Connecting Rods & Components

Connecting Rod Bolts, Caps & Kits

As their name implies, connecting rods connect the pistons to the crankshaft. They convert the reciprocating motion of the pistons to the rotating motion of the crankshaft, which turns the gears in the transmission, the axle shafts, and ultimately the wheels. A connecting rod consists of a small end, attached to the piston by a wrist pin, and a big end, which is split so it can be attached to the crankshaft throw, separated by the beam.

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Although cast iron was commonly used on older engines, today most connecting rods are forged from steel or powder metal. Rods for high-performance and racing applications can be made from billet steel, aluminum, or even titanium. The wrist pin can be a press fit in the rod small end bore and free to turn in the piston bore, or it can be “fully floating” and free to turn in each component. Some fully floating small ends have bronze bushings, and all such pins must be retained in the piston with circlips to prevent sideways movement and contact with the cylinder wall. The bottom portion of the split big end bore is called the rod cap. The bore is machined to accept plain bearings that ride on the crank journal on a thin film of pressurized oil. The cap is retained by bolts that thread into the rod or by bolts and nuts at each side of the big end bore.

Oil thrown off at the crankshaft connecting rod journals onto the cylinder walls lubricates the piston skirts and rings. However, some connecting rods have oil squirt holes in the beam or grooves at the rod cap parting line to lubricate the cylinder walls. The big end of the connecting rod is chamfered to clear the radius on the crankshaft journal. Rods that share a journal, such as on V6 and V8 engines, have a larger chamfer on the side facing the crank cheek. When the engine is assembled, these features require that the rods be oriented properly. Connecting rods are usually numbered according to cylinder position at the factory. The numbers should appear on the side of the connecting rod adjacent to the big end parting line. In-line engines usually have the numbers on the same side, while on V-type engines the numbers will face the pan rails.

When an engine is rebuilt, each connecting rod must be thoroughly inspected to ensure it can be reused or if replacement is required. Rods that are bent or twisted are evidenced by abnormal wear patterns on pistons skirts and rod bearings, but all rods should be checked for these conditions on a special alignment fixture. Bent or twisted rods can be straightened but are generally replaced. It is common for the big end bore of the rod to stretch due to the millions of direction reverses it undergoes during normal operation. The bore can be made round again by honing. The rod bolts are removed and the rod and cap parting faces ground on a special grinding fixture, to reduce the diameter of the bore. The rod is then reassembled with the bolts torqued to specification, and resized on a rod honing machine.

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