Mercedes-Benz M123 engine

The M123 engine family was a straight-6 automobile engine from Mercedes-Benz, used from 1976 to 1984 in the models 250 and 250 T of the 123 series. For this application, the M123 engine took the place that the M180 in the 230 (6-cylinder version) and the M130 in the 250 (2.8 liter version) had within the 114 series.

The M123 was succeeded by the M103, introduced in 1984 with the 124 series.

Some fundamental characteristics of the M180 remained unchanged with the M123: It only has four crankshaft bearings and the oil pump, fuel pump and distributor are driven by a shaft inclined at the front end of the motor, which in turn is driven through a sprocket and a helical gear pair.

On the other hand, different from the M180 where the cylinders are cast together in pairs, the M123’s cylinder spacing is uniform like in the M130. So, instead of cooling ducts cast between the cylinders, there are slots sawed in the cylinders’ partition walls, traversed by cooling water.

The engine has a single overhead camshaft (SOHC), driven by a chain. The vertical hanging valves are arranged in two rows, operated by rocker levers. As with the M115, inlet and outlet both are on the right side of the engine. Within the cylinder head there is (on the left side of the engine) a recessed combustion chamber (with a flat piston crown), in which the exhaust valve and the spark plug are located while the remaining area (on the right side) forms a squish area that is intended to swirl the fuel-air-mixture in the combustion chamber.

Fuel-air mixture was generated by a Solex 4A1 carburetor. It is of the double-register type, as with the previous model W114. That means each of the two sets of mixing systems feeds three cylinders and each of these two sets is composed of a first relatively small mixing system for idling and a second, larger mixing system for higher load and higher speeds. At idle and up to about one-third load only the smaller tube of each set is mechanically operated by the accelerator pedal, while the second tube of each set is enabled by a vacuum cell only at high load and high engine speed. All four (2 x 2) tubes are supplied with fuel by a central float chamber. The engine is equipped with gasoline feed line and gasoline return line (which usually was more typical for fuel injection engines) achieving a cooling effect which protects from vapor locks, that would cause starting problems in hot weather.

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