An engine block, or as is often called a cylinder block or crankcase, is the primary component of any internal combustion engine. Though much of the block may be visible in an engine compartment, its purpose and operation are often a mystery to the average vehicle owner. Because of this, we will dedicate some time to answer what an engine block is and how it allows the internal engine parts to work in harmony to produce power.

The Engine Block Explained

Engine Block
An engine block is typically made out of cast iron or aluminum and is responsible for housing internal engine parts that must maintain very close tolerances. One area where close tolerances must be maintained is the cylinder walls. The cylinder walls allow pistons and rings to perform their duties as the operating engine goes through its induction, compression, power and exhaust stroke phases. The engine block’s cylinder walls must have a sufficient piston to wall clearance that allows lubricated piston travel to occur in an environment where thermal expansion occurs. For example, a forged aluminum piston will expand faster than its surrounding cast iron cylinder wall.

Engine Block Tolerances and Design

Another critical dimension area inside an engine block is its crankshaft main bores. Collectively, these main bores are known as the line bore. The horizontal (inline) main bores must be precise and allow for the proper fitment of engine main bearings. The housing bore diameter directly influences the inside diameter of the bearings, which will allow for proper bearing to crankshaft journal clearances to be maintained. The clearance that occurs between the crankshaft main journals and bearings is pressurized with oil while the engine is in operation, which allows for proper lubrication.

The deck surface of the engine block is also an important area where dimensions must be maintained. Aftermarket pistons, for example, may protrude outside of the bore and create clearance issues with the intake and exhaust valves. Therefore, when an engine is rebuilt it is often necessary to compute the deck height (centerline of the crankshaft main housing bore to the top of the cylinder head deck surface) to determine if the pistons have adequate clearance from the valves. If adequate clearances are not present, a thicker cylinder head gasket or shorter connecting rod may used. Additionally, custom compression height pistons or machining valve reliefs in existing pistons may be used to provide for adequate piston to valve clearance.

Liquid cooled four stroke engine blocks also are cast with coolant passages that allow the block to stay cool under operation. Most blocks reserve the largest coolant passageways for around the cylinder bores, in proximity to freeze plugs, which are the hottest areas inside an engine. If temperatures are not properly maintained, and thermal expansion is not controlled, premature failure of the engine may occur. Such failures include cylinder wall warping, which impacts tolerances, and cracking that can cause an immediate catastrophic failure.

Oil galleys inside the block are also a very important part of an engine’s lubrication system. For example, blocks often have primary oil galleys that supply oil to the crankshaft, camshaft, cylinders and provisions for sending oil up through the lifters and into the pushrods to lubricate the rocker arms.

Engine blocks also have threaded provisions for mounting cylinder heads, a timing cover and an oil pan rail where the oil pan is bolted down. Additional threaded bolt hole bosses may be cast or machined into the block to allow for mounting pulleys, brackets and studs that are used only for securing hoses and wires.

There are two primary types of four stroke engine blocks found in cars, trucks and motorcycles. An inline block has all of the cylinders located in one bank inline to each other, which is quite common on four cylinder and some six cylinder engines. These types of blocks are often called straight four or straight six engines because of how the cylinder walls are positioned. V style engines, which have two separate banks of cylinders, are quite common in today’s vehicles and also in race cars. Most Chevy, Ford and Chrysler V6 and V8 engine blocks have cylinder banks where the decks are located at a 90° angle to each other. However, the Ford Duratec V6 engine and some General Motors V6 engine blocks are manufactured with a 60° angle while a 409 Chevy block has a unique 74° angle.

As mentioned towards the beginning of this page, engine blocks are primarily made of cast iron or aluminum. In the case of aluminum blocks, steel sleeve inserts are used inside the cylinder walls. The main advantage of aluminum blocks is that they are lighter and easy to machine. Particularly in cases of aftermarket performance blocks, where additional modifications by an automotive machine shop are required, the ease of machining aluminum is usually preferred.


Engine Block Tips

In this section of our website you will find important tips that will help you to identify and purchase the best engine block for your application.
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Engine Block Facts