A piston is a cylindrical component that slides back and forth in the engine cylinder. It has a head at the top that is subjected to heat and pressure, and a skirt at the bottom. Piston features include ring grooves that hold piston rings to seal against the cylinder wall. Pistons must withstand pressure and heat while maintaining proper clearance in the cylinder. They are usually made of aluminum alloys and have features like struts and a cam ground shape to control expansion under operating conditions.

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