Mazda Training manual - part 238

 
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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

TC010-05-01S

 

 

 

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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39

Piston Engine Fundamentals

TC010-05-01S

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

TC010-05-01S

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