fuel, which results in expanding gas inside the cylinder.
The piston transfers the pressure of combustion to the
piston pin, connecting rod, and crankshaft. It also holds
the piston rings and piston pin.
During engine operation, the piston slides up and
down in the cylinder at tremendous speeds. At a vehicle
speed of about 55 mph (88 km/h), the piston can accel-
erate from zero to 60 miles an hour and then back to
zero in one movement from top to bottom in the
cylinder. This places tremendous stress on the piston and
its related parts.
Piston Rings
The piston rings fit into grooves machined into the
sides of the piston. These rings keep combustion pressure
from entering the crankcase and engine oil from entering
the combustion chamber. Look at Figure 1-4.
The compression rings seal the clearance between
the block and piston. They are normally the two upper
piston rings. Their job is to contain the pressure formed in
the combustion chamber, Figure 1-5. Without compres-
sion rings, pressure would blow past the outside diameter
of the piston and into the lower area of the engine block.
The oil ring fits into the lowest groove in the piston. It
is designed to scrape excess oil from the cylinder wall to
keep it from being burned in the combustion chamber,
Figure 1-6. If oil enters the area above the piston and
burns, blue smoke blows out of the tailpipe.
Piston Pin
A piston pin, also called a wrist pin, allows the con-
necting rod to swing back and forth inside the piston. The
pin fits through a hole machined in the piston and through
a hole in the upper end of the connecting rod. Refer to
Figure 1-7.
12 Auto Engine Repair
Piston
clearance
Piston
clearance
Piston
diameter
Compression
rings
Oil ring
Piston pin
Figure 1-4. The clearance between the piston and cylinder
allows the piston to move freely in the cylinder. Rings seal the
clearance.
Combustion
pressure
Compression
ring
Ring
tension
Cylinder
wall
Piston
Figure 1-5. The compression rings use combustion pressure to
help seal against the cylinder wall. This keeps pressure in the
combustion chamber and out of the crankcase.
Cylinder wall
Piston
Oil control
ring
Escaping
oil
Oil to crankcase
Oil film
Ring
groove
Figure 1-6. Oil rings act as a scraper to keep oil out of the com-
bustion chamber. (Deere & Co.)
Piston
head
Piston
Ringlands
Piston
pin
Piston
pin bore
Connecting
rod
Swinging action
Figure 1-7. The piston pin fits into a hole bored in the piston.
The pin attaches the piston to the connecting rod.