1. Rear main seal (RMS)
The RMS sits between the crankcase and the clutch housing. On most mass-production engines, where crankshaft and crankcase are cast and machined together, the RMS rarely causes problems.
However, because its new water-cooled engine was very high power by 1997 standards (300bhp from 3.4 litres), Porsche employed a much higher-tech approach by basing the crankshaft in special alloy inserts to give the crankcase greater rigidity. This entailed machining the case and the crank individually, rather than together, which meant that alignment of the bolt threads was not always micron-perfect. Distortion of the rubber seal over time could then cause an oil leak.
Experts such as Hartech’s Barry Hart have long emphasised that this isn’t serious and provided the leak doesn’t worsen, it can often be left until the clutch is due for replacement. RMS failure became less common on later M96/97 engines as Porsche moved to a new seal made from a compound of rubber and plastic, whose inherent elasticity made it self-realigning.
This solves the problem, says Hart, who’s rebuilt “about 2,000” engines from the M96 family, but he’s not necessarily critical of this aspect of
