Most often, the loose nut in any old BMW is behind the wheel. Driving a virtually worthless Bavarian sports sedan is liberating, and can give typically sensible drivers the confidence to do some not-so-sensible things. However, if you drive certain BMWs hard enough, another nut can come loose loose with the potential for disastrous consequences.
So what does this nut do? Does it hold the steering wheel on, keep the differential input yoke attached, or hold the crank pulley to the engine? Nope, but it’s just as critical as anything you could imagine. I’m talking about the oil pump nut.
Under prolonged hard use, the oil pump sprocket retaining nut (number 11 in the image below) can back out on M54, M52, M50, and S52 engines installed in, well, pretty much every inline-six-powered BMW made between 1990 2006. In fact, it’s easier to list exceptions: The E32 735i, the E34 535i, the E46 M3, and the 2000 to 2002 BMW Z3 M Roadster and Coupe.
Mind you, this issue is most commonly reported on models with the M54B30 three-liter inline-six, such as the E46 330i, E39 530i, and pre-facelift E85 Z4 3.0i, so owners of cars with those engines are particularly attentive to this issue, with a litany of forum threads dedicated to the topic.
Before we get into what happens when the oil pump nut backs out, we need to answer why the M54B30 seems to be the most affected. Well, there may be some bias in failure reporting since the three-liter variant is the most powerful M54 and therefore the most preferred engine for performance driving. Performance driving causes this particular failure, so it makes sense that an outsized number of oil pump nut failures happen on the most powerful motor. Perhaps unsurprisingly, if you drive a 2.5-liter car hard enough, the oil pump can still back off.
One forum post might not seem like the greatest of evidence, but it comes from a reliable source. Let’s just click that link and see where it goes.
Yep, it’s Throttle House with Thomas but without James. Small world! So, what actually causes the oil pump nut to back off in the first place? Well, our friends at FCP Euro have succinctly detailed exactly why this failure mode happens.
Now, the reason why the oil pump nut has the tendency to come loose on the engines mentioned above is mostly due to the lack of a chain guide/tensioner for the oil pump chain. It does have some slack in it (which isn’t a problem per se), but the problem ultimately comes down to how the sprocket is mounted to the pump. The nut itself is relatively small and isn’t torqued very tightly (only 25Nm). The nut is reverse threaded which means it tightens as you spin it counterclockwise, and while this does help keep the nut in place, it’s not a surefire guarantee. When any of the listed engines go through constant and rapid RPM changes (such as being used on the track), the slack on the oil pump chain in a way acts as an impact gun on the nut, so over time, it can come loose.
Needless to say, the oil pump nut coming loose is a very bad thing because once it lets go; nothing’s holding the oil pump sprocket in, which means it can slide off of its splines and stop spinning the oil pump. Considering that every moving part inside the engine needs a film of oil to not self-destruct, losing the pressure that delivers that film of oil results in disastrous consequences. With the average 20-year-old BMW having the Kelley Blue Book value of a used Trojan, once that oil nut backs off, it could be the end of the car.
Even crazier? BMW may have known all along. E46 330i models exclusively with the ZHP performance package got an oil pump nut with thread locker applied. All other cars with the M54 inline-six? No thread locker. For those wondering about part numbers, the ZHP oil pump nut is part number 11417897238, while the standard oil pump nut is part number 0711990857. However, thread locker isn’t foolproof, so the aftermarket has come up with its own solutions.
One such solution is to safety wire the oil pump nut to the oil pump sprocket. It’s cheap, it’s relatively easy, it requires no special tools, and it’s a complementary upgrade to replacing a leaky oil pan gasket. Some people prefer to weld the oil pump nut to the shaft of the oil pump, but that’s really up to whatever you feel comfortable with.
If you have more than, I don’t know, $20, there are other, fancier upgrades out there that use extra hardware to prevent the sprocket from coming loose. How about three Allen bolts instead of a nut on a splined shaft? Yeah, those ought to hold really nicely. Granted, these solutions are probably overkill, but if you’re building an engine rather than just replacing an oil pan gasket and doing a complementary upgrade, why not?
If you aren’t actively competing in motorsports or doing a ton of trackdays, you probably don’t have to worry about your BMW’s oil pump nut coming loose. However, if you’re a particularly enthusiastic driver with a current Snell-rated helmet and own an older BMW, you might want to tend to your oil pump nut before you’re replacing an engine in your driveway.
(Photo credits: BMW, Amazon, E46 Fanatics, eBay)
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