Connecting Rod Failure
Connecting rods connect the piston to the crankshaft, are very strong and made of hardened forged steel, and are designed to last. Typically a bent rod is caused by overrevving, detonation, or fluid entering the cylinder ending in a hydrolocked engine.
A slightly bent connecting rod may not be noticeable during the initial inspection. However, it may appear as one-sided diagonal wear on the skirt located on the major thrust side of the piston. Typically the rod and piston are cleaned and mounted in a special tool, and bends are measured with a feeler gauge.
Bent Connecting Rod
A bent connecting rod is usually the result of incompressible coolant or fuel entering the cylinder resulting in hydrolock. Never scratch or mark a connecting rod. After removal, screw the nuts onto the bolts to keep the two parts together.
Broken Connecting Rod
A broken connecting rod is catastrophic. Typically the momentum of the engine causes the broken rod to thrash around the engine causing unrepairable damage. Even scratching into one of these rods will weaken it and result in failure. Most of us have seen a thrown rod protruding out of someone's engine block.