Of all of the reasons that people avoid purchasing aging BMW M cars, the fear of rod bearing failure is, without doubt, the most pervasive. It looms over any prospective E46 M3, E90 M3, E60 M5, or E63 M6 buyer’s head like a bomb with a random fuse, ready to explode and cause irreparable damage to a bank account at any time in the future. In this article, we’ll take a look at what BMW rod bearing failure is, why it happens, and what you can do to prevent it from happening.
What is BMW Rod Bearing Failure?
Let’s kick things off by talking about what rod bearing failure actually is and why it is essentially a death sentence for an engine. To do that, we need to cover some engine anatomy first. We’ll try to keep from using too much technical garbage, but understanding how the piston assembly functions is important to understanding rod bearing failure as a whole.
First and foremost, what is a connecting rod? An engine’s connecting rods are responsible for connecting the pistons to the crankshaft. The larger end of the connecting rod is where the piston assembly meets the crankshaft. However, it isn’t as simple as just clamping the connecting rod around the crank journals. That’s where rod bearings enter the picture.
In order for the reciprocating motion of the piston moving up and down to be turned into rotational motion to turn the crankshaft, the area where the connecting rods meet the crankshaft journals needs to be smooth. That is the job of the rod bearings. In the case of the S54, S65, and S85, these rod bearings were initially made from layers of different materials including copper and lead. While that provides a pretty smooth surface between the con rod and crank, it still needs to be well-lubricated so that there is no metal-on-metal contact.
Typically, a lack of proper lubrication leads to rod bearing failure, where too much contact is made between the crank and the rod bearings, causing damage and scoring to the bearings themselves. That damage can then lead to other connecting rod damage that can do fatal damage to the engine. We’ll go into further detail about that in a bit.
Which BMWs Are Affected by Rod Bearing Failure
While there are reports of rod bearing issues in some lower-tier BMW engines like the N55, rod bearing failure occurs most often in high-performance BMW M engines that were designed for sustained high rpm operation. Rod bearing failure was very persistent in 2000s and 2010s M engines, as they all used similar rod bearing designs. The design and material of BMW rod bearings were later revised, reducing the frequency of the issue on newer M cars. Here are the cars and engines that were primarily affected by rod bearing issues:
- 2001-2008 S54-powered BMWs including E46 M3s, Z3Ms, and Z4Ms (extremely high rate of failure in 2001-2003 S54 models)
- 2005-2010 S85-powered BMWs including E60 M5s and E63 M6s
- 2007-2013 S65-powered BMW M3s
What Causes Rod Bearing Failure?
To this day, the cause of BMW rod bearing failure is a contested topic. If you venture over to the forums for a quick research session, you’ll see how heated the arguments can actually get. Regardless, most BMW enthusiasts, technicians, and BMW themselves agree that rod bearing failure occurs as the result of multiple different factors, including tighter than average rod bearing to crank journal clearances, incorrect oil choice, and improper maintenance.
Tight Rod Bearing to Crank Journal Clearances
One of the most commonly cited causes of rod bearing failure on S54, S65, and S85 engines is the tighter-than-normal clearance between the rod bearings and the crank journals. As we talked about a bit ago, the rod bearings are never supposed to make direct contact with the crank journals. There is always supposed to be a thin layer of oil that separates the two parts from coming into contact with each other, as that is what causes damage to the rod bearings.
All three engines that were heavily affected by rod bearing issues had extremely tight rod bearing clearances which hampered oil flow between the bearings and crank journals. Let’s use the Chevy LS3 found in the C6 Corvette for comparison. The LS3’s rod bearing clearance should be somewhere in the 0.0023 to 0.0043 inch range. In contrast, the S54 has an approximate .001-inch bearing to journal clearance, which is extremely tight compared to most performance engines.
Calculating rod bearing clearance for a particular engine is an entire science on its own, so we won’t put you to sleep with the details here. However, it is important to note that the reason that the clearances on S54, S65, and S85 engines were so tight boils down to how high they were designed to rev. Regardless, the extremely tight rod clearances make it difficult for oil to properly saturate the surface and prevent the bearings and crank journals from making contact. It’s even more of a problem if you fail to let the oil reach operating temperature before romping on it.
The most common argument against clearances being the only cause is that it doesn’t explain why some engines are affected early in their service life and others have made it over 100,000 miles on the original bearings.
Using the Wrong Type of Oil
As with any performance engine, the type of oil that you choose to use is extremely important. BMW has some pretty specific guidelines as far as oil requirements are concerned, and those requirements have changed over the years. BMW outlines that only 10W-60, BMW LL-01 specification oils should be used in the S54, S65, and S85. Additionally, it is crucial to use an oil that contains Zinc dialkyldithiophosphate (ZDDP) additives, as they are a crucial element in creating a protective layer between the bearings and crank journals.
In some experts’ opinion, newer 10W-60 oil formulas aren’t as effective at providing a protective coating as the oils that were available at the time that the engines were designed. That is due to the decreased phosphorus content in modern oil, as it helps to extend the life of O2 sensors and catalytic converters. Since the zinc in ZDDP additives are reliant on phosphorous to provide an adequate protective barrier, newer oils with less phosphorus don’t do as good of a job.
That isn’t to say that there aren’t still good oil options out there for the S54, S65, and S85. Castrol TWS 10w-60 is the best bet for all three engines, as it was specifically designed with a higher shear strength following the reports of rod bearing issues on early S54 engines. Shell 10W-60 is another common option for all three engines, but it does fall on the lower end of the SAE 60 viscosity range, meaning that it shears down into the SAE 50 range quickly, necessitating more frequent oil changes.
There’s no doubt that using the right kind of oil is extremely important, but changing your oil and staying on top of regular maintenance is just as, if not more, important. While rod bearing clearance and incorrect oil choice are definitely two valid places to look as far as rod bearing failure is concerned, waiting too long for an oil change is another huge factor too. With that being said, BMW is partially to blame for that too.
BMW’s official oil change interval recommendation for the S54, S65, and S85 was 15,000 miles. Enthusiasts and technicians alike agree that 15,000 miles is way too long to wait. Remember when I said in the previous section that some BMW-recommended oils were prone to shearing down to a lower viscosity early? Well, a 15,000-mile oil change interval exacerbates that issue, as some oil loses a significant amount of lubricative properties by then which significantly increases the chances of rod bearing damage.
Testing for BMW Rod Bearing Failure
Unfortunately, BMW rod bearing failure often occurs randomly and without warning, making it an even more stressful issue, especially if you daily drive your M-car. While there aren’t usually any warning signs before rod bearing damage becomes an issue (aside from maybe rod knock), there are ways to test the condition of your rod bearings to get a heads up if they are on the verge of failure.
BMW rod bearings in the S54, S65, and S85 are made from a number of different materials, including copper and lead. As rod bearing material wears away due to metal-on-metal contact between the bearings and crank journals, trace amounts of copper and lead are collected by the engine oil. One way of testing the condition of the rod bearings is to do an oil analysis that checks for the concentration of copper and lead in the engine oil.
In most cases, labs measure the contents of the engine oil with a provided sample and give a breakdown of their findings. Blackstone Laboratories is the most well-known oil testing company and is well known for testing BMW engines for rod bearing issues. Generally speaking, lead content of over 50 parts per million indicates that the rod bearings are excessively worn and will need to be replaced imminently. The same goes for copper, where over 50 ppm indicates excessively worn bearings.
How Do You Prevent Rod Bearing Failure?
To a certain extent, rod bearing issues on the S54, S65, and S85 are a luck-of-the-draw kind of thing. While some owners run into issues as early as 40-50,000 miles, others never run into issues. While there isn’t a rhyme or reason for BMW rod bearing failure, there are certain things that you can do to reduce the chances of it occurring.
Depending on how often and how hard you drive your car, BMW’s oil service interval should be halved. On the recommendation of many enthusiasts and reputable shops, daily driven S54, S65, and S85 BMWs should have an oil change interval closer to 10,000 miles at the most. If you are frequently tracking your BMW and spending a lot of time higher in the rev range, 5,000-mile oil changes are a very good idea.
Along the same lines as the other oil-related concerns, it is also crucial to let your engine, whether it be the S54, S65, or S85, warm up before doing any kind of spirited driving and ideally any driving at all. Cold oil has a higher viscosity than warm oil, making it even harder for oil to properly saturate the bearings which can lead to damage.
Avoiding excessively high revving is another solid preventative measure, as revving past 8,700RPM exceeds the designed load limit of the factory rod bearings, meaning that oil flow/pressure is inadequate to avoid metal-to-metal surface contact.
The last important preventative measure is ensuring that you stop your engines as soon as you see a low oil/low oil pressure light on the dash, as an inadequate oil supply or inadequate oil pressure is the fastest way to destroy rod bearings.
BMW Rod Bearing Failure Solutions
Outside of the preventative measures that I listed above, replacing your S54, S65, or S85’s rod bearings before they fail is another very common solution to make sure that no larger problems arise in the future. Since rod bearing failure is such a common problem on those three engines, there are a ton of independent shops that are well-versed in rod bearing replacement and aftermarket rod bearing options available for all three engines.
While there isn’t a set interval where you should preemptively change your rod bearings, the results of an oil analysis should be a pretty good indicator of when you should start to think about replacing them. If the analysis comes back with copper or lead anywhere near the 50 ppm mark, it is a good idea to start thinking about a replacement. Other enthusiasts say that 60-70,000-mile intervals are a good bet if you don’t want to do an oil analysis.
Aftermarket Rod Bearing Options
Some enthusiasts swear by the factory rod bearings, not because they hold up very well, but due to the fact that their wear can be easily measured with an oil analysis. While there are some aftermarket rod-bearing options available that are made of harder compounds, it is harder to tell their condition without disassembling the engine. As a result, softer rod bearings from manufacturers like BE-Clevite and ACL are both common options that are well supported by favorable reviews.
Since rod bolts can’t be reused on some engine variants, like post-February 2002-produced S54s, most S54, S65, and S85 owners opt to replace the OEM rod bolts anyway. There are a few reputable rod bolt options available for all three engines, APR is the route that most owners stick with. The ARP bolts are much stronger than the factory bolts, providing some peace of mind. It is important to note that, depending on the production date, the same engines can use different size rod bolts, so make sure you check the size of the bolts on your particular engine before ordering them.
Replacing the rod bearings on any of the listed BMW engines is a pretty in-depth job, especially for those who don’t have very much experience wrenching. For that reason, if you aren’t familiar with doing the job yourself, it is a good idea to find a reputable BMW-specific shop in your area to perform the replacement. Depending on the engine, rod bearing replacements can cost anywhere between $2,000 and $5,000. While that might seem like a lot, it’s nothing compared to the price of a new engine.
While it is a difficult job to DIY, it is still very possible if you have some engine-building experience. FCP made a fantastic video outlining the process on the S54 engine if you are interested in attempting the job.
So, How Big of a Deal Is Rod Bearing Failure Really?
While the frequency of rod bearing failure on S54, S65, and S85 engines is somewhat exaggerated online, it is a problem that should be taken seriously. A damaged or spun rod bearing can spell the end of an engine which is a worst-case scenario.
If you are looking to purchase an E46 M3, E9X M3, or S85-powered M5/M6, it is crucially important to find a car with complete service history. While rod bearing failure can be largely attributed to the design and tolerances of the bearings themselves, their health also depends on proper maintenance, correct oil choices, and diligent care. It is also a good idea to do an oil sample analysis to ensure that the rod bearings aren’t already in a compromised state.
Rod bearing failure is a pretty randomized issue that can affect S54, S65, and S85 engines somewhat indiscriminately. However, there are some steps that you can take to limit the chances of it occurring. Reducing the oil change interval to 5,000-10,000 miles instead of BMW’s recommended 15,000-mile interval is a good idea, using BMW-approved 10W-60 oil of course. Limiting the time that you spend at high RPMs and letting the engine oil warm before hooning is also crucial to rod bearing health.