Free Automotive Guide: Internal Combustion Engine-Short Block

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Early engineThe history of crankshaft and rod mechanisms date back as far as the third century, where the apparatus was used in industrial sawmill applications. By the end of the eighteenth-century the crankshaft and rod mechanism had evolved into the internal combustion engine, a version of which is used in the modern automobile. With current advancements in engine technology, the limitations placed on internal combustion engines as recently as ten years ago, now cease to exist. Engine components, which were once used only in competitive racing and high-speed research applications, have become almost redundant as they are now used routinely in mass vehicle production.

The internal combustion engine consists of four basic core groups of components: The bottom end, the top end, the lubrication system, and the combustion (fuel, air, and spark delivery) components. In this, the first of a three-part series, we will cover the bottom end, or short block assembly.

Bottom End

Bare engine blockThe engine block, or crank case, consists of the actual engine block assembly. It is typically forged from cast iron, steel, or aluminum, and then drilled lengthwise (for the crankshaft journals) and crosswise or diagonally (for cylinder walls). The crankshaft journals are then cut from the casting to form two separate halves, which will be bolted back together to form a series of nearly perfect circles, in which the crankshaft will rotate.

crankshaftThe crankshaft is the heart of the engine. It is hollow forged from cast iron or steel. The hollow design of the crankshaft allows for oil to be pumped into its center and to exit from rotating journals located under every rod and main bearing. It is made with a multiple off-set design. When it is rotated in the block it pushes one connecting rod, with the piston attached, up and into its respective cylinder while pulling another from its cylinder. As the piston is pushed upward, it creates either a compression or exhaust stroke. As the piston is withdrawn, it makes either an intake or power stroke. We will cover this more when we reach the operation phase, contained in combustion components.

connecting rodThe connecting rods are also forged, usually from cast iron or steel, with holes on either end. The larger hole is cut into two separate halves, which will be bolted back together to form a nearly perfect circle around the crankshaft. On the opposite end of the connecting rod, a smaller hole is precisely drilled. Through this hole a wrist pin will be pressed. The wrist pin is a pivoting style pin that will also secure the piston to the connecting rod. It is called a wrist pin because of its singular flexible design. This process will be repeated for each connecting rod. Four-cylinder engines have four connecting rods, six-cylinder engines have six connecting rods, and so on.

pistonThe pistons fit snugly into the cylinders of the block. The shape of the typical automobile piston resembles an inverted drinking cup, with a flat top and a hollow bottom, in which the top of the connecting rod is recessed. The piston is precision drilled crosswise and the wrist pin is pressed in; through one side, then through the connecting rod, and then through the other side of the piston, where it is press-fit into place.

ring gapThough the piston is designed to fit snugly into the cylinder bore, the final thousandths of an inch of tolerance are closed using compression piston rings. These rings are made from steel and they are forged into a semi-circular design. The pistons (which are normally made from aluminum) have a series of grooves cut into the upper, outer edge. The rings, which are manufactured to very exact specifications, are placed into the ring grooves, along with an oil ring which ring installationprevents pressurizedrod and piston oil from being forced up from the crankcase. At this point the rings are slightly loose on the piston and a noticeable gap exists between the two ends of the ring. As the piston, with rings installed, is inserted into the cylinder bore a ring compressor is used to close this gap. This allows the piston and rings to be pushed slowly into the cylinder without damaging either component in the process. Once the piston and rings are pushed out of the ring compressor and into the cylinder bore the rings spread slightly, providing a tight spring like compression/oil seal. Multiple rings are used so that when the rings are positioned correctly the slight gaps in the individual rings are never aligned with each other.

short blockThe assemblage of these parts is known as a “short block.” Auto technicians often refer to it as the bottom end due to its location in the automobile.

S.M. Darby

S.M. Darby

I am a freelance author with over 25 years of experience as a professional, ASE certified automotive technician and shop owner, muscle car enthusiast, avid street racer, and classic car restoration specialist.