worn set of rod bearings

The E9x M3 and Rod Bearings: What to Know

Introduction: The BMW E9x M3 is an iconic car that has won the hearts of many automotive enthusiasts and critics around the world. With its impressive handling performance and exhilarating driving experience it's a fan-favorite for a reason. However, like any high-performance vehicle, it can have its share of problems and requires regular maintenance to ensure optimal performance and longevity. Among the critical maintenance tasks specific to the E9x M3 platform, one that deserves special attention is the replacement of the rod bearings. In this article, we will explore the importance of changing out the rod bearings and the impact they have on the overall performance and reliability of the E9x M3.

Understanding Rod Bearings: Rod bearings play a crucial role in the engine's functionality by connecting the crankshaft to the connecting rods. These bearings provide a smooth surface for the crankshaft to rotate on and ensure proper alignment and support for the connecting rods and pistons. The bearings are essentially 2 halves of a circle that meet together to form a perfect circle over the crank, and these halves are held in place by the connecting rod, connecting rod cap, and connecting rod bolts.

How Failure Occurs: Over time, the constant high-speed rotation, extreme temperatures, and stress placed on the bearings can cause wear on the bearing, leading to catastrophic failure known as "spinning a bearing" or "shooting a rod". The OEM bearings have allegedly been found to be of improper clearance and will often wear very unevenly, this is visible with the naked eye by simply observing the spotting or friction marks along uneven portions of the bearing. In Pre-LCI models, the base material of the bearings was made of copper, and could easily be found via copper traces in oil analysis tests. For those with LCI E9x M3's, the bearings were changed to aluminum, a more commonly found material, making oil analysis tests with aluminum content inconclusive. This is why an oil analysis test is not always an indication of bearing health and preventative service should still be highly considered. The severity and result of rod bearing failure will vary from needing an engine rebuild with a new crank and connecting rods, to the worst case scenario needing an entire new short block if a hole is blown in the motor, or long block if head/valvetrain damage has occurred. The cost of a new engine from BMW at the time of this writing is around $20,000 with used options ranging from $6,000-$14,000 depending on mileage and condition.

What To Look Out For: Symptoms of faulty rod bearings include abnormal engine noise, particularly during cold startup or under load, decreased oil pressure, and increased oil consumption. Unfortunately every case is different and not everyone will be fortunate enough to get a warning sign. For a majority of instances of rod bearing failure there is usually no tell-tale signs that it was going to occur. Neglecting the warning signs that do present themselves can lead to more extensive engine damage if not taken care of promptly.

The Importance of Rod Bearing Replacement: Replacing the rod bearings on the E9x M3 is crucial for maintaining its performance and preventing engine failure. Upgrading to high-quality aftermarket bearings, such as those made of stronger materials and featuring improved clearance, can provide increased durability and reliability, not to mention save you thousands of dollars. Due to alleged suspicions of improper clearance of the OEM bearings it is not recommended to use them as a replacement as there is still a large chance of premature failure, unlike aftermarket bearings with proper clearance that have been documented to go over 80k-100k miles safely, and is often considered a "permanent solution" by many. There is no guaranteed mileage when the original bearings may fail, however in all my time working in a BMW performance shop I have seen M3's spin a bearing as early as 28k miles, and others lasted well past 100k miles before failure. By proactively replacing the rod bearings you can mitigate the risk of engine damage and enjoy a longer-lasting, smoother-running M3.

Performance Enhancement Opportunities: Upgraded rod bearings can offer improved load-bearing capabilities, reduced friction, and enhanced life-span. These factors can contribute to a smoother engine operation for years to come and reduced wear on your crank. For enthusiasts seeking to optimize their E90 or E92 M3's performance, selecting high-performance rod bearings designed for track or spirited driving can be a worthwhile investment. Examples include ACL and BE Bearings. 

Connecting Rod Bolt Upgrades: One more thing you may want to consider upgrading along with your rod bearings is the connecting rod bolts. Changing them is required as the original rod bolts are known as "stretch bolts" and cannot be reused under any circumstances. While brand-new OEM connecting rod bolts could be used, it is almost never advisable as there are far better options on the market. Connecting rod bolts are responsible for clamping the rod bearings down onto the crank by holding together the connecting rod cap and connecting rod. With a component like this that is equally important to your rod bearings it is recommended to go with a high quality upgrade. High performance examples of upgraded rod bolts can be found from ARP and Carrillo. 

Doubts And Skepticism: Online forums are littered with conflicting opinions and views, such is life. However mechanical knowledge and first-hand accounts easily disprove many claims that rod bearing failure is a myth. Aftermarket bearings designed with extra clearance, such as the ones offered on our site, are known to last upwards of 80k-100k miles safely, a significantly higher average range than factory bearings. This alone assumes a possible correlation to bearing clearance issues from factory, as well as other allegedly questionable practices and the lack of a nationwide recall. Many of the same people who once doubted the possibility of rod bearing failure have suffered it themselves at one point or another by missing their window to remedy the issue. There are plenty of supporting articles, forum posts, and videos to prove the failure is all too real, and we hope all of our readers never have to experience it. 

Conclusion: Upgrading the rod bearings is an essential part of caring for this legendary car. By replacing the bearings while you still can you ensure the engine's longevity, reliability, and optimal performance. Upgraded rod bearings not only mitigate the risk of engine failure but also offer an opportunity for performance enhancement for all those track days and canyon runs. Consulting with experienced automotive professionals is crucial for proper install and service, and selecting the most suitable replacement bearings and bolts. By prioritizing the health of your E9x M3's rod bearings, you can continue to enjoy the thrill and excitement this vehicle offers for years to come!

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