BMW Coupes and Sedans 1970-1988 Repair Guide



See Figures 1 and 2

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Fig. Fig. 1: Cut-away of 4-cylinder engine-2002 model

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Fig. Fig. 2: Cut-away of a 6-cylinder engine-3 Series model

The BMW 2 liter 4-cylinder engine and the larger 6-cylinder engines are of an in-line design. The block is made of a special gray iron, while the heads are of an aluminum alloy to save weight. The crankshafts are forged steel and are nitrite hardened. 4-cylinder engines use 8 counterweights, while 6-cylinders use 12, for optimum balance. 4-cylinder engines use 5 main bearings while the 6-cylinders employ 7; all main bearings are a replaceable, 3 layer design.

The cylinder heads employ valves located in an inverted V-pattern, allowing the use of hemispherical combustion chambers for optimum, swirl assisted combustion with minimum heat transfer to the cylinder head. This kind of combustion chamber also allows for use of the largest possible valves. The valves are actuated by a nitrite hardened overhead camshaft. Since they are not located directly below the shaft, they are actuated via light alloy rockers. This makes valve adjustment procedure very simple. The only special tool required is a simple piece of wire. Valve adjustments are made via a unique cam, located in the end of the rocker arm. Since the adjusting cams are separate from the lockbolts, and the lockbolts are prevented from turning via a flat at one end, it is easy to tighten down on the adjustment mechanism without changing the clearance. This minimizes the time required in performing a valve adjustment. Other engine features that prolong life are the use of chromium plated rings and valve stems, and forged connecting rods.

The outstanding feature of BMW engines is maximum output per cubic inch. This is possible through painstaking design of camshafts, valves, and the crossflow form taken by the intake and exhaust systems; the ram-type manifolds and fuel injection used on later models, which permits elimination of the restricting effect of the carburetor venturi; and the toughness and precise balance required to permit high run up to 6,400 rpm. The result is high performance without the fuel economy penalty usually associated with it.