Crank Hub Failure in BMW S55 & N55 – What can I do?

At Progressive Parts, we understand the concerns of BMW owners regarding the infamous Crank Hub Failure affecting S55 and N55 engines found in high-performance M-models such as the M4, M3, and M2 Competition. These engines, renowned for their power and tunability, have faced criticism due to the crank hub issue, which seems out of place for such finely engineered vehicles.

What’s the deal with the S55 & N55 Crank Hub issue?

In the intricate workings of modern engines, like the S55 & N55, the crankshaft plays a crucial role in driving the camshafts through a chain or belt mechanism. Any unintended slippage in this setup can wreak havoc on the engine’s timing, potentially leading to severe consequences, as pistons collide with swiftly moving valves.

What’s puzzling is BMW’s engineering choice in securing the S55’s crank hub, responsible for driving the timing chain with its toothed gears. Rather than employing traditional methods such as keyways or pins for precise alignment, BMW opted for a seemingly simplistic approach: bolting the hub tightly with a high-friction sintered washer in place. This strategy relies heavily on torque and friction, leaving little room for error.

Enter the notorious ‘S55 crank hub failure’ – an unfortunate event where either abrupt shock loading, like during gear shifts, or prolonged exposure to engine vibrations gradually loosens the crank bolt. Once this occurs, the hub loses its grip, leading to rotational slippage and a significant deviation from accurate timing synchronization.

Which BMW models does it affect?

Let’s put things into perspective: based on our estimates, less than 10% of S55 engines will encounter a spun crank hub issue throughout their lifespan. Interestingly, this problem appears to affect both modified vehicles and those maintained to BMW’s standards, though the risk escalates with higher levels of engine tuning. Each S55 engine behaves uniquely, much like every driver’s style behind the wheel.

It’s noteworthy that the S55 engine shares the same flawed crank hub design found in the popular N55 engine (with many parts also overlapping with the N54). However, instances of spun crank hubs are relatively rare in N54 and N55 engines compared to the S55. Why the discrepancy when the design is identical? The answer lies in the S55’s capacity to achieve significantly higher power outputs, placing greater strain on internal components and consequently increasing the likelihood of failures.

Therefore, whether you’re driving a high-powered or aggressively used N55 or N54, we strongly advise considering crank hub upgrades to safeguard your engine, just as we recommend for S55 owners.

Here’s the problem: the factory crank-hub, showing the individual pieces. These are the geared rings that eventually slip.

How can I tell if my BMW has had any Crank Slip?

Unfortunately, expect no warning signs of crank hub failure.

It doesn’t manifest gradually over time nor is it detectable through routine checks. If your S55’s crank hub decides to slip, it will do so abruptly.

Should this occur, your car’s ECU will promptly detect the timing discrepancy, triggering a “Drivetrain Failure” warning on the dashboard accompanied by rough running, unstable idling, and decreased engine performance. Consider this the best-case scenario.

As the timing drifts further from its optimal setting, the ECU takes a preventive measure by refusing to start the engine. Though inconvenient, it’s the ECU’s way of safeguarding the engine from potential damage, sparing you from hefty repair bills.

In instances like these, resolving the issue entails replacing the crank hub and meticulously recalibrating the timing. Once the hub has slipped, there’s no recourse but to swap it out, as BMW deems them single-use items. A mere tightening of the crank bolt won’t suffice. While the fix may not be budget-friendly, rest assured that internal engine damage is unlikely.

However, in severe cases, a spun crank hub can wreak havoc on the engine’s internals, necessitating a comprehensive engine rebuild. Such catastrophic damage typically arises if the hub slips at high engine RPM, like during aggressive downshifting on the track, or if the vehicle continues to operate with a compromised crank hub.

How to fix my S55/N55 once Crank Hub Slip has happened?

This answer is divided into two parts: proactive measures to prevent crank hub slippage in your S55 and steps to repair your engine in the event of crank hub failure, ensuring it doesn’t recur.

Let’s address preventive measures first: A Crank Bolt ‘Capture Plate’ serves as an economical initial defense for vehicles showing no signs of slippage and still operating smoothly. After confirming the engine’s timing is spot-on, the capture plate is installed over the crank bolt’s head and securely fastened to the crank pulley assembly with 8 bolts. The outcome? With the capture plate tightly secured, the crank bolt remains firmly in place. This solution proves effective for most vehicles, and though installation typically demands 4-5 hours of labor, it can potentially save considerable costs and anguish in the long term.

• MMX Crankhub Capture Plate BMW S55/N55 £144.13

For the utmost in preventative measures—particularly if you intend to engage in rigorous track activities or push your engine to high power thresholds—we strongly advocate for the installation of a ‘Single Piece Crank Hub,’ crafted from a solid billet of hardened steel. Pairing this with the Capture Plate ensures absolute peace of mind throughout your car’s lifespan. While the upgrade may require significant expertise for installation, you can rest assured knowing that you’ve permanently rectified BMW’s issue.

Now, you face a decision: should you opt for a pinned or unpinned Single Piece Crank Hub?

At MMR Performance, we now offer two variations of the Single-Piece Crank Hub, leaving you to ponder: pinned or unpinned? For most S55 & N55 engines, ranging from standard to Stage 1 tuning, the unpinned MMR Single-Piece Crank Hub suffices. Its installation is more straightforward, as it doesn’t necessitate precision drilling of the crankshaft’s end to accommodate the four locating dowels.

• MMX One Piece Crank Hub Upgrade With Capture Plate BMW S55/N55 £653.89

Nonetheless, for engines boasting high power and torque levels surpassing this threshold, we advocate for the upgraded pinned version of the Crank Hub. This advanced iteration incorporates a 4-pin dowel drive mounting system, ensuring unparalleled engine security.

Installing the pinned Crank Hub involves drilling four holes into the end of your crankshaft to accommodate the 4 pins dowels utilized by the MMR hub. While this may be considered the most extreme solution, there’s no doubt it stands as the ultimate answer in terms of safeguarding your engine.

• MMX Crank Hub 4-Pinned Single Piece Upgrade BMW S55/N55 £818.99

What can I do if Crank Slip has already happened?

In the unfortunate event of crank-slip occurrence, bad luck.

Even if your engine has escaped damage, your only recourse is to replace the affected components, as they are deemed single-use parts by BMW. Once any slippage, no matter how minimal, has transpired, the bolt cannot be re-tightened.

Now comes the moment of decision. While BMW offers to rectify the issue with a new set of identical replacement parts, many informed owners opt against re-installing these factory components, mindful of the looming possibility of a recurrence of the same problem.

Instead, the wiser course of action is to install our Single Piece Crank Hub, fortified with MMR’s Capture Plate. Although addressing the crank hub slippage entails financial strain, you can now relish absolute peace of mind throughout your car’s lifespan.

• MMX One Piece Crank Hub Upgrade BMW S55/N56 £562.16
• MMX Crank Hub 4-Pinned Single Piece Upgrade BMW S55/N55 £818.99