Engine Block | Engine Block-Construction | Structure of Engine Block | Engine Block Design

The engine block, which is the supporting structure for the entire engine, is made from one of the following:

■ Gray cast iron

■ Cast aluminum

■ Die-cast aluminum alloy

Cast iron contains about 3% carbon (graphite), which makes it gray in color. Steel is iron with most of the carbon removed. The carbon in cast iron makes it hard but brittle. Cast iron is used to make engine blocks and cylinder heads for the following reasons.

■ The carbon in the cast iron allows for easy machining, often without coolant.

■ The graphite in the cast iron also has lubricating properties.

■ Cast iron is strong for its weight and usually is magnetic.

The liquid cast iron is poured into a mold made from either sand or Styrofoam. All other engine parts are mounted on or in the block. This large casting supports the crankshaft and camshaft (on OHV engines) and holds all the parts in alignment. Newer blocks use thinner walls to reduce weight. Blocks are often of the mono block design, which means that the cylinder, water jacket, main bearing supports (saddles), and oil passages are all cast as one structure for strength and quietness. Large-diameter holes in the block casting form the cylinders to guide the pistons. The cylinder holes are called bores because they are made by a machining process called boring. ● SEE FIGURE.1.

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Fig.1 The cylinder block usually extends from the oil pan rails at the bottom to the deck surface at the top.

Combustion pressure loads are carried from the head to the crankshaft bearings through the block structure. The block has webs, walls, and drilled passages to contain the coolant and lubricating oil and to keep them separated from each other. Mounting pads or lugs on the block transfer the engine torque reaction to the vehicle frame through attached engine mounts. A large mounting surface at the rear of the engine block is used for fastening a bell housing or transmission.

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Fig.2 An expansion (core) plug is used to block the opening in the cylinder head or block the holes where the core sand was removed after the part was cast.

The cylinder head(s) and other components attach to the block. The joints between the components are sealed using gaskets or sealants. Gaskets or sealants are used in the joints to take up differences that are created by machining irregularities and that result from different pressures and temperatures.

Block Manufacturing

Cast-iron cylinder block sand casting technology continues to be improved. The trend is to make blocks with larger cores, using fewer individual pieces. Oil-sand cores are forms that shape the internal openings and passages in the engine block. Before casting, the cores are supported within a core box. The core box also has a liner to shape the outside of the block. Special alloy cast iron is poured into the box. It flows between the cores and the core box liner. As the cast iron cools, the core breaks up. When the cast iron has hardened, it is removed from the core box, and the pieces of sand core are removed through the openings in the block by vigorously shaking the casting. These openings in the block are plugged with core plugs. Core plugs are also called freeze plugs or frost plugs. Although the name seems to mean that the plugs would be pushed outward if the coolant in the passages were to freeze, they seldom work this way. ● SEE FIGURE 2.

One way to keep the engine weight as low as possible is to make the block with minimum wall thickness. The cast iron used with thin-wall casting techniques has higher nickel content and is harder than the cast iron previously used. Engine designers have used foundry techniques to make engines lightweight by making the cast-iron block walls and bulkheads only as heavy as necessary to support their required loads.

Aluminum Blocks

Aluminum is used for some cylinder blocks and is nonmagnetic and lightweight. Styrofoam is often used as a core when casting an aluminum block. The Styrofoam vaporizes as soon as the molten aluminum meets the foam leaving behind a cavity where the aluminum flows. ● SEE FIGURE 3.  Aluminum block engines usually require cast-iron cylinder walls for proper wear and longevity. Aluminum blocks may have one of several different types of cylinder walls.

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Fig.3 Styrofoam casting mold used to make the five cylinder engine blocks for the Chevrolet Colorado and the Hummer H3. The brown lines are glue used to hold the various parts together. Sand is packed around the mold and molten aluminum is poured into the sand which instantly vaporizes the Styrofoam. The aluminum then flows and fills the area of the mold.

■ Most cast-aluminum blocks have cast-iron cylinder sleeves (liners) such as Saturn, Northstar, and Ford modular V-8s and V-6s. The cast-iron cylinder sleeves are either cast into the aluminum block during manufacturing or pressed into the aluminum block. These sleeves are not in contact with the coolant passages and are called dry cylinder sleeves. ● SEE FIGURE 4.

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Fig.4 Cast-iron dry sleeves are used in aluminum blocks to provide a hard surface for the rings.

■ Another aluminum block design has the block die cast from silicon-aluminum alloy with no cylinder liners. Pistons with zinc-copper-hard iron coatings are used in these aluminum bores (in some Porsche engines).

■ Some engines have die-cast aluminum blocks with replaceable cast-iron cylinder sleeves. The sleeves are sealed at the block deck and at their base. Coolant flows around the cylinder sleeve, so this type of sleeve is called a wet cylinder sleeve (in Cadillac 4.1, 4.5, and 4.9 liter V-8 engines). ● SEE FIGURE 5. Cast-iron main bearing caps are used with aluminum blocks to give the required strength.

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Fig.5 A dry sleeve is supported by the surrounding cylinder block. A wet sleeve must be thicker to be able to withstand combustion pressures without total support from the block.

Bedplate Design Blocks

A bedplate is a structural member that attaches to the bottom of the block and supports the crankshaft. The oil pan is mounted under the bedplate which in most cases is also part of the structure and support for the block assembly. ● SEE FIGURE 6.

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Fig.6 A bedplate is a structural part of the engine which is attached between the block and the oil pan and supports the crankshaft.

Casting Numbers

Whenever an engine part such as a block is cast, a number is put into the mold to identify the casting. These casting numbers can be used to check dimensions, such as the cubic inch displacement, and other information, such as year of manufacture. Sometimes changes are made to the mold, yet the casting number is not changed. Most often the casting number is the best piece of identifying information that the service technician can use. ● SEE FIGURE 7.

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Fig.7 Casting numbers identify the block.

Block Deck

The cylinder head is fastened to the top surface of the block. This surface is called the block deck. The deck has a smooth surface to seal against the head gasket. Bolt holes are positioned around the cylinders to form an even holding pattern. Four, five, or six head bolts are used around each cylinder in automobile engines. These bolt holes go into reinforced areas within the block that carry the combustion pressure load to the main bearing bulkheads. Additional holes in the block are used to transfer coolant and oil, as seen in ● FIGURE 8.

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Fig.8 The deck is the machined top surface of the block.

Cooling Passages

Cylinders are surrounded by cooling passages. These coolant passages around the cylinders are often called the cooling jacket. In most cylinder designs, the cooling passages extend nearly to the bottom of the cylinder. In some engine blocks where the block ends at the centerline of the crankshaft, the cooling passages are limited to the upper portion of the cylinder.

Some engines are built with Siamese cylinder bores where the cylinder walls are cast together without a water jacket (passage) between the cylinders. While this design improves the strength of the block and adds stability to the cylinder bores, it can reduce the cooling around the cylinders. ● FIGURE 9 is a typical V-8 engine cutaway that shows the coolant jackets and some of the lubrication holes.

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Fig.9 Cutaway of a Chevrolet V-8 block showing all of the internal passages.

Lubricating Passages

An engine block has many oil holes that carry lubricating oil to the required locations. During manufacture, all oil holes, called the oil gallery, are drilled from outside the block. When a curved passage is needed, intersecting straight drilled holes are used. In some engines, plugs are placed in the oil holes to direct oil to another point before it comes back to the original hole, on the opposite side of the plug. After oil holes are drilled, the unneeded open ends may be capped by pipe plugs, steel balls, or cup-type soft plugs, often called oil gallery plugs. These end plugs in the oil passages can be a source of oil leakage in operating engines. ● SEE FIGURE 10.

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Fig.10 Typical oil gallery plugs on the rear of a Chevrolet small block V-8 engine.

Main Bearing Caps

The main bearing caps are cast or manufactured from sintered or billeted materials, separately from the block.

■ They are machined and then installed on the block for a final bore finishing operation.

■ With caps installed, the main bearing bores and cam bearing bores (on OHV engines) are machined to the correct size and alignment. On some engines, these bores are honed to a very fine finish and exact size.

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Fig.11 Two-bolt main bearing caps provide adequate bottom end strength for most engines.

■ Main bearing caps are not interchangeable or reversible, because they are individually finished in place.

■ Main bearing caps may have cast numbers indicating their position on the block. If not, they should be marked with numbers and arrows pointing toward the front of the engine.

Standard production engines usually use two bolts to hold the main bearing cap in place. ● SEE FIGURE 11.

Heavy-duty and high-performance engines often use additional main bearing support bolts. A four-bolt, and even six-bolt, main cap can be of a cross-bolted design in a deep skirt block or of a parallel design in a shallow skirt block. ● SEE FIGURES 12 AND 13.

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Fig.12 High-performance and truck engines often use four-bolt main bearing caps for greater durability.

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Fig.13 Some engines add to the strength of a four- bolt main bearing cap by also using cross bolts through the bolt on the sides of the main bearing caps.

Expansion force of the combustion chamber gases will try to push the head off the top and the crankshaft off the bottom of the block. The engine is held together with the head bolts and main bearing cap bolts screwed into bolt bosses and ribs in the block.

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Fig.14 A girdle is used to tie all of the main bearing caps together.

The extra bolts on the main bearing cap help to support the crankshaft when there are high combustion pressures and mechanical loads, especially during high engine speed operation. Many engines use a girdle which ties all of the main bearing caps together to add strength to the lower part of the block. ● SEE FIGURE 14.




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