
PTO Journal
Bearing Journal

7. Piston
Rings
Skirt
Piston Head
Piston Pin Bore

8. 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.

9. Ring Grooves
Ring Lands

10. Crankpin Journal Bearing Surface
Connecting Rod-transfers motion from the piston to the crankshaft and functions as a lever arm
Piston Pin
Piston
Piston pin Bearing Surface
Crankpin Journal Bearing Surface
Rod Cap

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

12. 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
Timing Mark

13. Intake Valve- allows the air-fuel mixture to flow into the cylinder
Valve Spring
Valve Retainer
Valve Stem
Valve Head
Intake Valve- allows the air-fuel mixture to flow into the cylinder
Exhaust Valve- allows exhaust valves to flow out of the cylinder

14. Flywheel
Flywheel Nut

15. Magneto
Spark Plug Wire
Armature
Magneto

16. Air Filter

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

18. Intake Manifold

19. Muffler
Muffler Guard

20. Breather Assembly
Breather Cover

21. Shields
Cylinder Shield

22. Dipstick

23. Starter Housing

24. Gas Tank

25. Blower Housing Cover