You have probably read on the internet and various forums about the ‘Crank Hub issue’ which affects the 3 litre S55 engine found in the F8x BMW M3 and M4, and the M2 Competition models. The question you are probably asking as an owner, is should you be concerned? The answer unfortunately is yes as this issue affects both tuned cars and those that have remained completely stock with a full BMW service history. More DCT cars seem to be affected than 6 speed manual, but we shouldn’t forget that more DCT cars were actually sold by BMW!

So what is the issue?

In traditional engines (read: pretty much any other engine made in the last 100 years) the crankshaft attached to the pistons turns the camshaft(s) attached to the valves, via a belt or chain. The gears are locked in place on the crankshaft and camshafts with a keyway, so that the relationship between the pistons and valves is always the same i.e. there is no chance of the valves opening at the wrong time and hitting the pistons. This is known as the ‘timing’ of the engine.

Unfortunately BMW in their wisdom have thrown out the past 100 years of development on the internal combustion engine and instead decided to lock the crank gear to the crankshaft with nothing more than a diamond sintered washer, which offers high friction, and a load of torque from the crank bolt to hold the timing in place. In certain circumstances, a high shock load from changing gear, or harmonics over time loosening the crank bolt, the bolt moves and no longer holds the timing accurately. This may happen over time, or all at once, it seems that every engine is different as everyone’s driving style is different. When the timing goes out by a set amount, it triggers a “Drivetrain Failure” on the dashboard which results in rough running, rough idle and reduced engine performance. When the timing goes out by a further amount the ECU will actually prevent the engine from starting, because of risk of damage. We are not aware of any complete engine failures with OEM components from the crank hub slipping yet, as the ECU does seem to do a good job of shutting the engine down. However once it has slipped, the only course of action is replacement of the affected parts, as they are ‘one use’ only from BMW.

We have fitted many other solutions and have been unhappy with most of them.

There is a ‘fix’ available that machines a portion away from the crank gear in order to make a keyway to lock to the crank. On the bench this is a good solution, but we have seen three fail in short order, and in a much worse way than the OEM BMW parts. Typically they crack where the material has been removed, and part of the crank gear then separates and falls inside the engine. At best this requires immediate removal, and at worst it allows the pistons and valves to hit. It can be an expensive repair.

Machined ‘Fix’:

A) This flat face is not actually flat and machined 90 degrees to the shaft

B) The splines that are machined into the top of the hub are designed to cut into the crank to help keep it secure.  These splines don’t even put a mark on the crank face and the teeth disintegrate.

C) These pre-cut teeth have 0.2mm run out, which will cause the chain to run incorrectly.

The other solution available uses splines on a replacement crank hub that get driven in to the crankshaft as the bolt is tightened. Again a good idea, but we have seen some where the splines have not been machined accurately and as a consequence, the whole hub runs out of true and moves around in the front crankshaft oil seal, resulting in a permanent oil leak! We have also seen this solution where the material used was softer than the crank itself, so as it was driven in, the splines disintegrated!  

At Litchfield we have developed a two part solution: 

We have a developed a crank bolt capture plate, which differs from those already available on the market. If your car has shown no issues to date and is running fine, then this is a good first step. With our BMW diagnostic laptop, we can plug in to your car and ascertain if the crank bolt has slipped at all – remember, the ECU only shows an error once it has slipped a set amount. Assuming it checks out, the capture plate can be fitted over the top of your crank bolt and secured to the crank pulley assembly. This process typically takes 4-5 hours labour. This is enough for many vehicles and we provide a 12 month warranty on the part, such that should you see a crank hub failure in that time, we will deduct the cost of the labour to install the capture plate from the cost to install the full crank hub replacement, the second part of our fix.

If your crank bolt has slipped and you have an error on your dashboard then you automatically need the second part of our fix which is a replacement crank hub assembly, with oil pump drive pulley and cam drive pulley already installed. This is due to the fact that the OEM parts are all one use only, and if any slipping has occurred the bolt cannot just be re-tightened. As part of the re-assembly, we will also fit a crank bolt capture plate. This complete solution is still a drive in/drive out solution but we would require the car for two days. We have developed our own service tools* to fit these parts, which make the process quicker than using the BMW shop tools (which also aren’t up to the job), as the crank bolt must be set to a very specific high torque. Within the cost, we include and replace all consumables including a full oil and coolant change, new crank bolt, new crank main seal and cam cover gaskets. Our solution does not require drilling of the crank.

*Because the full fix requires installation with own tools, torque settings and knowledge this isn’t something we can send out.  All work would need to be completed in our workshop but trade enquires are still welcome.

If you have any question about our F8x tuning or crank hub options just let us know.