If you’re reading this, you’re probably considering purchasing (or currently own) a BMW vehicle with the 5.0-liter S85B50 V10 engine. If you want to learn more about its issue with connecting rod bearing failure, keep reading. What is the S85 V10 engine? Inspired by BMW’s previous involvement in Formula One, the S85 engine is a high-performance V10 produced from 2005 through 2010 and exclusively powered the automaker’s M-series variants of the E60/E61 5-series sedan (called M5) and E63/E64 6-series coupe and convertible (called M6). The S85 was also used in the Wiesmann GT MF5 2-seat roadster for a limited time from 2009 through 2010. It uses a DOHC valvetrain layout with the company’s version of Variable Valve Timing, called VANOS. As of today, it was BMW’s first and only V10 engine. What is the issue? Despite the S85’s relatively small production quantity and use in very few vehicles, a well-known issue impeding long-term reliability is excessive connecting rod bearing wear. Yes, even an expensive, meticulously designed engine is not exempt from catastrophic problems and design oversights. The root cause of this problem, though, is debated, especially in owner forums. Connecting rod bearing failure occurs over time if insufficient oil cannot properly lubricate the bearings. Some believe the issue stems from the bearing design itself, the engine’s tight tolerances, or the oil weight the automaker recommends. All three combined may be responsible for the issue. By far, the most prominent theory is the oil viscosity BMW recommends because it may be too thick to lubricate the bearings properly. With the S85, BMW recommends an oil viscosity rating of 10W-60, which is unusually thick for a modern engine. Most gas engines used in cars today require a viscosity between 0W-20 or 5W30. To put this in perspective, at operating temperature, BMW uses oil in this engine that’s three times thicker than a 20 weight and twice as thick as a 30 weight. 30-weight oil is already reasonably thick. Because this engine is high-performance, its manufacturing processes result in tolerances much tighter than what is found in average, everyday engines. Tight tolerances mean smaller gaps and space between internal components. This in and of itself isn’t a bad thing, but problems can arise if the passageways are too small and especially if the oil used is too thick, where it can’t easily flow through the passageways to lubricate the rod bearing and journal. This inadequate lubrication may lead to accelerated wear on other parts, such as the engine’s VANOS system, since its operation depends on correct oil pressures. Too thick of oil could result in too high of a pressure, which can damage the system. What should I do if I own or want to buy a vehicle that has this engine? Unfortunately, there’s not a simple one-time fix to prevent this engine’s rod bearing wear because several different things are likely contributing to it, and some of which cannot be changed, such as tolerances. With this engine, it is not a question of if the rod bearings will fail, but when, and maintenance and luck determine when it will happen. Replacement could be deferred as long as 80,000 miles or more or as early as 50,000 miles. At first, the solution to preventing this issue may seem to use a slightly thinner oil, but it’s essential to consider the implications a thinner oil has on its other components, such as VANOS. Like rod bearings, variable valve timing requires oil to flow through tiny passageways and could have even tighter or looser tolerances than the rod bearings. For example, a 30 weight oil may adequately lubricate the VANOS, but not the rod bearings, but a 40 weight may lubricate the rod bearings, but not the VANOS. In other words, it’s possible that, because of the way different components are designed and their differing tolerances, one oil weight cannot satisfy all lubrication requirements this engine has. If this is the case, it’s a lose-lose situation for both the owner and the engine. The engine is slowly destroying itself from the inside out, and the only thing the owner can do is try to slow the rate of wear. Regardless if owners decide not to use thinner oil, the best actions they can take to slow down rod bearing wear are to change the oil at a shorter interval than BMW recommends and let the engine fully warm up before driving. When the engine reaches operating temperature, the oil becomes slightly thinner than it was when it was cold, where it will have somewhat better flow properties. It’s also worth mentioning that engine oil containing more Zinc may help with the metal-to-metal contact this engine experiences. Zinc helps prevent this contact between engine parts by forming a protective film, and is commonly used in racing engines (the S85 is considered one). Using oil containing more Zinc won’t guarantee the complete elimination of this premature rod bearing wear, but it can extend the time until the part’s replacement is required. With this engine, other symptoms of worn bearings include metal shavings in the engine oil and/or a ticking sound at low RPM and idle (though ticking could stem from VANOS instead). If metal shavings are present in the oil, the rod bearings need replacing as soon as possible to prevent other problems. Preventative maintenance is critical with this engine. This repair is serious and cannot be put off for too long, or it will only cost more money. Should I buy a vehicle with this engine? Before answering this question, I think a better question to ask first is, “Do I want a fire-breathing, 500 horsepower car with a top speed of 190 miles per hour?” If your answer is no, this isn’t the engine for you, and something more mainstream would be a wiser choice. If your answer is yes, you should then ask, “Am I willing to spend thousands of dollars on maintaining this car, with a lot of the cost going toward the engine?” Owning a BMW M5 or M6 with this engine isn’t for the faint of heart, as its reliability is poor and maintenance costs are sky-high. Even within the owner community, it’s regarded as one of the least reliable or worse BMW engines ever made. If you want this engine, you desire a car that’s track-ready (if you can even find a track to race on) and seek a performance thrill leaving your heart pounding, no matter how many reliability headaches it will cause you. The S85 is one of my favorite engines due to its performance and unmistakable sound, but I would be very hesitant to own one because of its cost of ownership. If you want to buy one, I strongly suggest requesting all maintenance records the owner has before buying, hoping the engine’s rod bearings were already replaced. That alone will save you thousands of dollars. It may be worth trying different oil weights other than BMW’s recommendation in hopes of mitigating the rod bearing wear, but this is risky. When I researched the correct oil for this engine, I briefly came across a comment from an owner that a 10W-40 weight with extra Zinc mitigates the bearing wear, but more research is needed. Given I don’t own this engine, I can’t personally verify this as a definitive fix. Opinion The S85 V10 is one of BMW’s most beloved yet hated engines, depending on whether you ask a car enthusiast or a mechanic who works on them. Historically, BMWs aren’t known for long-term reliability, but the engineers could have made this engine more reliable had they designed it a little differently, starting with slightly looser tolerances. This engine is a great example of the importance of using an engine oil that can adequately lubricate all components rather than only a few. With every engine, it’s essential to use an oil viscosity that’s not too thick and not too thin, but also is ok to use depending on geographic location (i.e., utilizing a thinner oil in the winter, thicker in the summer). In most cases, it’s best to follow the automaker’s recommendation stated in the owner’s manual, but there are cases, such as the S85, where recommendations are not correct. Interestingly, with the next generation M5 (F10 and F90) using the twin-turbocharged S63 V8 rather than the S85 V10, the recommended oil weight is no longer a 60 weight and is now a 30 or 40 weight. This makes me wonder if BMW realized the oil used in the S85 was too thick and partly responsible for catastrophic damage, so they decided to use a thinner weight for their future engines and design tolerances accordingly. A question that may never be answered, though, is how long the automaker knew about this problem, as they had little to no interest in addressing the issue despite a mountain of owner complaints. Additional resources Rod Bearing Question | M5 Board Forum Upgrading Rod Bearings | M5 Board Forum Now you know the truth behind BMW’s troublesome S85 V10 engine. I hope you have found this information informative and helpful. Still have questions or comments about this issue? I'd love to help. Feel free to leave them in the comments section below.
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