The power unit has a longitudinal arrangement in the engine compartment and can only be removed upwards using a special crane.
The gray cast iron engine block has cylinder bores. With significant wear or scratches on the walls, the cylinders can be honed, that is, ground in a special workshop. After that, only oversized pistons should be installed. At the bottom of the engine block is a crankshaft mounted on its bearings. The connecting rods that communicate with the pistons are mounted on the crankshaft on plain bearings. The lower part of the engine is completed by an oil crankcase, in which the oil necessary for - lubrication and cooling of the engine is collected. On top of the engine block, a cylinder head made of light metal alloys is bolted. It is aluminum, since this metal, in comparison with cast iron, has a higher thermal conductivity and a lower specific gravity.
The cylinder head is built according to the so-called cross-flow principle. This means that the freshly prepared air-fuel mixture enters the cylinder head from one side, while the burnt gases are pushed out from the other side of the cylinder head. Cross flow design provides faster gas exchange. At the top of the cylinder head is the camshaft. The camshaft is driven from the crankshaft via a single-row roller chain on a four-cylinder engine or via a toothed belt on a six-cylinder engine. The camshaft controls the V-shaped intake and exhaust valves.
Engine lubrication is provided by an oil pump mounted on the four-cylinder engine at the front of the engine crankcase. On a six-cylinder engine, the oil pump is located in the oil sump and is driven through an intermediate shaft. Oil from the oil sump enters through the channels to the crankshaft and camshaft bearings, as well as to the working surfaces of the cylinders.
The water pump is located at the front of the engine block. The water pump is driven through a V-belt, which also serves to drive the generator. An additional V-belt drives the power steering pump. It should be borne in mind that the cooling system must be filled with a mixture of antifreeze and anti-corrosion concentrate with lime-free water throughout the year.
A carburetor or fuel injection device is used to prepare a flammable air-fuel mixture, which, as a rule, does not require maintenance.
The ignition spark is generated by a transistorized ignition system that keeps the ignition timing almost constant. Depending on the type of engine, the ignition distributor is mounted either on the left side of the engine block or on the rear side of the cylinder head. On models 320i, 323i, it is driven through an intermediate shaft. In turn, the intermediate shaft is driven from the crankshaft through a toothed belt. On models 316, 318i, and 325i, the ignition distributor is driven directly from the camshaft.