What does it mean to "throw a rod"?

Dear Car Talk

Dear Car Talk | Apr 01, 1990

Dear Tom and Ray:

A friend of mine told me that he had ruined his car engine while driving very fast on a country road. He claims he "blew a rod" and the motor was worthless after that. In more technical terms, could you please tell me what happens when one "blows a rod?"

RAY: To set the record straight, in the field of auto mechanics, one doesn't "blow a rod," one "throws a rod." Now that that's clear, here's what happens.

TOM: When gasoline explodes in a cylinder, the violent force of that explosion pushes the piston down. The motion of that piston is what makes the crankshaft turn--and ultimately what makes the car move. The metal part that connects the piston to the crankshaft is called a "connecting rod."

RAY: When you "throw a rod," it means that one of these rods has literally broken off. When that happens, one of two things occurs. If the rod breaks while the piston is on its way up, the piston keeps going up until it jams itself permanently into the cylinder head. If the rod breaks while the piston is coming down, the broken rod can pierce a hole right through the engine block (like a compound bone fracture breaking through the skin). Either way, the engine is instantly ruined, and you'll have to "blow a wad" to get the car back on the road.

RAY: There are two theories about why connecting rods break. The first is divine revenge. This theory states that people who rev the heck out of their cars get what they deserve. We call that the "divining rod theory." Mechanical Atheists, on the other hand, argue that thrown rods have more to do with the age of the car and the accompanying low oil pressure. Since there is a divine element involved, we play it safe by subscribing to both theories.

TOM: So if your friend wants his next car to last, he should cover all of the bases. First, he should live a good clean life. That'll cover the "divining rod theory" and all of its corollaries. Second, he should change the oil frequently, and make sure the level is always adequate. That should take care of the Atheists. And finally, he should slow down on country roads so he can enjoy the scenery. That'll satisfy the Audubon Society and the local constabulary.

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