Mazda Training manual — part 238

3 – SHORT BLOCK

37

Piston Engine Fundamentals

TC010-05-01S

Construction

Pistons are usually made of aluminum alloys, which are lighter than cast
iron or iron-steel alloys. While lighter weight is an advantage, aluminum
alloys also tend to expand more with heat. To overcome this problem,
some pistons have a steel strut cast into the bottom of the piston head.
This strut helps control expansion.

Clearance

Although the piston fits closely in the cylinder bore, it does not seal the
combustion chamber. The compression seal is made by the piston rings
installed in grooves near the top of the piston. To allow room for the piston
rings and lubricating oil, a clearance of a few thousandths of an inch must
be maintained between the outside edge of the piston and the cylinder
wall.

This clearance lets lubricating oil into the upper part of the cylinder. The
clearance also prevents the engine from seizing if one of the pistons
expands too much from overheating.

To maintain a consistent clearance from the top to the bottom of the
cylinder, the piston usually has a slightly tapered shape, as shown in
Figure 24. The top diameter of the piston is slightly smaller than the
bottom diameter when the piston is cold. When the engine operates, the
top of the piston gets much hotter than the bottom, and the expansion at
the top evens up the diameter.

FIGURE 24. The
upper part of the
piston gets very
hot, and its
smaller diameter
expands to
maintain
consistent clear-
ance from top to
bottom.

Top
diameter
(smaller)

Bottom
Diameter
(larger)

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Piston Engine Fundamentals

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FIGURE 25. The
oval shape of a
cam-ground
piston allows
different parts of
the piston to
expand at
different rates.
When the piston
heats up, it
becomes round.

Piston pin
bore

Large
diameter.

Small
Diameter

Another technique used to make the piston fit better in the cylinder and
control heat expansion is called cam grinding. Cam-ground pistons are
slightly oval shaped, as shown in Figure 25.
















Surfaces of the piston expand at different rates because they are made of
different materials. For example, the area around the piston pin bore is
made of denser material to maintain strength. Therefore, this area
expands more when it is heated. The oval shape of the piston allows for
this expansion. As the piston heats up and expands, its shape becomes a
perfect circle.

If the piston clearance becomes too large, the piston can rock in the
cylinder, striking the cylinder wall loud enough to be heard. This condition
is called piston slap. Piston slap usually occurs in older, high mileage
engines with worn cylinders.

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Piston Engine Fundamentals

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PISTON RINGS

The piston rings seal the combustion chamber, forming a closed, sealed
space where the air-fuel mixture is ignited. In addition to sealing the
combustion chamber, the piston rings also:

•

Scrape oil from the cylinder walls so it doesn’t get into the upper

cylinder and burn.

•

Carry heat from the piston to the cylinder walls.

Figure 26 shows the rings on a typical piston. The top two rings are
called compression rings. They are made of cast iron with chrome plating,
and come in various shapes. The bottom oil ring is made of two separate
scuff rings and an expander.

FIGURE 26. A
typical piston
has two
compres- sion
rings on top and
an oil ring on the
bottom.

Top
compression
ring

Second
compression
ring

Scuff rings
on oil ring

Expander on
oil ring

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Piston Engine Fundamentals

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FIGURE 27.
Compression
rings scrape the
cylinder wall
and seal the
combustion
chamber.

Compression
rings scrape
cylinder wall

Compression
rings skim
over oil film

Compression Rings

Compression rings seal, scrape, and cool the cylinder, as shown in Figure
27. Compression rings are designed to twist slightly when they are not
under pressure. The top edge of the ring will be twisted away from the
cylinder wall, while the bottom edge presses against the wall. So while the
piston is moving down during the intake stroke (View A), the lower edge of
the compression rings scrape off any oil that was missed by the oil ring.
















On the compression and exhaust strokes (View B), the twist lets the
compression rings skim over the oil film so the oil is not pushed into the
combustion chamber.

During the power stroke (View A), the combustion pressure on the rings
forces them to untwist, so both the top and bottom edges of the rings
contact the cylinder wall. This creates a tighter seal for the combustion
chamber. The untwisted rings also create a path for heat to flow from the
piston to the cylinder wall.

View A Intake or

power stroke

View B Compression and

exhaust strokes