So, you've heard about the dreaded S55 crank hub failure which affects the 3-litre S55 engine found in the F8x BMW M3, M4, and M2 Competition models.
As an avid BMW Stan, you'll find yourself asking what are the symptoms of the crank hub on the S55? What's the common failure of the S55? What causes crank hub failure? Let's find out...
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What's the crank hub issue?
Take the vast majority of modern engines (high performance or otherwise), the crankshaft drives the camshaft via a belt or chain with gears locked in place on both shafts with a keyway. The piston and valve relationship should always stay the same to maintain piston-to-valve clearance. Any unintended slippage of the chain/belt will vary the timing - ultimately leading to catastrophic engine failure, resulting in a rising piston meeting several fast-moving valves.
It's a mystery why BMW's engineers secured the crank hub with a single bolt, accompanied by a friction material. No keyway or pins, just friction, and prayers.
The infamous S55 crank hub failure can occur after a high shock load from changing gear, or harmonics over time loosening the crank bolt. This may happen over time, or all at once. It seems that every engine is different as everyone’s driving style is different. Putting those two points aside, the failure is catastrophic.
What BMW models are affected?
Let's keep this in perspective, we estimate that under 10% of S55 engines ever suffer from a spun crank hub; standard or modified. That being said, clearly the higher the state of tune, the higher the risk of failure.
The 3-litre S55 engine shares the same flawed crank hub design as the ever-popular N55 and N54 engines, however, it's much less frequent an issue. Why's that? Simply, the S55 is much easier to extract higher power outputs from, making it more susceptible to failure once tuned. They're so similar that the S55 and N55 share identical OE crank hub part numbers and the N54 is followed closely behind on similarities too.
So if you're running a high-powered, or hard-driven S55, N55, or N54, we'd certainly recommend our crank hub upgrades to protect your engine and wallet!
What are the symptoms of a slipped crank hub?
Don't expect any warning symptoms of a crank hub failure. This isn't something that occurs gradually over time, nor is it something that you can check for. If your S55, N55, or N54 hub is going to slip, it will occur all of a sudden.
If the ECU senses that the timing has slipped even a minute amount, it will trigger a drivetrain failure EML on the dash. Expect a rough idle and reduced engine performance. This is the best-case scenario.
The next stage in the 6-cyl self-destruction is to prevent the engine from starting after further slippage has occurred. Annoying, we know - but one that will save you from some really big bills. At this stage, the problem is fixed by replacing the OE crank hub and to carefully re-time the engine. Once the crank hub has slipped like this, the only course of action is to replace the failed hub, as they're a one-time-use part from BMW.
However, in the absolute worst case, a spun crank hub will cause serious internal engine issues, 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 with an already slightly spun hub.
How to fix a crank hub failure and what are my options?
Right, there are two parts to this answer. Firstly, preventative steps to take before your M-power engine experiences the dreaded crank hub failure. Secondly, assuming crank hub failure has already occurred, how to repair your crank assembly to make sure it doesn't 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 still runs smooth. 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 eight bolts.
The result? With the capture plate bolted up tight, the crank bolt simply cannot come undone. This alone is enough to save you an awful lot of future headaches and wallet pain - thank us later.
If you would like more specific info on our capture plate and purchasing options, click here.
For the ultimate preventative measure, which we'd strongly recommend if you're planning serious track use or tuning your S55, N55, or N55 powered 6-cyl; install a single-piece crank hub. We highly recommend installing this along with our capture plate, so you've got absolute peace of mind of no future failures occurring.
In the world of crank hubs, you have two options. Either a pinned or an unpinned crank hub. In essence, a pinned crank hub contains a selection of pin dowels that require holes to be drilled into the end of your crankshaft, uniting your crank and crank hub. An unpinned crank hub doesn't utilise the above mentioned pin dowels.
There is no doubt that a pinned crank hub is the absolute ultimate preventative and upgraded part to use when pushing your tuned M-Car to the limit. It's also worth pointing out that it's the most expensive, time-consuming, and risky to install compared to the capture plate or unpinned options.
So, my crank hub has slipped. What now?
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 for the lifetime of your car.