5-ton 6x6 truck

The 5-ton 6x6 truck, officially "Truck, 5-Ton, 6x6", was a class of heavy-duty trucks used by the US Armed Forces. The basic cargo version was designed to transport a 5-ton (4,536 kg) 14 ft long cargo load off-road in all weather. In on-road service the load weight was approximately doubled. Through three evolutionary series (M39, M809, and M939) there have been component improvements, but all trucks were mechanically very similar. They were the standard heavy-duty truck of the US military for 40 years, until replaced by the Medium Tactical Vehicle (MTV) beginning in 1991.

History
In 1949 the Army set a requirement for a 6×6 off road truck to replace World War II 4 and 6 ton 6x6’s built by Brockway, Diamond T, Mack, and White with a single standardized chassis. International Harvester and Mack submitted designs, the International design was selected. Because of the Korean War it was rushed into production 1951, before it was standardized in 1953. Soon Kaiser also became a major manufacturer, with Diamond T and Mack building smaller numbers. In 1963 Kaiser-Jeep began building the final order, production was completed in 1965.

The M39 series originally had a gasoline engine, then were upgraded with diesel, then multifuel engines, with mixed results. AM General (successor of Kaiser-Jeep) developed an updated version of the M39 series. Standardized as the M809 series, the primary difference was the engine. The hood and grille were lengthened to make room for the larger engine, and the lighting system was updated to meet new US safety regulations. All had an air cleaner on the left front fender, a way to tell them from the earlier M39 series. AM General built all M809s between 1970 and 1982.

The M939 series was a Product Improvement Package of the M809, with updated engine, transmission, and brakes. A new, larger cab and tilt-forward hood were a major visual change from earlier trucks. In 1982 AM General began production of M939s.}

Engines
The M39 series were originally powered by a Continental R6602, a 224 hp 602 cuin inline 6 cylinder gasoline engine. The -A1 upgrade had a Mack ENDT-673, a 210 hp 673 cuin turbocharged inline 6 cylinder diesel engine. The -A2 had a Continental LDS-465-1A 175 hp 478 cuin turbocharged inline 6 cylinder multifuel engine.

The M809 series used a Cummins NH250 engine, a 240 hp 855 cuin naturally aspirated inline 6 cylinder diesel engine.

The M939 and M939A1 models used the Cummins NHC 250 engine of the M809 series, the M939A2 models use a modern Cummins 6CTA8.3 240 hp 504 cuin turbocharged inline 6 cylinder diesel engine.

Driveline
The M39 and M809 series had a Spicer 5 speed manual transmission with a "low" 1st. The transmission in the gasoline engine trucks was not well-matched to diesel and multifuel engines, so a different model was used. Still with a "low" 1st gear, 4th was direct and 5th was overdrive, requiring an unusual shift pattern. The M939 used an Allison automatic, for better engine speed control and driving ease.

A two speed transfer case also engaged the front axle. M39s and M809s used the same model, the M939s had an improved type.

Chassis
A ladder frame with three live beam axles, the front on semi elliptical leaf springs, the rear tandem on quarter elliptical leaf springs with locating arms. Brakes on the M39 and M809 were air over hydraulic with drum brakes on all wheels, M939s were full air. Many trucks were available with a front mounted 20,000 lb capacity winch.

There were three wheelbases (Measurements are from the centerline of the front axle to the centerline of rear tandem). The short, used for tractors and dumps, was 13 ft, the long, used for cargo, wreckers, and bolsters, was 14 ft, and the extra long, used for long cargo, tractor wreckers, and expansible vans, was 17 ft.

Most models had 11.00x20 size tires with dual rear tires, bridge trucks and some chassis-cabs had 14.00x20 with dual rear tires. Early M939s used 11.00x20s with dual tires, but M939A1s and A2s had 14.00x20s with single rear tires and a Central tire inflation system.

Cargo Truck
Standard cargo trucks had a 7 ft x 14 ft low sided box with a bottom hinged tailgate. It could secure a load but could not be loaded from the side by forklifts, so a body with drop sides was also standardized.

Long cargo trucks, with an extra long wheelbase, had a 20 ft long box. There were no drop side versions.

Dump Truck
Dump trucks were used to haul sand, gravel, dirt, rubble, scrap, and other bulk materials. They had a 5 cuyd dump body with cab protector and a tailgate that could hinge at either the top or bottom.

Tractor Truck
Semi-tractors were used to tow semi-trailers up to 37,500 lb lbs. The fifth wheel load rating is 15,000 lb.

Medium Wrecker Truck
Wreckers were used to recover disabled or stuck trucks and lift large components. A rotating, telescoping, and elevating hydraulic boom could lift a maximum of 20,000 lb. Although the truck was not meant to carry a load, the boom could support 7,000 lb when towing.

Tractor Wrecker Truck
Tractor wreckers, with an extra long wheelbase, were a wrecker with a fifth wheel mounted behind the boom. This let the truck perform wrecker duties and to load and tow semi trailers.

Expansible Van Truck
Expansible vans had a 17 ft long van body with a slide out section on each side. When the sections are extended the working floor was over 12 ft wide.

Bridge Transporting Truck
Bridge trucks had a stake body 20 ft long for carrying bridging equipment and components. In the M939 series there were no standardized bridge models, instead specialized bodies were mounted on chassis-cabs.

Bolster Truck
Bolster trucks, with a bolster trailer, were used to carry long loads like logs, poles, and bridge sections. When unloaded the trailer could be loaded onto the truck. There were no bolster trucks in the M939 series.

Chassis cabs
In addition to standardized models, bare chassis cabs were produced for specialty bodies. The largest, the M39 series M139C/D, was an Honest John rocket launcher.