Although Ford’s 390 cubic inch eight-cylinder engine was first produced for the 1961 production year, it was not a popular engine choice until the mid-1960s. The 390 is perhaps most notable for being the first big-block engine installed in the Mustang in 1967, although it was replaced by the 428 engine in the middle of the following year. The 390’s design varies slightly from most other engines of the time, such as the use of rocker arm shafts rather than pedestal-mounted rocker arms. For this reason, the 390’s factory torque specifications should be followed.
Tighten each of the 390’s spark plugs to between 15 and 20 ft-lb of torque.