The cylinder block and head of an engine is commonly referred to as "the box". Both the cylinder block and the cylinder head are parts that do not actually do anything. The pistons and camshafts perform work inside. For this reason, it is considered the box, inside which the parts that do the work. Since the birth of the Subaru's horizontally-opposed engine 36 years ago, the cylinder block and cylinder head have been made of aluminum.

When compared to in-line engines with the same number of cylinders, the horizontally-opposed and V-type engines have two cylinder heads making them slightly heavier. Subaru successfully reduced weight of its engines by using aluminum. The engine is called all-aluminum to distinguish it from engines in which only the cylinder head is aluminum. Even now there aren't many all-aluminum engines, but 36 years ago only special aircraft engines and racing cars adopted such material technology.

Subaru has a long history in developing mass-produced, all-aluminum engines. With a wealth of technical knowledge and experience, Subaru is well placed to promote research and development in this area. The cylinder block, which is the box containing the piston and the conrod, is cast at high pressure. This method involves pouring molten aluminum at high pressure into a metal mold. With this manufacturing method a thin precisely finished product is possible.

The cylinder block has a wide range of functions. If the cooling water jacket and lubricating oil-line does not function well the engine will overheat. In addition, the cylinder block contains the pistons and the crankshafts, which are very high-speed moving parts. This means that if the cylinder block is not rigid and highly precise the engine will not move smoothly. The precision required for parts such as a cylinder is in the order of a few microns. The cylinder head is bolted to the block by six bolts and a force of more than 6 tons is exerted on each bolt, so if they are not of high tensile strength, they will deform. The cylinders, precisely aligned in the cylinder block, are subjected to heat due to both combustion and piston movement and, although cooled, can be deformed easily. In addition, the attachment of the cylinder head is easily deformed. Even if deformed slightly, the pistons will not move smoothly. One difficulty in developing a cylinder block is that it must be rigid, but not heavy. Just because a part is not subjected to a load does not mean it can make it thinner. Reducing the thickness contributes to increased vibration and noise.

Complex technology must be focused and developed to satisfy a range of requirements related to the conrod, piston, cooling and lubrication. During development, simulations are conducted, but the cylinder block is a good example of vehicle engineering which incorporates elements of empirical science. The cylinder head contains the combustion chamber and a valve system, and therefore has more complicated requirements. These are the key mechanisms for engine performance and therefore the development of the cylinder head, which contains delicate tuning technology, is one of the most complicated parts. Compared to an in-line engine with the same number of cylinders, the horizontally-opposed engine has smaller cylinder head. Therefore this cylinder head is less likely to deform, a significant advantage. However, the complicated layout of air intake and exhaust are major design considerations. The cylinder head incorporates such parts as valves, the camshafts and a variable valve timing system. Therefore, a high level of rigidity and precision is required. In the combustion chamber, repeated explosions occur, requiring the cylinder head to be precisely fitted to the cylinder block. Since the intake and exhaust ports must allow good flow, the internal wall should not have any irregularities or obstructions. If enough oil doesn't t circulate around the camshafts, the part is worn away. In addition, cooling the cylinder head is of the utmost importance. To increase the effectiveness of cooling, it is necessary to keep the wall of the water jacket thin. However, if it is too thin rigidity may be insufficient. The cylinder head is where the balancing of each function must be brought together at a high level of precision.