Connecting Rod Basics

Most passenger cars used forged iron or steel connecting rods. They are rigid, light and relatively inexpensive. Rod length, piston diameters and crank pin diameters vary with the application and expected power output. Connecting rods are made in two pieces — a cap and beam section, held together with nuts and bolts.

Cast semi-steel connecting rods are making a return in production engines thanks to their comparatively low cost and ease of manufacture. Their main drawback is their weight, typically some 100 grams heavier than their forged counterparts.

Parts of a Connecting RodSome manufacturers are now producing a one-piece rod made from compressed and sintered powdered metal (P-M technology). The cap is either cut away from the beam section (as would be the case with all other rods), or broken away and bolted back into place using the fracture line as the parting edge.

Forged steel rods are essential in high-output spark-ignition (SI) and compression-ignition (CI) engines. They provide the extra strength needed for performance under heavy load conditions.

Forged rods get their strength from the forging process, which gives them a definite grain structure. Cast rods, on the other hand, have no grain structure; cast-iron nodules appear randomly distributed, forming no pattern. Steel rods can be made from the steel alloy best suited for a particular application.

Forged-billet steel connecting rods are found in high-output and endurance racing engines. They are machined form a solid forging of SAE-4130 or SAE-4340 alloy steel. They take advantage of the hammered-in grain structure established at the steel mill.

Drag racing engines use forged aluminum rods. They are light and allow rapid acceleration of RPMs. Aluminum rods are also common in small air-cooled engines and air compressors.

To allow for installation and service, connecting rods are made in two pieces; a cap and a beam section.

In some cases, the parting edge (where the cap and beam mate) is serrated or made with tangs. This decreases the chance of the cap becoming loose and moving out of position.

Occasionally, straight-cut rods will include hollow or solid dowel pins between the beam and cap. Both measures prevent cap walk — the shifting around of the cap on the beam while the engine is running. Loose caps can cause fretting, a condition marked by small dents on the cap and rod parting edge.

Some rods are made with an off-set or angled cap as the parting edge. This allows the use of smaller diameter pistons while retaining a sufficient diameter for the rod bearing. This type of rod may use any of the parting edges discussed above.

When engine must fit a tight space, the designer may need to use small bore diameters and a long stroke crankshaft (under-square engine). This combination can require off-set beam rods. Such rods are common to some import and industrial engines, but are also used in many V-type engines too. With the beam off-set to one side of the bearing housing, the crankshaft can be made stronger and accommodate wider rod or main bearings.

The beam is that part of the rod between the piston and the bearing. The most common is the I-Beam, which when cut in half, resembles the letter “I”. The I-Beam construction allows the connecting rod to be substantially stronger and lighter than solid metal would.

Many performance engine connecting rods use an H-Beam. The beam, when cut in half, resembles an “H”. The H-Beam offers even greater strength-to-weight ratios. Round beams were once common in performance connecting rods, but their use has faded.

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