The BMW E12 BMW 5-Series was made between 1972 and 1981. The E12 was the first series to bear the 5 Series name: the '5' denoting BMW's fifth 'New Class' platform. Designed as a replacement for the popular BMW New Class mid-size sedan, the E12 5-Series models were smaller than the large BMW E3 sedan but larger than the two-door 2002 models.
Designed to compete with the Mercedes-Benz W114 sedans, the E12 models were fitted with a variety of engines. 1.8 L and 2.0 L M10 I4 engines from the older sedans were used in the 518 and 520, respectively. A six-cylinder version of the 520, built with a 2.0 L M20 belt-driven engine was also available from 1977 on.
Source: Wikipedia, 2012