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Tech - N55/ s55 Crank Hub issue and fix

BMW N55/S55 engines, found in all of the "35i" cars, M2/3/4 and x3/4M from 2009 to 2016 are amazing engines capable of producing power far beyond their factory levels, however they have a fatal flaw. The timing chain, which mechanically controls the relationship between the crankshaft and the camshafts, is turned by a gear on the crankshaft. The gear is locked onto the crankshaft by a "clutch style" plate, and herein lies the problem.

For decades, all timing chain gears have been mechanically locked to the crankshaft via a "key-way", or a small chunk of metal that is slotted into a perfectly carved opening in the crankshaft and timing chain gear, locking them together with no way for one to break free from the other. the N55/S55 "clutch style" system instead uses a friction disc in between the crankshaft and the timing chain gear, which is held in place by the tension supplied by the crank bolt when everything is torqued down to a very high torque spec. The issue with the system is that if there is a failure of this friction disc due to mileage/wear, increased power from aftermarket parts or aggressive driving, the timing chain gear can spin independently of the crankshaft or 'slip' from its correct orientation, causing significant drivability issues and potentially complete engine failure.

As always, the aftermarket has come up with a solution- the pinned crank hub. There are many pinned crank hub kits on the market, but our favorite is the kit from VAC motorsports. It is a comprehensive kit which includes the crank hub, a crank guard (in case a belt ever slips off the pulleys and tries to get pulled into the engine), a keyed oil pump gear, a new OE crankshaft bolt and the necessary drill bits for installation. We installed this kit recently on a customers M235i along with a full set of supporting modifications and an upgraded turbocharger, we can't wait for it to hit the dyno for tuning and put down some big numbers!

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