Metal fatigue, poor lubrication, or over-revving of an engine are the most common causes of thrown rods. When gasoline ignites in a cylinder, the piston head descends, pushing the connecting rod downward and spinning the crankshaft. The connecting rod between the head and the crankshaft may break if the metal is weakened. Friction can be caused by a lack of lubrication, and over-revving can generate more force than the connecting rod can bear.
When a rod is thrown, the engine usually fails catastrophically. It usually happens when the piston is on the downstroke, and the broken end of the rod can force its way into critical engine components. It can press the piston head into the top of the cylinder with enough force to fuse the two sections together if it happens while the piston is going upward. In either situation, the engine will almost always need to be replaced.
One approach to help prevent this type of damage is to keep an engine properly oiled. Low oil pressure might result in insufficient oil inside the piston to grease the piston head’s bearings. A shortage of lubrication, when combined with the metal fatigue prevalent in a high-mileage engine, can be the last trigger for a thrown rod.