The M123 engine family was a straight-6 automobile engine from Mercedes-Benz, used from 1976 to 1984 in the W123 series models 250 and 250 T. It replaced the 6-cylinder engines M180 in the 230 and M130 in the 250 versions from the W114 series.

The Mercedes-Benz M103 is a straight-6 15 degrees, crossflow cylinder head with an inclination to the right, automobile engine produced from 1984 to 1997. It is a single overhead cam design with 2 valves per cylinder. The M103 was replaced by the M104 starting in 1989. This engine received several updates over the years of production, one of the updates was a change from a spherical combustion chamber to a "heart shaped" combustion chamber.