Mazda Training manual — part 234

3 – SHORT BLOCK

21

Piston Engine Fundamentals

TC010-05-01S

The short block includes the cylinder block, crankshaft, bearings, connecting rods, and
pistons. (A partial engine, or long block, is a short block plus cylinder head(s) and cover(s),
timing belt and covers, and oil pan.) A low mileage engine with excessive oil consumption or
lower end knocking is usually replaced with a short block. This section describes how each
part of the short block works.



OBJECTIVES

After completing this section, you will be able to:

•

Identify the important parts of the cylinder block and describe their functions.

•

Identify the important parts of the crankshaft and describe their functions.

•

Describe how main bearings secure and lubricate the crankshaft.

•

Describe how the piston is attached to the crankshaft.

•

Identify important parts of the piston and describe their functions.

•

Describe how pistons are constructed to overcome the problems of heat expansion.

•

Identify the piston rings and describe their functions.

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Piston Engine Fundamentals

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FIGURE 12. A
short block is a
cylinder block
fitted with a
complete crank-
shaft assembly.

Cylinder
block

Piston

Connecting
rod

Crankshaft

OVERVIEW

The major parts of the short block are shown in Figure 12.




















The engine’s cylinders are bored in the cylinder block, which is a large
casting made of iron or aluminum. The combustion chambers are the
spaces where the air-fuel mixture is compressed and burned. Usually, the
combustion chambers are formed in the cylinder head, although some
engines have chambers in the tops of the pistons.

The pistons are fitted closely inside the cylinders, where they form the
“floor” of the combustion chambers. The pistons are linked to the
crankshaft by connecting rods and bearings. The crankshaft is mounted in
bearings in the cylinder block.

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Piston Engine Fundamentals

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CYLINDER BLOCK

Figure 13 shows a typical cylinder block for a 4-cylinder in-line engine.

The cylinder block is the main supporting member of the engine. Almost
every other engine component is either connected to or supported by the
cylinder block. The pistons, connecting rods, and crankshaft work inside the
cylinder block.

The cylinder block contains the cylinders, internal passages for coolant and
oil, and mounting surfaces for attaching engine accessories, such as the oil
filter and water pump. The cylinder head is mounted on top of the cylinder
block, and the oil pan is mounted on the bottom.

FIGURE 13. The
cylinder block is
the engine’ s
main supporting
part.

Cylinder
bores

Cylinder head
mounting
surface

Crankshaft
Opening

Front

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Piston Engine Fundamentals

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Construction

Cylinder blocks are usually made of cast iron or aluminum alloy. Cast iron cylinder liners
are cast into aluminum alloy blocks. These materials can easily be machined for the
cylinder bores and other mating surfaces. Coolant and oil passages are cast into the block
when it is manufactured. The block is heavy and rigid, so it can with- stand the vibration
and heat stress of engine operation.

The cylinder bores are carefully machined and angled correctly to match the crankshaft.
The block material is designed to handle the wear that occurs in the cylinders as the
pistons move up and down.

Crankcase

The crankcase contains and supports the crankshaft and main bearings. The bottom of
the cylinder block forms the upper part of the crankcase. The oil pan attached to the
bottom of the cylinder block forms the bottom part of the crankcase.

Crankshaft Main Supports and Bearings

The crankcase includes several support surfaces for the crankshaft. The number of
supports varies, depending on the length of the crankshaft. For example, a four-cylinder
engine will usually have five of these support surfaces.

The crankshaft mounts on insert bearings that are installed on the support surfaces and
attached with bearing caps. The supports have oil passages that lubricate the crankshaft
as it spins against the bearings. These passages align with oil holes in the bearings. The
cylinder block also includes a groove for the rear main oil seal, which keeps the oil from
leaking out at the rear of the crankshaft and engine.

In automotive technical vocabulary, the term main refers to bearings, seals, and other
mounting hardware for the crankshaft itself. Main distinguishes these mounting parts from
other mounting parts that connect to the crankshaft, such as connecting rod bearings.