What Causes Rod Bearing Failure – The Reasons Behind It!

What Causes Rod Bearing Failure – The Reasons Behind It!

May 18, 2023 0 By CarGuy

Are you experiencing some knocking noises coming from underneath the hood and you think that you have a rod knock caused by bad rod bearings? Well, if that is the case, then you are at the right place because, in this article, we are going to cover quite a bit when it comes to what causes rod bearing failure.

Doing your own research on a problem should be one of your top priorities. Why we are saying this? Well, because it is true.

At least you will understand why this problem appeared on your car and how you can solve it. Or at least like in the case with the rod bearing sound, how to avoid it in the future. Mainly because rod bearing knock is often not repairable. Since if you try to repair it, it will probably cost you an arm and a leg to do so. Still, if it’s a classic engine, it might be worth saving it.

In this article, first, we are going to cover rod bearings and learn more about the basics of these components and the differences between rod bearings and main bearings. Then we will cover what causes rod bearing failure and the reasons behind this situation. After that, we will cover some of the symptoms and also how to diagnose bad rod bearings. So, if you want to learn more, follow along.

What Are Rod Bearings?

Now before we learn more about what causes rod bearing failure, let’s take a look at what are rod bearings in general. Why do we think that this is necessary? Well, a lot of people are struggling with similar issues with these bearings and we need to elaborate on the topic thoroughly before we dive deeper into what causes rod bearing failure.

So, what is a rod bearing? Well, a rod bearing in simple words is a split sleeve bearing type. So, what does this mean?

Well, this type of bearing is made out of two identical halves and when these two halves are put together, form a circle.

Inside this circle when the bearings are installed on the engine, the crankshaft is rotating. So, you see what these bearings do.

They basically allow the pistons to move up and down and most importantly, they allow the motion to be transferred smoothly onto the crankshaft, so the engine can spin and create power that is then transferred from the crankshaft into the transmission and then to the wheels.

The bearings are doing all this heavy lifting in order to keep everything functioning nicely and smoothly and also keep the pistons nice and tight.

The rod bearings usually come in two designs. A steel back and aluminum alloy, or steel back and copper-nickel design. Both of these types are rather common. But the aluminum alloy bearings are more widely used in modern engines.

But what causes rod bearing failure? More on this, we are going to cover later on in the article, now let’s take a look at the difference between rod bearing vs main bearing.

Rod Bearing VS Main Bearing

Another topic that we would like to cover before we learn what causes rod bearing failure is the topic of the differences between rod bearings and main bearings. This is something that confuses a lot of people out there. So, let’s cover this topic real quick.

So, what is the difference between rod bearing vs main bearing? Well, in terms of function and design, they are very similar. But there is one big difference.

The main bearings have the task to support the crankshaft and allow it to spin freely. The main bearings usually slide between the engine block and the main bearing caps. Then the crankshaft can be tightened and the next step in assembly is to be taken.

And the next step when it comes to the assembly is installing the rod bearings. Rod bearings unlike main bearings are not seated on the engine block but on the piston rods. Thus, the name rod bearings.

They sit between the rod and the crankshaft allowing the pistons to rotate the crankshaft in the process.

So, to sum things up, the main bearings allow the crankshaft to rotate and keep it in place. Rod bearings make sure that the pistons and rods move up and down. Once you visualize this stuff, things are rather easy to comprehend. So, looking for a diagram online might help you visualize these components. Now let’s move on and learn what causes rod bearing failure.

What Causes Rod Bearing Failure

So, we have covered the basics and learned more about what is a rod bearing and the differences between rod bearings and main bearings.

Now, we think that it is time to move on and learn more about what causes rod bearing failure and learn the reasons why these components fail. As with anything out there, there are some reasons behind why it’s happening and so is the case with the rod bearings. So, let’s cover what causes rod bearing failure in the following chapters.

1. Oil Starvation

Now let’s learn what causes rod bearing failure and start with the most common cause for this problem.

The most common reason for rod bearing failure is definitely oil starvation. So, what is oil starvation? Well, oil starvation is a situation when your engine is deprived of getting enough oil.

This could be caused by a number of problems with the lubrication system, as well as problems with the engine design. Some engines by design are more prone to suffer from oil starvation. Subaru engines for example, like the EJ255 engine that we elaborated on previously.

But oil starvation can also be caused by a low amount of oil as well. So, if you have an oil leak and you just don’t care about the level of oil inside of the engine and you never check the dipstick. There is a high chance that the engine will run low on oil and the engine will basically starve from oil.

So, in this case, what happens is the rod bearings and piston rings will take the toll and fail prematurely. And you definitely want to avoid this from happening because this damage could scrap the whole engine. So, we covered the first cause on what causes rod bearing failure, now let’s move on to the next.

2. Contaminated Oil

Now let’s move on to the next probable cause of what causes rod bearing failure. And this is also another very common cause of rod bearing damage and rod bearing failure.

More precisely, contaminated oil. Contaminated oil can really harm the rod bearings and also the main bearings.

When your engine is working, there will be a lot of dirt and debris in it. Whether metallic flakes, gasket material, or some other impurities, these sharp microscopic particles can really damage the bearings.

Another thing that can contaminate engine oil is coolant. If you have a blown head gasket, then the engine oil is probably mixed with the coolant.

And in this situation, the engine oil has lost its oiling characteristics. So, in this case, the bearings will take the toll and your engine will suffer from rod bearing failures. And also, if you run the engine like this for very long, the whole engine will be destined for scrap. So, pay close attention to the engine oil and make sure that the coolant level is also good. Now let’s move on to the next reasons for what causes rod bearing failure.

3. Less Frequent Oil Changes

Another reason of what causes rod bearing failure is the problem with poor maintenance. That’s right, poor maintenance is one of the prime causes of wear on these components.

Namely, less frequent oil changes. Modern cars that run synthetic oil require an oil change every 10,000 miles.

So, if you don’t follow this interval and you keep delaying the oil change on your car, you risk a lot of damage to happen on your engine.

This old oil will lose the lubricating characteristics and there will be heat creation inside the engine. Since engine oil not only that it lubricates, it also dissipates heat out of the engine.

And if you keep delaying these services, you keep making things worse and worse. And eventually, you will have a prematurely worn-out engine.

Once delayed oil change for a few thousand miles, can easily remove 50,000 miles from the life of your engine. This is why you should never skip the oil changes and make sure that you always do the services on time. Now let’s move to the last causes in our list of what causes rod bearing failure.

4. Poor Manufacturing Process

Another thing that causes rod bearing failure is the poor manufacturing process of the rod bearings. Sometimes the rod bearings can be rather problematic and the engine could wear them down quite significantly.

And this is rather common in a lot of BMW engines to have rod bearing issues. The most common models to have this issue are the M3 and M5. More specifically the E46 M3, E90/E92 M3 and the E60/63 M5.

So, before you decide to get one of these engines, especially the M5, make sure that this engine has been serviced once in its lifetime. Especially if the engine is higher mileage.

We are saying this because the replacement costs can be astronomical and a new engine could cost you as much for the whole car. So, beware of this.

We have covered what causes rod bearing failure, now let’s move on and dive into the symptoms for this problem and see what are the common symptoms that you will experience when you are dealing with bad rod bearings.

Rod Bearing Failure Symptoms

Now as we learned more about what causes rod bearing failure. We can slowly move on and learn more about the rod bearing symptoms.

Knowing the symptoms is quite useful, especially for people who are new to cars and are suspecting that they have rod bearings on their cars. So, let’s cover the most common symptoms that are associated with rod bearing failure and then learn how to diagnose the problem.

1. Metal Shavings In Oil

So, we learned what causes rod bearing failure, now let’s take a look at the first symptom that you will notice when you have bad rod bearings.

The first in our list of symptoms is the situation when you have metal shavings in oil. Imagine that you remove the dipstick and you learn that on the dipstick there are metallic flakes. Or you change your oil and when you put your hand in the oil, you can feel metallic flakes as well.

In some cases, these flakes can be visible to the naked eye. But how you can know that these flakes are from the rod bearings?

Well, usually rod bearings are made out of either copper nickel or aluminum. When you are dealing with copper, these shavings are often yellowish in color. When it comes to aluminum, the flakes are silver in color.

Anyways, not a good situation and a clear example that you need to take things further and diagnose the problem before it gets too late for your engine. If diagnosed on time, you could get away with only replacing these bearings with new ones and call it a day. So, we learned what causes rod bearing failure and the first symptom. Now let’s move on to the next symptom on our list.

2. Rod Knock

The next symptom on our list of rod bearing failure symptoms is a more common one. And usually, the sign that you need to replace the bearings as soon as possible.

And this symptom is the rod knock. The symptom is quite self-explanatory. But let’s elaborate further on it and visualize how this happens.

So, the bearing as we noted is the buffer between the piston rod and the crankshaft. So, what happens is that this buffer wears out.

Once you have enough material from the bearing missing, there will be a play between the crankshaft and the rod.

In this situation, there could be a rod knock developing. This symptom can be characterized as a strong metal tap coming from the bottom of the engine. And when it comes to what causes rod bearing failure, in this situation, the problem is often poor oiling that causes the bearings to wear out. Now let’s move on to the last symptom.

3. Low Oil Pressure

So, we learned what causes rod bearing failure and some of the symptoms associated with this problem. Now let’s discuss the last symptom on our list.

The last symptom that we would like to discuss when it comes to the problem with the rod bearings and their failure is the low oil pressure. So, what is oil pressure, and why it is so important?

Well, oil pressure in general is the resistance that the oil faces when flowing through the engine. This pressure needs to be constant because if the oil flows too easily, you will have low oil pressure, and if the oil faces some obstacles inside of the engine it will be high.

And both of these situations are not good. When you have low oil pressure, this means that there is a lot of wear inside of the engine and the oil is moving too fast. On the other hand, when you have high oil pressure, you have oil sludge inside of the engine that is blocking the oil flow.

Proper oil pressure is somewhere between 25 and 60 PSI. Anything below 25 PSI is considered a low oil pressure and this low oil pressure can be caused by poorly, bad oil pump, sealing piston rings, as well as worn out rod bearings and also worn out main bearings.

So, you get the idea, if the oil pressure seems low and the engine struggles to run well. It is a top priority to take the car to a shop and diagnose the problem. And in the following chapter, we are going to learn how you can diagnose rod bearings.

Rod Bearing Diagnosis

So, we have covered what causes rod bearing failure and we learned the most common symptoms associated with this problem. We think that now it is time to learn more about how you can diagnose this problem. What is the right way to determine that you have bad rod bearings?

First things first are to make sure that the oil is clean and does not have metal particles, that pressure is low, or if you have a rod knock. If this is the case, you will have to move on to diagnosing the issue.

And in order for the rod bearings to be properly diagnosed, you have to remove the oil pan and then remove the rod caps.

Once the caps are removed, you can inspect all of the bearings one by one and see if there is any damage done to them.

Just make sure that you don’t mix them up because they will never work if you plan to reinstall them on the engine.
What is good in this case is that you can get away by only replacing the bearings. But if the crankshaft is too scratched, it might require some machining work in order to be put back in business. Overall, it is an expensive job. But it has to be done if you want to keep driving this engine.


In this article, we have covered quite a bit when it comes to what causes rod bearing failure. First, we learned what are rod bearings and why they are so important. Then, we covered the difference between rod bearings and main bearings.

Once we covered the basics we learned what causes rod bearing failure and also the symptoms of rod bearing failure. Both of these topics are really important for you if you have an issue with the bearings. And lastly, we covered the diagnostics process. If you have something to add, please write it down in the comments below.