‘ When the bolt isn’ t clamping the washers, there isn’ t the necessary friction and the timing sprocket moves on the crank. Once it has worked its way loose, if there’ s any movement on that diamond washer, that washer is trying to grip both sides; it’ s likely to break and a section will spit out of the side. Then you’ ve lost part of the height in the assembly that the bolt is clamping. They go really loose when that happens,’ says Ben.‘ Most of the failures tend to be just a drivetrain fault; they won’ t cause mechanical damage. Most of the cars we see for crank hub upgrades are more for peace of mind, and it adds value to know that it’ s all done.’
Some cars are more prone to the crank hub shifting.‘ Dual-clutch gearboxes, because they shift so quickly, they cause a shock through the engine when it’ s changing speeds so suddenly, which seems to worsen the effect. Also, late M2 Competitions seem to be the worst for some reason. The more torque the engine is making, the more it seems to affect it, too.’ Not ideal for an engine so loved by the aftermarket tuning scene.
A bolt that loosens and allows an engine’ s timing to be affected, that sounds like a significant design flaw. But, perhaps it’ s not quite as prevalent as the rumours would suggest.‘ In all honesty, I haven’ t seen any failures at all, but we’ ve done lots of upgrades. There are probably a lot of cars out there that haven’ t had problems. There are plenty of documented problems, but I do wonder if it’ s a bit of an internet scaremongering thing.’
There are cars that Ben has worked on where you’ d have expected to have seen issues, yet still nothing.‘ We had a car that was running, I think, 700hp. It had bigger turbos and all the relevant supporting modifications, but it had the standard crank hub setup; it didn’ t have anything done to it. The crank bolt is some ridiculous torque, and then you have to turn it another 270 °, so it’ s bloody tight. It’ s a two-man job to undo it or do it up. We have a huge scaffold pole that is our“ special tool”. But on this car, the bolt came undone with just me with a fairly small ratchet, it was that loose. If that was driving around with a bolt that loose and hadn’ t had a problem, then you wonder how far they have to go to actually have a failure.’ Ben’ s not suggesting it’ s all a myth, though.‘ There’ s certainly an issue which is well worth looking into, but not that many seem to get as far as having a timing slip.’
Late S55-powered M2 Competitions seem to suffer from crank hub slip more than any car, according to Hack Engineering
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