Mazda Training manual — part 235

3 – SHORT BLOCK

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Piston Engine Fundamentals

TC010-05-01S

CRANKSHAFT

The crankshaft is the component that changes the up-and-down motion of
the pistons into rotational motion that drives the wheels of the vehicle. The
crankshaft attaches to the cylinder block supports with main bearing caps,
as shown in Figure 14. When the bearing caps are bolted on, they hold
the crankshaft in place. When the block is machined, main bearing
surfaces are aligned and bored on the crankshaft’s center line (line-bored),
so bearing caps must never be interchanged.

FIGURE 14. Main
bearing caps are
used to attach
the crankshaft to
the cylinder
block.

Cylinder
block

Crankshaft

Main bearing

Thrust
bearing

Main bearing
cap

Main bearing
cap mounting
bolt

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Piston Engine Fundamentals

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FIGURE 15. Main
bearing journals
are used to
mount the
crankshaft.
Connecting rod
journals connect
the pistons to
the crankshaft.

Front end

Oil passage

Connecting
rod journal
(crankpin)

Flywheel end

Main bearing
Journal

Features

Figure 15 shows the important features of a crankshaft. The dashed line
represents the center of the shaft.

















A journal is a smooth, round bearing surface. A main bearing journal is a
surface that mounts on the supports in the crankcase, with an insert
bearing between the journal and the support. The main bearing journals
actually support the crankshaft in the crankcase. In Figure 15, the main
bearing journals are all in a line defined by the center line of the shaft.

The other journals — called connecting rod journals or crankpins — are
offset from the center line of the crankshaft. These journals are used to
attach the connecting rods from the pistons.

The connecting rod journals are positioned on the crankshaft so that the
pistons can be at different points in the combustion cycle as the crankshaft
turns. This spaces out the power strokes from the cylinders as they fire in
turn. The result is a smooth flow of power, rather than a jerky, pulsing
motion.

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Piston Engine Fundamentals

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Construction

Because the crankshaft must handle the tremendous forces of the pistons’
power strokes, it is usually made of heavy, high-strength cast iron or
forged steel for high performance or heavy-duty applications. Some
crankshafts have counter- weights cast opposite the crankpins, as shown
in Figure 16. The counterweights help balance the crankshaft and prevent
vibration during high-speed rotation.

The main bearing journals on a crankshaft are highly polished and are
manufactured to exact roundness so they will rotate properly in the
bearing inserts. Oil passages drilled into the main journals receive oil flow
from the supports in the cylinder block.

Slanted oil passages are drilled from the main journals to the crankpin
journals to lubricate the connecting rod bearings. In addition, one of the
main journals — usually in the middle — is machined with a thrust surface.
This surface rides against a special thrust bearing that controls
front-to-rear movement of the crankshaft.

FIGURE 16.
Counterweights
on the crankshaft
improve balance
and prevent
vibrations.

Crankshaft
counter-
weights

Main bearing
journals

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Piston Engine Fundamentals

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Number of Journals

The crankshaft shown in Figure 16 has five main bearing journals and four crankpin
journals. This is a common design for a 4-cylinder engine. One piston is connected to each
crankpin journal with a connecting rod.

You might expect V-6 and V-8 engines to use longer crankshafts, with more crankpin
journals to connect the additional pistons. However, most V-6 and V-8 engines actually
have shorter crankshafts because they connect two pistons to each crankpin journal.

For example, a V-6 engine might use a crankshaft with three crankpin journals and four
main bearing journals. When the cylinder banks are arranged at 90 degrees from each
other, two pistons on the same crankpin journal will always be a half-stroke apart in the
cycle.

Vibration Damper

Even though the crankshaft is very strong, it must have a certain amount of “give” that
allows it to flex slightly. The downward thrust against the crankpin during the power stroke
actually twists the crankpin slightly. This force is immediately relieved, and the crankpin
returns to its original shape.

At a normal hot idle, this twisting and untwisting may repeat five times every second. When
the vehicle is accelerating under load, the cycle may occur 25 or 30 times per second. The
result of this constant twisting and untwisting is torsional vibration, or flexing of the
crankshaft.

To control torsional vibration, a vibration damper (or harmonic balancer) is usually mounted
on the front end of the crankshaft, often as part of the crankshaft pulley. Figure 17 shows a
typical torsional damper.