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Engine block

Engine block


All basic engines are fluid cooled 4-cylinder in-line engines with 4 valves per cylinder, twin overhead camshafts and 2 balancer shafts integrated in the cylinder block. The engines are equipped with multiport fuel injection and are cross-flow type, i.e. the intake is on one side of the combustion chamber and the exhaust on the other.

The engine is mounted transversely and is tilted forwards 8°.

A cylinder capacity of 2.0 litres (B204i/B204L) and 2.3 litres (B234i) provides high engine torque even at low speeds, which is an advantage in normal traffic conditions.

The balancer shafts neutralize vibrations and forces from the moving parts in the engine while reducing unwanted engine noise.

Cylinder block


The cylinder block is a special one-piece casting with the cylinder bores drilled directly into the block. Special oilways for the lubricating system are also drilled into the block.

There are two tunnels for the balancer shafts in the block. The tunnels have aluminium bearing bushings pressed into them. The bushings are replaceable without requiring machining.

Pistons


The pistons are made of light alloy and have grooves for two compression rings and one oil scraper ring.

The top compression ring has a straight edge and is chrome or molybdenum coated. The bottom ring has the characteristics of an oil scraper ring and is slightly wider than the top one, while the oil scraper ring itself consists of three parts.

Connecting rods


The connecting rods are forgings and incorporate bushings for the gudgeon pins. The gudgeon pin bushings and big-end bearing shells are replaceable.

The gudgeon pins are of fully-floating type, being free to turn in both piston and connecting rod. Axial movement of the gudgeon pin is limited by circlips in the gudgeon pin holes.

Crankshaft assembly


The crankshaft is forged and has induction-hardened ground journals. This produces a hard surface finish, which is resistant to wear. The crankshaft has five main bearings, of which the middle one also functions as a locating bearing axially. Oilways are drilled in the crankshaft for lubrication. All main bearing shells are replaceable.

Cylinder head


The cylinder head is precision-manufactured in light alloy and bolted to the cylinder block. The combustion chambers are hemispherical with 4 valves per cylinder and centrally located spark plug. This improves the flow of gases in the cylinders, providing efficient combustion and increased engine power.

Timing cover and end plate


The timing cover and end plate are both designed to suit the form of the cylinder block. The oil pump is mounted in the timing cover. The timing cover and end plate are both sealed to the block with Loctite 518.