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The MMX Performance S55 / N55 One-Piece High HP Crank Hub Upgrade Kit addresses one of the best-known failure points on BMW's turbocharged S55 and N55 engines. The factory crank hub is a three-piece assembly that relies on friction to hold its position on the crankshaft. Under high load it can slip rotationally, and once it does the timing moves far enough for piston-to-valve contact. The repair bill after that is substantial.
The MMX hub replaces it with a single-piece design machined from heat-treated billet EN36 steel. Rather than relying on friction, it is mechanically locked to the crankshaft by four aerospace precision-machined dowels. That is the difference: the hub cannot rotate independently of the crank, so the timing relationship holds regardless of load, boost or how hard the engine is being driven. This four-pin version is the one to specify on high-horsepower builds.
Installation requires drilling the crankshaft, so the kit is supplied complete with the drilling jig and drill bit needed to do it accurately. This is not a job for a home garage: the crankshaft is being permanently modified and the alignment has to be right, so professional installation is required.
The kit is available with or without the MMX capture plate. The capture plate is a billet aluminum hat that bolts over the entire head of the crank bolt, making it physically impossible for the bolt to back out under the engine's vibrations and harmonics. Selecting the version with the plate covers both failure modes at once.
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