(Updated on June 10, 2021)
A failed connecting rod is one of the most catastrophic forms of engine failure. This failure is commonly referred to as a “thrown rod” or “throwing a rod”.
When a connecting rod fails, there is a very good chance the piston or connecting rod could smash right through the engine block at very high speed, damaging anything in its path.
What Are Connecting Rods?
Connecting rods are the link between each piston and the crankshaft in an internal combustion engine. As the crankshaft spins, the pistons move back and forth very rapidly, subjecting the connecting rods to immense tensile forces.
Connecting rods have bearings that sit between the rod and the crankshaft. These bearings are lubricated and prevent metal on metal contact as the crankshaft rotates.
Likewise, there is a pin that connects the piston to the top of the connecting rod. This pin is called a wrist pin, piston pin, or gudgeon pin.
Top Symptoms of a Bent Rod
A connecting rod needs to be strong and durable for the engine to survive. If it is too weak to handle the load of the combustion, then the extreme tensile forces on the connecting rod may bend or even break it.
This would be a very bad thing for the engine because there would be no more intake strokes, exhaust strokes, compression strokes, or power strokes.
As for the driver, they will be able to notice when this occurs because there will be some very noticeable symptoms present. Here are some of the most common symptoms indicating connecting rod failure.
1) Low Compression
If the connecting rod is slightly bent, you may be able to drive on it without too much of an issue. However, the cylinder with the bent rod will probably have lower compression than the others.
If you’re worried about the possibility of a bent rod, you could start by getting a compression test. If your compression numbers are within about 10% of each other across all cylinders, your rods are probably fine.
Do note that you may experience low compression for a number of reasons. After an unsatisfactory compression test, consider performing a leak down test to help you narrow down the possibilities.
2) Engine Knocking Sounds (Rod Knock)
When your engine is turned on and running, you may hear a knocking sound coming from it. This will be a repeated knocking sound that gets faster as your engine speed increases. This is colloquially referred to as “rod knock”.
The sounds may go away after a short while, once your oil has a chance to circulate through the engine components and lubricate them.
3) Low Oil or Oil Pressure
If you have a bad or failing connecting rod, your engine will probably lose a whole lot of oil very suddenly, which would have devastating effects on your engine’s oil pressure.
There may be an oil pressure gauge on your engine which will tell you when it has low oil pressure. You likely have a warning light on your dashboard that will tell you as well.
If you have low oil pressure, then it means oil isn’t circulating well through the engine. This might lead to an overheated engine if the problem is not resolved quickly, and you will experience drastically more wear and tear on the engine because it won’t be lubricated properly.
4) Visibly Bent or Damaged Rod
If you suspect that you have a bent or damaged connecting rod, based on the other symptoms which may be present, then you need to visually inspect the rod yourself to see what its condition is.
This would require your engine to be disassembled, which is a job that only an experienced mechanic should do.
5) Seized Engine
A thrown rod is likely to stall and seize the entire engine. You will know the engine is seized because you can’t turn it over by hand (using a wrench on the crankshaft pulley bolt). Even an engine that won’t start or crank should turn over by hand in this manner.
Chances are if you threw a rod, you’ll know it. A thrown rod is often accompanied with a loud bang and metal flying out of the engine bay. You may notice a huge puddle of oil under the car, and even a big hole in the block after this happens.
Causes of Connecting Rod Failure
1) Flooded Engine
Unlike air, liquids are not really compressible. If the engine is hydrolocked (that is, filled with a fluid instead of just air) you run the risk of bending the connecting rods when you go to start your vehicle. You will have to de-flood the engine before you can start the car normally.
2) Pre-ignition or Detonation
Pre-ignition and detonation are terms for abnormal combustion of the air fuel mixture inside the combustion chamber. Fuel may combust prematurely due to a variety of reasons: carbon buildup, ignition timing that’s too advanced, or running fuel with too low of an octane rating for the vehicle.
Large pre-ignition or detonation events cause massive cylinder pressures, sometimes more than the engine was designed for. Greatly exceeding expected cylinder pressures in this way could cause catastrophic engine failure.
3) Poor Engine Management
If you’re running aftermarket engine parts, chances are you had a tuner program your ECU. If this tune is too aggressive for your running conditions or you push your engine a bit too far, you may end up throwing a rod.
Most vehicles require a tune when certain bolt on parts are installed, such as an intake, exhaust, boost controller, or a bigger turbo. Driving the car hard before the engine has been tuned to account for the new hardware could break something pretty expensive, including a connecting rod.
4) Over Revving the Engine
As you rev the engine higher, the forces exerted on connecting rods increase because the pistons are moving faster. If you rev the engine higher than it was designed to handle, you’re probably going to be looking at buying a new engine.
Most engines prevent you from over revving the engine, and will cut fuel when you’re at or just above redline. The easiest way to over rev the engine is on a manual transmission.
If you downshift to a gear that’s too low for the speed you’re traveling, you’ll skyrocket the RPMs and your engine won’t be able to keep up. This is known as a “money shift”, because this type of miss shift ends up being very expensive.
5) Engine Modifications
Many factory connecting rods are made thinner than they used to be on older vehicles. This is done in the name of efficiency and cost savings. Thinner rods don’t pose a problem at stock power levels, but stock rods aren’t always sufficient if you’re trying to make a lot more power.
If you’re trying to build your car to go faster, make sure you do a lot of research on forums for your specific vehicle to see what the typical build path is for the power level you’re hoping to make.