Mazda Training manual — part 237

3 – SHORT BLOCK

33

Piston Engine Fundamentals

TC010-05-01S

5. The gap between the main bearings and the main journals is called
_____________________ .

6. The _____________________ half of the crankshaft main bearing has oil holes and fits
into the ___________________ .

Match the numbered items on the crankshaft drawing to the definitions below.

7. _______

A.

main journal

8. _______

B.

oil passages

9. _______

C.

crankpin

10. Which of these symptoms would you expect to find in an engine that has worn main
bearings? More than one answer may be correct.
A. low oil pressure
B. leaking gasoline
C. hard starting
D. low rattle noise from engine

3 – SHORT BLOCK

34

Piston Engine Fundamentals

TC010-05-01S

FIGURE 21. The
connecting rod is
attached to the
piston with a
piston pin.

Piston pin

Bushing

Connecting
rod

Connecting
rod bearing
cap

Upper
connecting
rod bearing

Lower
connecting
rod bearing

Oil holes

Bearing cap
nut

Bearing cap
bolt

CONNECTING RODS

The connecting rod transfers the movement of the piston to the crankpin
on the crankshaft. Figure 21 shows the connecting rod and its related
parts.


























A steel piston pin (commonly called the wrist pin) connects the piston to
the rod. The pin slips through the piston and through the bushing (a
circular, sleeve-type bearing) in the connecting rod. The pin must be free
to turn either in the piston or the connecting rod so the rod can rock back
and forth as the crankshaft turns. In some engines, the pin turns freely in
both the piston and the connecting rod. Some pins have retainers at both
ends to keep the pins from sliding out.

The large end of the connecting rod is connected to the crankshaft with a
connecting rod bearing cap.

This cap is very similar to the main bearing

caps, and the clearances are equally important.

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35

Piston Engine Fundamentals

TC010-05-01S

Construction

Connecting rods must be very strong and rigid to handle the force of the
piston on the power stroke. Connecting rods are usually made of
high-strength steel, with an “I” construction for extra rigidity. The rods are
kept as light as possible, and weight variation among the rods is carefully
controlled so the engine will be balanced.

Like the main bearings, the connecting rod bearing surfaces are
line-bored. They must be reinstalled in the same positions from which
they were removed. If they are not, it can cause premature and
uneven wear.

Usually, both the rod and cap are stamped to ensure

proper reassembly.

Cylinder Wall Lubrication

As shown in Figure 22, an oil jet in the connecting rod lubricates the
cylinder walls and cools the piston. The crankshaft oil passages deliver oil
to the connecting rod journals. When the bearing holes match up with the
oil hole in the connecting rod journal, pressurized oil is squirted through
the oil jet.

Answers to
Review
Exercise 3

1. cylinder

block

2. main bearing

caps

3. crankshaft

4. thrust

bearing

5. bearing

clearance

6. upper;

cylinder
block’s main
support

7. B

8. C

9. A

10. A and D

FIGURE 22. The
oil jet on the
connecting rod
squirts oil onto
the cylinder wall.

Connecting
rod

Oil holes

Upper
connecting
rod bearing

Lower
connecting
rod bearing

Oil jet

3 – SHORT BLOCK

36

Piston Engine Fundamentals

TC010-05-01S

FIGURE 23. The
piston forms the
bottom of the
combustion
chamber.

Head

Piston pin
bore

Piston ring
grooves

Thrust
surface

“ Front”
mark

Center of
piston

Center of
piston pin
bore

Offset

Skirt

PISTONS

The piston forms the bottom of the combustion chamber in the cylinder. It
transfers the power created by the burning air-fuel mixture to the
crankshaft. Figure 23 shows a typical piston.



















The top surface of the piston is called the head, or crown. The upper part
of the piston contains several grooves where the piston rings fit. The
lower part of the piston, under the rings, is called the skirt. Thrust
surfaces

on the skirt guide the piston in the cylinder bore and prevent the

piston from rocking back and forth in the cylinder. Most pistons have a
mark on one side that identifies the side of the piston that faces the front
of the engine.

The piston pin bore is drilled through the piston. The piston pin is
inserted through this bore to attach the piston to the connecting rod. In
some piston designs, the pin bore is offset slightly from the center of the
piston. The offset — usually no more than a few thousandths of an inch
or 0.5-2 millimeters — helps keep the piston from “slapping” against the
cylinder walls during the power stroke.