When a rebuild is performed on an engine having a cast iron block and an aluminum head, it is necessary to always, and without exception, replace the head bolts.

This is recommended by the engine manufacturer and must be done or there is a high probability of engine failure. Head bolts in bi-metallic engines are of necessity specially designed. Since the thermal expansion rate of aluminum and cast iron are greatly different, and aluminum grows much more it makes the head bolts role very critical in maintaining correct clamping pressures on the head gasket. As the engine is started and begins to heat up the head bolt must stretch as the head grows in thickness. When the engine is stopped and allowed to cool the head bolt must shrink to its original length to keep the head gasket sealing for the next cold engine start. When the bolt had been submitted to months and years of repeated stretching and shrinking eventually it will fail. If the bolt is removed for overhaul and reused, the re-torque procedure can further increase chances of the bolts failure.

When failure occurs and the head gasket allows antifreeze to enter the combustion chamber and lubricating oil serious engine damage will result.

(P55)