An ANCHORED, or fixed, piston pin is locked
into the piston pin bosses by a screw. The rod
pivots freely on the connecting rod, which is
fitted with a bronze bushing.
A SEMIFLOATING pin is locked to the
connecting rod by a screw or by friction. The pin
pivots freely in the piston pin bosses.
The FULL-FLOATING piston pin pivots freely
in the connecting rod and piston pin bosses. The
outer ends of the piston pins are fitted with lock
rings to keep the pin from sliding out and
contacting the cylinder walls.
Piston rings serve three important functions (fig.
3-31). They provide a seal between the piston and the
cylinder wall to keep the force of the exploding gases
from leaking into the crankcase from the combustion
chamber. Blow-by is detrimental to engine
performance because the force of the exploding gases
merely bypasses the piston, rather than push down on it.
It also contains the lubricating oil. They keep the
lubricating oil from passing the piston and getting into
the combustion chamber from the crankcase. Also, they
provide a solid bridge to conduct heat from the piston to
the cylinder wall. About one third of the heat absorbed
by the piston passes to the cylinder wall through the
Piston rings are secured to the piston by fitting into
grooves. They are split to allow for installation and
expansion, and they exert pressure on the cylinder walls
when installed. They fit into grooves that are cut into the
piston and are allowed to float freely in these grooves. A
piston ring that is formed properly, working in a
cylinder that is within limits for roundness and size,
exerts an even pressure and a solid contact with the
cylinder wall around the entire circumference. There
are two basic classifications of piston rings. The
COMPRESSION RING (fig. 3-32) that seals the force
of the exploding mixture into the combustion chamber
and the OIL CONTROL RING (fig. 3-32) that keeps
engine lubricating oil from getting into the combustion
chamber. These rings are arranged on the piston in three
basic configurations (fig. 3-33). They are as follows:
The three-ring piston has two compression rings
from the top, followed by one oil control
ringthe most common configuration.
Figure 3-31.Purpose of piston rings.