Porsche M96 / M97

Porsche's first water-cooled flat-six — the engine of the 996 and 997 911 and of the Boxster and Cayman of that era, and the design that ended thirty years of air cooling.

It is defined by the intermediate shaft bearing, universally known as the IMS. The bearing supports a shaft that drives the camshaft chains; when it fails, debris circulates and the engine is generally destroyed. Failure rates were low in absolute terms but the consequence is total, the bearing design changed several times across production, and whether a given car's bearing has been replaced is the single most valuable piece of history about it. Later engines moved to a non-serviceable design.

Bore scoring on the larger-displacement versions is the second theme, presenting as ticking and oil consumption. Both faults are internal, both are expensive, and both mean an oil analysis and a filter inspection are worth far more on this engine than a scan tool is.