Changes during the 67-72 run were minimal, but notable. In 1968, to comply with FMVSS (Federal Motor Vehicle Safety Standards) marker lights were added to the rear bedside and front sides of the hood. 1970 saw a switch from metal bar style grillwork to the plastic eggcrate style. Sixth generation (1973–1979) The sixth-generation F-series was introduced in 1973. This version of the F-series continued to be built on the 1965 fourth-generation's revised platform, but with significant modernization and refinements, including front disc brakes, increased cabin dimensions, full double-wall bed construction, and increased use of galvanized steel. The FE engine series was discontinued in 1976 after a nearly 20-year run, replaced by the more modern 335 and 385 series engines. In 1975, the F-150 was introduced in between the F-100 and the F-250 to avoid certain emission control restrictions. For 1978, square headlights replaced the previous models' round ones on higher trim package models, such as Lariat and Ranger, and in 1979 became standard equipment. Also for 1978, the Ford Bronco was redesigned into a variant of the F-series pickup; 1979 was the last year that the 460 engine was available in a half-ton truck. Seventh generation (1980–1986) The seventh-generation F-Series was introduced for 1980, marking the first ground-up redesign of the model line since 1965. Alongside an all-new chassis, the pickup trucks received a completely new body. While distinguished by straighter body lines, the aerodynamics of the exterior were optimized to improve fuel economy. Sharing their cab structure with F-Series pickup trucks, medium-duty trucks (F-600 through F-800) underwent their first redesign since 1967. The powertrain line of this generation underwent multiple revisions through its production. At its launch, the engine line was largely carried over from 1979. While the 7.5 L V8 was dropped entirely, a 4.2 L V8 was introduced as the smallest V8 engine. For 1982, a 3.8 L V6 became the standard engine for the F-100. For 1983, to improve the fuel efficiency of the model line, the M-Series engines (the 5.8 L 351M and 6.6 L 400 V8s) were dropped; the latter was replaced by the return of the 7.5 L V8. In response to low demand and poor performance, the 4.2 L V8 and 3.8 L V6 were phased out in 1982 and 1983, respectively. For the F-250 and F-350, a 6.9 L diesel V8 (sourced from a partnership with International Harvester) became an option