LO 7-7 Identify and explain the purpose, function, and construction features of connecting rods

1.3. Connecting Rod Features

Connecting Rod Features

Connecting rods use an I-beam cross section, which has good weight-to-strength characteristics and resists bending. The smaller end of the rod will have a bushing pressed into it if it is used with a full-floating piston. When the small end of the connecting rod is shaped like a triangle, it is called a trapezoidal rod . The individual weight of a set of connecting rods is critical to engine balance. If the connecting rods vary in their mass, engine vibration will result. To adjust the weight of connecting rods, balance pads are located at each end for grinding to balance the rod’s weight between the rod ends and around its center point. Often, select fit rods are used together in sets in an engine. In select fit rod sets, rods are weighed after fabrication; those closest in weight are used together in sets. These sets can be identified by the paint marks that uniquely identify the weight variation of the rod (a group of connecting rods may have white, yellow, red, or other paint markings to identify them as belonging to a particular weight classification). Such rod sets should be kept together when assembling an engine. If a service rod is required, it will often have a different paint marking that is in the middle of the weight distributions. Because the final finish bore dimensions of the big end of the rod are made after the parting halves of the rod and cap are completed, the two parts must be assembled in the same way, as they were finished. If the cap is fastened to the rod 180 degrees away from the original position, the bore will not be concentric, even though the two pieces can be bolted together. Matching marks on a connecting rod identify the correct assembled position. Failure to align these marks will result in improper bearing clearances, and the engine may not rotate after the pieces are assembled. If the engine is operated when the components are assembled incorrectly, a tight connecting rod bearing will result in an insufficient oil film on the connecting rod journal, which will produce catastrophic engine damage Figure 48. To further ensure a concentric bore in the big end of the connecting rod, the parting halves of the cap and rod will have various machined features, serrations, or alignment dowels Figure 49.

It is also critical that rods be installed in an engine with the correct front-to-back orientation. Connecting rod caps will strike the block and severely damage an engine the first time it is rotated if the rod is not installed in the engine correctly Figure 50.

FIGURE 07-48: Proper alignment of the connecting rod and cap is made by matching up the markings on the cap and rod.

FIGURE 07-49: Machine features on the parting halves of connecting rods ensure proper alignment and bore concentricity.

FIGURE 07-50: Parting halves of these rods are serrated. Note the directional marking on the rod.

Diesel engines can have a passageway drilled through the connecting rod to the piston pin bushing to supply oil to the piston pin. In V6 and V8 engines, connecting rods also share a common crankpin journal, which means rods must also be correctly oriented. A chamfered edge in the rod cap is installed toward the crankshaft fillet radius, the edge between the journal and the crank cheek. The flat sides of each rod match each other, providing a surface for each rod to glide over one another. Even in in-line engines, the connecting rod must be correctly oriented front and back due to piston pin and rod cap split offsets.