M96 Engine failures

[ladismantler]

996 engine suffers from a perceived lack of strength from its cylinders, which are susceptible to cracks.
The M96 (and also M97) engines use Lokasil, a metal matrix composite preform cast into the crankcase block, as a cylinder bore material.

Lokasil would be great if the surrounding material was strong enough to retain its shape and the cylinder block was sufficiently cooled.
Unfortunately, because its weaker and unsupported, the bore can warp into an oval shape until it either seizes (because the increased blow overheats the piston to the cylinder interface)
or cracks (when the shape exceeds the elastic limit of the material)

Incedently, the 2.5 & 2.7 Boxster and the 3.2 Boxster S had thicker supporting walls than the 996 (which was effectively the Boxster S casting bored out another 3mm in diameter)
Resulting in the M96 being more susceptible to failure usually after around 60 000 miles of use.

The chance of this happening to your 996s slim but if you do suffer from a cracked or seized cylinder, they can be reformed by fitting restraining rings that not only prevent further movement and cracks.
Restraining rings also stabilize the liner by converting it to a closed deck design.

Aside from cylinder bores warping or cracking, there are also reported issues of cylinder heads cracking. When a cylinder head is cast, the thickness of the aluminium between the outside and the inside of it is governed by the design of the casting. For some reason on both the M96 $ M97 engines there is an area in which the wall suddenly gets thin. Right near the spark plug location.

As the engine starts to warm up, the inside of the cylinder head expands first, then the coolant and finally the outside faces.
This results in different expansion rates that put strain on the parts.Eventually leading the formation of a fatigue crack.
It usually appears between the oil gallery under oil pressure and the coolant gallery inside. This causes the oil to flow into the coolant under pressure creating a horrible mixture of sludge that is eventually forced out from the overflow leading to overheating inside the engine and potential piston seizure.

The main preventative measure to uphold here is driving style. Cars that are driven too hard before they have warmed up sufficiently and those that have been used for short trips will have a shorter life span. Those warmed up steadily and driven for long journeys at constant speed will last the longest. Proper maintenance will also help prolong your engines life.