Connecting RodDear Ole Mechanic;

My car, a 2008 Ford Fusion, developed a loud knocking noise that seemed to be coming from the engine compartment.  It was much more noticeable as the engine went faster but not nearly as loud at idle.  In fact, it almost went away at idle.  I was worried, so I stopped by a local mechanic and had him listen to it.  He said that it sounded like a rod knock.  He also said that I should not drive it and have it towed back to the dealer that I brought it from as it should still be covered under the warranty.  I did drive it home very carefully and called the dealer.  They towed it to the dealership, checked it and called me back saying that it was a rod knock.  They asked me when the oil was changed last and I reminded them that they had done all of the service work.  They said that it would be covered under warranty and it would be fixed in a few days.

What I am wondering is just what is a “rod knock”?

Thank you.  Not a Mechanic


Dear Not;

I can explain what a rod knock is, but some basic engine information needs to be covered to help you understand the explanation.  In the engine there are round holes called cylinders and there are moveable plugs in the cylinders called pistons.  When gasoline is burned in a cylinder, it moves the piston down in the cylinder.  This downward movement needs to be turned into a turning motion to turn the wheels and move the car.  To do this, the piston attaches to one end of a rod called a connecting rod.  The other end of the connecting rod attaches to a crank called a crankshaft.  As the burned gasoline pushes the piston down, the connecting rod turns the crankshaft, which continues to turn and pushes the piston back up into the cylinder to push out the burned gasses so that more fuel can be put in and burned to make more power and move the car.  While this is an oversimplification of engine operation, it is enough to understand a rod knock.

The connecting rod that attaches the piston to the crankshaft requires a very close and precise fit between the rod and the crankshaft.  There must be some clearance between the rod and the crankshaft for lubricating oil as they do move very fast.  However, if the clearance is too much, then there can be a knocking noise produced.  The problem is that the knocking is the hammering sound produced by two pieces of  metal banging together.  The more they knock, the bigger the clearance gets and the louder the noise.  Had you continued to drive the car, the clearance would have gotten bigger and bigger until it broke apart.  At that point, broken pieces of metal could have been thrown out so hard that they could have broken out the side of the engine.  What is the proper clearance between the rod and the crankshaft?  About .001 inch or about one half the thickness of a human hair.  How much clearance does it take to cause a knock that you can hear?  About .010 inch or about the thickness of 3 sheets of paper.  What can cause a rod to knock?  Usually it is a lack of lubricating oil, hence the dealers question about oil changes.  However, in an almost new engine like yours, it is more likely the improper installation of the rod or a small chunk of casting that was left in the engine during assembly or--well, there are too many items to list.  How will they fix your car?  If there is not any other damage, they may just install a new crankshaft and connecting rod.  If there is other damage, they will probably install a new engine.  How can you keep this from happening again?  There really is no way for you to prevent assembly problems.  About all you can do is follow the maintenance schedule--especially the oil and filter changes.

While assembly problems are not all that common any more, they can still happen. That is why there is the factory warranty.

Herr Professor Nuzanbolts