Connecting Rod

Connecting rods are forged or sintered steel, sometimes aluminium or titanium in racing engines. The small end holds the piston pin; the big end wraps around the crankshaft journal with a pair of plain bearing shells. Many modern rods are "cracked" at the big end during manufacture, so the cap fits back exactly on its rod. Rod bolts are often single-use and must be replaced whenever they are undone.

As the piston is pushed down, the rod swings and pushes the crankshaft round. The rod bearings run on a thin film of pressurised oil, so metal parts never touch in normal running.

A worn rod bearing causes a deep knocking noise that rises with engine speed, often called rod knock. If ignored, the bearing can seize and the rod may break through the engine block, so the engine should be stopped and inspected.