3 – SHORT BLOCK
41
Piston Engine Fundamentals
TC010-05-01S
Oil Rings
Oil rings help lubricate the cylinder walls and control the flow of oil. Oil is
constantly sprayed or splashed onto the walls of the cylinders, where it
keeps the metal parts from wearing against each other. Oil also helps cool
the cylinder, and it washes carbon and dirt particles from the cylinder
walls.
So much oil is on the walls of the cylinders that it won’t all fit into the small
clearance next to the piston when the compression rings scrape the walls.
To provide a place for the extra oil to go, the oil ring is usually segmented
— made of three pieces — as shown in Figure 28.
As the oil is scraped, it flows behind the expander ring to holes in the ring
groove. These holes direct the oil to the open space inside the piston skirt,
where it eventually drains back into the crankcase.
The expander ring is a steel ring installed between the two oil rings. These
oil rings are made of more flexible material than compression rings. The
expander ring holds the oil rings tightly against the cylinder wall, improving
their ability to scrape the oil.
FIGURE 28. Oil
rings are
usually made of
three pieces, or
segments.
Oil rings
Expander
ring