1. Pistons

2. Piston
 A piston is a cylindrical engine component that slides
back and forth in the cylinder bore by forces produced
during the combustion process.
 The piston acts as a movable end of the combustion
chamber.
 The stationary end of the combustion chamber is the
cylinder head

3. Piston features
 Piston head,
 Piston pin bore,
 Piston pin,
 Skirt,
 Ring grooves,
 Ring lands, and
 Piston rings

5.  The piston head is the top surface (closest to the cylinder
head) of the piston which is subjected to tremendous forces and
heat during normal engine operation.
 A piston pin bore is a through hole in the side of the piston
perpendicular to piston travel that receives the piston pin.
 A piston pin is a hollow shaft that connects the small end of
the connecting rod to the piston.
 The skirt of a piston is the portion of the piston closest to the
crankshaft that helps align the piston as it moves in the cylinder
bore.

6.  A ring groove is a recessed area located around the
perimeter of the piston that is used to retain a piston ring.
 Ring lands are the two parallel surfaces of the ring
groove which function as the sealing surface for the piston
ring.
 A piston ring is an expandable split ring used to
provide a seal between the piston an the cylinder wall.

7. Piston Design Considerations
 Pistons must:
◦ Contain cylinder pressure
◦ Transmit the pressure created by
combustion to force on the connecting rod
◦ Provide a place for oil control and
compression rings to be located

8.  Aluminum expands when heated, and
 proper clearance must be provided to maintain
free piston movement in the cylinder bore.
 Insufficient clearance can cause the piston to seize
in the cylinder.
 Excessive clearance can cause a loss of
compression and an increase in piston noise

9. Piston Design Considerations
 Pistons must:
◦ Be rigid enough to not deform under the
tremendous pressures and forces
encountered
◦ Be ductile enough to absorb pressure
peaks and not shatter
◦ Retain the proper shape under the
extreme temperatures encountered.

10. Piston Construction
 Materials
◦ Cast iron (used in very old engines)
◦ Cast aluminum (most common)
◦ Forged aluminum
◦ Hypereutectic alloys (high silicon content
aluminum)
◦ Carbon Graphite (being tested)

11. Piston Construction

12. Piston Construction
 Piston head
◦ Round
◦ Approx. .040 undersized
 Ring grooves
 Ring lands
 Drain holes and slots
 Wrist pin boss

13. Piston Construction
 Reinforcing struts – cast pistons
contain steel struts to control
expansion and aid in wrist pin support
 Lock ring grooves – for full-floating
piston pins
 Piston skirt
◦ Full skirt
◦ Partial skirt

14. Piston Construction
 Cam ground pistons.
◦ Most pistons are cam ground, this means
they are not perfectly round.
◦ The diameter is larger on the non-thrust
sides, because they tend to expand more
as the temperature of the piston
increases.
 Piston taper – pistons may be tapered
from top to bottom

16. Piston Construction
 Piston Pin Offset
◦ The piston pin is offset towards the major
thrust side of the piston to reduce piston
wear, and piston noise
◦ The major thrust side is the side that is
loaded during the power event
◦ The minor thrust side is the side loaded
during the compression event

18. Major/Minor Thrust

19. Piston Direction
Pistons should be positioned so arrow, dot or other marking
faces the front of the engine (except reverse rotation
engines)

20. Piston Head Design
 Pistons may have a dished, flat or
domed design

21. Piston Inspection
 Abnormal skirt wear
 Scoring
 Melted aluminum or pits in the head –
indicates detonation
 Cracks – check ring lands, pin boss,
and skirt closely
 Pin bore – check for scoring and wear