Engine Connecting Rods

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Automotive Connecting Rod
Automotive Connecting Rod

A connecting rod is a component in an engine that connects the piston to the crankshaft. This rod is designed to convert the linear motion created in a vehicles engine into rotating motion. Connecting rods are rigid which allows the rod to both push and pull the crank through both halves of a revolution.

Most connecting rods are made of steel, titanium, or aluminum alloy depending on the size of the engine and application of the vehicle. Steel rods are extremely durable and are most commonly used in a regular engine that sees everyday use. An aluminum alloy rod is used for its lightness and ability to absorb high impact. Titanium rods provide the benefits of both a steel rod and aluminum rod but come at a higher price which is why they are only commonly found in high performance engines.

Picking the correct kind of connecting rod for the vehicles application is very important because a failure of the connecting rod is the main reason for most catastrophic engine failures. A failure of a connecting rod is often called “throwing a rod” and is often represented by a broken rod in the side of the crankcase. Connecting rods are usually casted but can also be machined out of a solid billet of metal which is commonly done in racing engines for added strength.

The small end of the connecting rod attaches to the piston pin, gudgeon pin, or wrist pin, while the larger end of the connecting rod connects to the bearing journal on the crank throw. A pin hole is typically bored through the bearing and big end of the connecting rod to allow pressurized lubricating motor oil to spray out on the thrust side of the cylinder wall which helps lubricate the pistons and piston rings. Smaller two stroke engines and some single four cylinder engines avoid using this type of lubricating system by using a roller element bearing instead.

Connecting rods are constantly placed under tremendous amounts of stress created by the reciprocating load the piston presents. The load on the connecting rods varies depending on the speed of the engine. This is why a connecting rod will often break while the engine is at peak power. Common methods for reducing connecting rod stress include eliminating stress risers, shot peening, and balancing all connecting rod/piston assemblies to the same weight and magnafluxing to reveal small cracks.

One of the most common sources of wear on an engine is from the piston being driven up in down by the connecting rod in the cylinder. A shorter connecting rod will exert more sideways motion on the piston which will eventually wear the cylinder into an oval like shape, causing the engine to lose effectiveness. To combat this, a longer connecting rod can be used to reduce the amount of sideways force. Increasing the length of a connecting rod in an engine is very difficult because the length of the connecting rod plus the piston stroke is a fixed distance.

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