Mack R Model: History, Engines, Variants and How to Spot One

A Four-Decade Run

Development of the R series began in 1965, and production overlapped with the outgoing B series to ease the change for customers. Model years run from 1966 to 2005. The R was built in Allentown, Pennsylvania, moving to a plant in Lower Macungie Township after 1975, and some versions came out of Hayward, California, until Mack closed that plant in 1980. Mack Australia and New Zealand assembly, plus licensed production in Iran before 1979, carried the design overseas.

The R arrived with ideas that later looked normal: an air-ride cab, an integral engine compression brake, and a drivetrain built to cut the need for an auxiliary transmission. Its one-piece, front-hinged fiberglass hood became an industry standard. In 1973 the cab got a deeper rear wall for more room and a new dashboard.

The R Family: R, RD, RL, RW and More

The letters matter when you are reading a title or a data plate. The plain R was the standard highway model. The RD was the heavy-duty version, and it received a new nose in 1991. The RS and RL were Western models with steel and aluminum frames, with the RL introduced in 1967. The RM and RMM were 4x4 and 6x6 municipal and maintenance chassis, and the RB, a setback front axle version, came along in the 1990s. Suffix letters add detail: T for tractor, S for six-wheel, L for lightweight and X for extreme duty.

The short-hood, offset-cab U model also belongs to the family, and so do the DM and DMM, the heavy-duty construction versions with the DMM adding all-wheel drive.

The RW is the branch that surprises people. Mack's RW700, sold as the Super-Liner, began as a Brockway project with a squared-off hood to fit a bigger radiator and engine. When Mack closed Brockway in 1977, it finished the truck itself. The Super-Liner used the R-series cab and model designation but rode on the frame of the WR Cruise-Liner cab-over. North American production ran from 1977 to 1993, and the Super-Liner has been sold in Australia since 1980, where locally built versions shared their frames with the R series.

Maxidyne, Maxitorque and the E6

The engine story is why the R became a legend among drivers. Mack offered other engines in the R, but the Maxidyne diesel defined it, starting with the ENDT-675 in 1966. It was an 11-liter inline six making 237 horsepower, modest on paper, with 906 lb-ft of torque at 1,200 rpm. The point was the torque rise: about 52 percent, against the 15 to 20 percent typical of the era. Power stayed close to constant from roughly 1,200 to 2,100 rpm.

That flat curve let Mack pair the engine with the five-speed Maxitorque transmission. With so wide a working band, a driver needed fewer, more widely spaced gears and less shifting than with the multi-speed gearboxes common at the time. The intercooled Maxidyne 300 series followed in 1973, making 285 to 315 hp and 1,080 lb-ft.

Mack's engine codes read like a recipe. EN meant gasoline, END diesel and ENDT turbocharged diesel, followed by a three-digit number loosely tied to displacement in cubic inches. Around 1980 the naming shifted to the E series, and in the 1980s the Maxidyne family evolved into the E6, built around a 672-cubic-inch block. By 1988 an M marked Maxidyne wide-power-range tuning, C marked California certification and L marked the low-speed Econodyne. The 728-cubic-inch (11.9-liter) E7 arrived in the late 1980s. Across the range, R-series ratings ran from about 200 to 440 hp, with Mack/Fuller manuals and Allison automatics listed.

How to Identify an R Model

Start with the hood. A one-piece fiberglass hood that tips forward from the front is the classic R arrangement. Some heavy-duty and municipal versions, often those carrying plows or front-mounted gear, used steel butterfly hoods instead, so a steel hood does not rule a truck out.

Next, check the nose. Round headlamps mark the original 1965-style nose, while the RD's 1991 front end, with square sealed-beam headlights, is the later face. A squared-off hood built around a big radiator points to an RW Super-Liner rather than a plain R.

The surest answer is on paper. The model code on the title or data plate, with its R, RD, RL or RW prefix and any T, S, L or X suffix, tells you more than any photo, and the engine tag gives the EN, END, ENDT or E6 designation.

The End of the R and the Granite

The highway versions were phased out in the 1990s in favor of the CH and CL. Severe-duty and construction versions held on much longer. Mack introduced the Granite vocational truck to dealers and press in Las Vegas in June 2001, and trade coverage at the time noted that the RD, then some 35 years old, had the smallest cab in the market. Mack called the RD the last of the R series and set its retirement for the end of 2003, marked by the R-Model Legend special edition with its own paint, badges, chrome, steering wheel and embroidered seats. How many Legends were built is unclear. The RB and DM were scheduled into later production, which is why 2005 is usually given as the final year.

The R left a long shadow. Its fiberglass tilt hood became standard practice, its Maxidyne and Maxitorque pairing changed how heavy trucks made and used power, and plenty of R models are still working dump, mixer and plow routes today.