Auto Repair Help
KYLE’S CORNER – AUTO REPAIR HELP

ENGINE BLOCK, PISTONS, & CRANKSHAFT
by Kyle McFadden

The engine block is the foundation of the engine. It is constructed of cast iron or aluminum. The engine block is a platform in which the components necessary for the operation of the engine are mounted. The engine block has passages cast into it that distribute oil for lubrication and coolant to prevent overheating.

The crankshaft is mounted in the lower section of the engine block. This section is called the crankcase. The crankshaft is bolted in place with small sections called main bearing caps. The portion of the crankshaft that is contained by the main bearing caps is called journals. The main bearing caps contain the bearings that allow the crankshaft to rotate freely. The bearings are divided into two semi-circular halves, with one half mounted in the engine block and the other in the main bearing cap. The bearings contain a soft metal alloy bonded to a steel outer shell. The soft surface faces the crankshaft journal. The crankshaft rides on a film of oil between the crankshaft main journal and the bearing surface. The crankshaft has weights built into its design that along with the flywheel, help to balance the rotating motion of the engine. It also provide inertia to keep the engine rotating at slow speed.

The engine block also contains the cylinders that contain the pistons. Pistons are generally constucted of cast or forged aluminum. This material expands at about twice the rate of the cast iron engine block, as the engine warms up to operating temperature. Specific clearance is machined into the cylinder to allow for expansion of the piston. The top of the piston is designed to withstand the tremendous heat and force present when the air/fuel mixture is ignited in the combustion chamber. There are grooves or slots under the top of the piston. These grooves contain the rings which seals the piston in the cylinders. Rings are constructed of metal such as iron or chrome-alloy. They are almost completely circular with a small gap in the ring. The gap is designed to prevent overlap of the ring as the engine reaches operating temperature. Most automobile and light truck engines use a piston with three rings. The top two rings are used to prevent leakage of the compessed air fuel mixture past the piston. The bottom ring is used to control oil loss by preventing oil from reaching the combustion chamber.

The connecting rod is attached at one end of the piston and to the crankshaft at the other. It transmits the up and down motion of the piston in the cylinder bore to the crankshaft. It is generally constructed of forged or cast iron and is rigidly designed to withstand the force generated by the downward thrust of the piston. As each cylinder fires the crankshaft is subjected to torsional force that causes it to wind and unwind as if it were a spring. While the actual distance that the crankshaft winds and unwinds can only be measured with a micrometer, the action of the torsional forces on the crankshaft is enough to produce vibrations that could be felt by the driver. To control this a dampner is used to absorb vibration. This is usually a weighted two part metal disc. It consist of an outer ring attached to an inner ring using a thin layer of rubber. The vibration dampner is placed on the front part of the crankshaft that protrudes from the engine.

(Kyle has an affinity for Pale Ale and tooling on his 1956 Chevrolet Nomad Station Wagon.)

Ask our techs your questions on the BAT Auto Forums!

One Response to “Engine Block Pistons”

  1. lieu ma says:

    I need engine block for Honda passport 2000,6cyl 3.2

Leave a Reply