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The engine blocks used in the Porsche 928 and 944 are well made pieces with very good cooling capacity.  These blocks are cast from an aluminum alloy referred to as Reynolds 390.  This was referred to as Alusil on the Porsche (and other) engine blocks.  While difficult to cast, Porsche's supplier got these right.  The basis for the all aluminum engine block is to maximise heat transfer and to run extra tight piston to wall clearances.  All 928 engine blocks are stamped with a tolerance group number afte the finish honing process is completed.  Each piston was selected based on the bore tolerance group.  While most engines have the same tolerance group for all bores, it was not impossible to get an engine block with two different tolerance groups.

In order for a piston to ride on the all aluminum bore, the blocks had to be finish machined and hones and then etched to expose the silicon particles.  Basically, the aluminum is etched away leaving the silicon particles standing proud of the aluminum substrate.  The piston skirts and rings are basically riding on the silicon particles.  This design allowed Porsche to run a very tight clearance between the piston and the bores which gives the pistons, rings and blocks and exceptionally long service life.  It is not uncommon to see these engines with 300,000 or even more miles on them without the need for a rebuild.

While the engines have proven themselves for decades, they are not without some inherent weaknesses.  The Alusil bores can be very sensitive to stuck rings or pistons and do not tolerate foreign objects very well.  The walls can be easily damaged.

If we fast forward to the requirements placed on a forced induction engine today, once can see that there is a need to reduce compression in many instances.  It is also a good idea to go with a forged piston to allow a custom design with a custom set compression ratio for the build.  A major problem surfaces here because there are no known good coatings that can be applied to aftermarket pistons that will tolerate riding on the Alusil.  Attempting to install a non coated aftermarket piston on an Alusil bore will always result in a disaster, one that may even render your engine block totally useless!

The good news is that there is a proven and cost effective solution to convert the Alusil engine blocks over to allow them to run any performance piston, coated or not.  The process of plating the block's cylinder walls with Nikasil (Nickel Silicon Carbide) allows one to restore the engine to the stock bore size, correct any out of round issues on the bores and run any piston possible.  The Nikasil has further advantages, in that it reduces friction because it has an inherent trait of holding onto the oil that is spread onto the cylinder walls.  Additionally, the coating is incredibly hard, thus making the bores very wear resistant.

The process is fairly straightforward.  Your engine block is compeltely stripped, then cleaned and masked.  Bores are plated, then rough honed and finally diamond honed to achieve the desired piston to wall clearance.  For a 95 to 100 mm bores running a 2618 forged piston, typical clearance is 0.0025", somewhat less for a 4032 forged alloy piston.  This method is, by far, the best possible option when you are looking to build an all-out boosted 928 or 944 engine.