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  • Writer's pictureMMR Performance


You’ve no doubt heard about the dreaded Crank Hub Failure on BMW’s S55-engined M-cars. As the S55 is fitted to the high-performance, very tuneable M4, M3 and M2 Comp models, it seems like an uncharacteristic error by Munich’s finest engineers.

So, as the owner of one of these cars, should you be aware of this problem? Unfortunately, yes, you should.

What you need to know about the BMW Crank Hub Failure.
BMW S55 M2C I M3 I M4: Is Crank Hub Failure something to worry about?

So what’s the S55 & N55's Crank Hub problem?

In pretty much any modern engine, high-performance or otherwise, the crankshaft drives the camshafts via a belt or chain. Any unintended slippage of this chain will vary the timing - which can ultimately be pretty catastrophic, as a rapidly rising piston gets to meet several fast moving valves.

So it’s a bit of a mystery to the world quite why BMW’s engineers would decide to simply secure the S55’s crank hub, with its geared teeth driving the timing chain, by simply bolting it up really, tightly with a high-friction sintered washer in between. No keyway, no pins used to locate it, just a load of torque on the bolt - and hope for the best!

The infamous ‘S55 crank hub failure’, as it has become known, is that fateful moment where either a high shock loading from a gearshift or over time, the engine’s vibrations and harmonics loosen the crank bolt. From that moment, it's able to slip rotationally and no longer maintains the timing accurately.

This clearly shows the difference between the failure-prone factory Crank Hub (you can see the joins, this is where it will eventually slip). On the right, the MMR Single Piece unit, with no possibility for slippage.
Left: the OE Crank Hub I Right: the MMR Single-Piece Crank Hub

Which BMWs does it affect?

Let’s keep this in perspective – our estimate would be that under 10% of S55 engines ever suffer from a spun crank hub in their lifetime. The issue seems to affect modified cars as well as completely standard ones with full BMW service history, but clearly the higher the state of tune, the higher the risk. It seems that every S55 engine is different, just as everyone’s driving style is different.

It’s worth adding that the S55 shares the same flawed crank hub design as the ever-popular N55 engine (and also many of those parts are also shared with the N54). So although the N54 and N55 can occasionally experience spun crank hubs, it’s more of an isolated and uncommon issue on these engines. Why is that, when the design is shared with the S55? Simply because it’s so much easier to extract much higher power outputs from the S55 engine – and these additional internal loads inevitably mean a higher likelihood of failures elsewhere.

So if you are running a high-powered, or hard-driven N55 or N54, we’d certainly recommend the Crank Hub upgrades to protect your engine just as much as we would for S55 owners.

Here you can clearly see the problem waiting to happen. The two gear rings are the problem - once the bolt's grip loosens, these rings are free to rotate.
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 Crank Hub Slip has happened?

Don’t expect any warning symptoms of crank hub failure. It's not something that occurs gradually over time, nor is it something that you can check for. If your S55 crank hub is going to slip, it will occur all of a sudden.

If it happens to you, when the ECU senses that the timing has slipped by a certain small amount, it triggers a “Drivetrain Failure” on the dashboard with accompanying rough running, rough idle and reduced engine performance. This is your best-case scenario.

The next stage is when the timing goes out by a greater amount the ECU will actually prevent the engine from starting, to protect itself from the risk of damage. Annoying as that may seem, it’s the ECU doing a good job of shutting the engine down to save you from some really big bills.

In the cases above, the problem is fixed by replacing the crank hub and carefully resetting the timing. Once the crank hub has slipped like this, the only course of action is replacement of the failed hub, as they are ‘one use’ only from BMW. Simply retightening the crank bolt will not work. This won’t be a cheap fix, but you can console yourself that no internal engine damage is likely.

However, in the worst cases, a spun crank hub will cause serious internal engine problems, requiring a full engine rebuild. Such damage will likely occur if the crank hub slips at high engine rpm, such as a downshift on-track, or if you continue to drive your S55 with an already spun crank hub.

Here you can see the two elements of the S55 N55 Crank Hub protection. The Single Piece Crank Hub is on the left, the Capture Plate is the part on the right.
Left: the Single Piece Crank Hub I Right: the Capture Plate

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

There are two parts to this answer. Firstly, preventative steps to take before your S55 experiences the dreaded Crank Hub slippage. Secondly, assuming crank hub failure has happened to you, how to repair your engine to make sure it doesn’t ever happen again.

Let’s start with preventative measures: a Crank Bolt ‘Capture Plate’ is an affordable first step to protect a car that has shown no slippage issues and is still running fine.

Having checked that the engine is timed perfectly, the capture plate fits over the head of the crank bolt and is secured to the crank pulley assembly via 8 bolts. The result? With the capture plate bolted up tight, the crank bolt simply cannot come undone. This solution will be enough for most vehicles, and although fitting typically takes 4-5 hours of labour it could save you an awful lot more cost and heartache in the longer run.

For the ultimate preventative measure, which we’d strongly recommend if you’re planning either serious track use, or running your engine at high power outputs, install a ‘Single Piece Crank Hub’, machined from a solid billet of hardened steel. We highly recommend fitting this along with the Capture Plate, so you’ve got absolute peace of mind for the lifetime of your car. Whilst it’s an involved job for an expert to compete this upgrade work now, you’ll be safe in the knowledge that you’ve corrected BMW’s problem once and for all.

But you have a further choice to make: a pinned or an unpinned Single Piece Crank Hub.

As MMR Performance now offers two versions of the Single-Piece Crank Hub, which is best for you, pinned or unpinned? For most standard to Stage 1 tuned S55 & N55 engines, the unpinned MMR Single-Piece Crank Hub is more than sufficient. Installation is more straightforward as the end of the crankshaft does not need to be precision drilled to mount the four locating dowels.

The pinned Crank Hub, showing the 4 dowels that mechanically locate the hub.

However, for high power/torque engines tuned beyond this level, we recommend the upgraded pinned version of the Crank Hub that incorporates the 4-pin dowel drive mounting for the ultimate engine security. Fitment of the pinned Crank Hub requires four holes to be drilled in the end of your crankshaft to insert the 4 pins dowels that the MMR hub uses. It's the most extreme solution, but without question it's the ultimate answer.

What if Crank Hub Slip has already happened?

Finally, if crank-slip has already occurred, bad luck. Even if no engine damage has occurred, you now have no choice but to replace the affected components, as they are single-use parts from BMW. Once any slipping, even the slightest amount has occurred the bolt cannot be re-tightened.

This is your moment of choice. BMW will fix the problem with a new set of identical BMW replacement parts. But most knowledgeable owners opt not to re-fit these factory parts again, in the sure knowledge that the same problem occurring again is a distinct possibility.

Rather, the wiser choice is to now install our Single Piece Crank Hub secured with our Capture Plate. Although you’ve got the financial pain of fixing the crank hub slippage, at least you’ll now have absolute peace of mind that for the lifetime of your car.

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