M123 and M125 10-ton 6x6 trucks

The Mack M123 (G792) was a 10-ton 6x6 semi-tractor introduced in 1955. The Mack M125 was a heavy cargo truck version of the M123. The M123 was used to tow tank transporter trailers while the M125 towed field artillery pieces.

History
In 1949 the US Army set a requirement for a family of extra heavy-duty 10‑ton (9,072 kg) load rated, off-road, 6x6 tactical trucks. Mack's design, influenced by their WWII era NO ($7 1/2$-ton, 6x6 truck), was chosen. Designed as an entire family of trucks, only the semi-tractor and cargo/prime mover were built. 392 M123 were built between 1955 and 1957 and 552 M125 were built between 1957 and 1958.

In 1965 CONDEC began building 3188 diesel powered M123A, Mack began building 420 in 1968. In 1969 Mack then upgraded 210 gas engine models to diesel power.

With the Army using more self‑propelled artillery, the need for prime movers was reduced, no follow‑up orders for M125 were made.

Both M123 and M125 saw service in the Vietnam War. M123 tractors remained in Army service into the 1990s.

In addition to the US Army, the M123 was used by the U.S. Marine Corps and exported to U.S. allies in such countries as Australia and Spain.

Engine and driveline
All M123 and M125 built in the 1950s had a LeRoi T‑H844 engine, a 844 cuin overhead valve V8 gasoline engine developing 297 hp at 2600 rpm and 725 lbfft of torque at 1700 rpm. The M123A1 built in the 1960s had a Cummins V8‑300, a 785 cuin naturally aspirated V8 diesel engine developing 300 hp at 3000 rpm and 580 lbfft torque at 2100 rpm. This engine was also retrofitted into M123 to make M123E2. No M125 were converted to diesel power.

The drivetrain was a repeat of that in the Mack NO, with a Mack TRDXT72 5 speed transmission with the transfer case mounted solidly to the rear of the transmission. This transmission was direct in 5th. The transfer case had a 2.50:1 low range, engaged the front axle, and had a power take-off to operate the winch(es).

The front axle was an unusual triple reduction type which did not need universal joints on the outer ends and allowed a tighter turning radius. The two rear axles were a double reduction type. Final drive ratios were 9.02:1 for gas powered trucks and 10.11:1 for diesels. The driveline for every M123 and M125 was built by Mack, regardless of who manufactured the rest of the truck.

Chassis
A ladder frame chassis type was used. The front beam axle was mounted on leaf springs, the rear tandem beam axles were mounted on a leaf sprung "walking beam" type suspension. All models shared a 181 in wheelbase (measured from the 1st to the 3rd axles). Full air drum brakes were used. The M123 was able to control the trailer brakes independently of the tractor. The M125 had electrical connections for the artillery piece brakes. In both models, the tire size was 14.00x24.

The main differences between various chassis was the location of the 45,000 lb winches and fifth-wheels. Different M123 models had either one or two winches mounted behind the cab, all M125 had a single winch at the front.

The M15A2 was the trailer that the M123 was intended to tow. It had a higher load plate and larger coupling pin than a standard semi-trailer. Early M123 had a larger fifth wheel mounted above the frame rails, but as more standard trailers were used, the M123C and all following models had lowered fifth wheels. The pin remained larger, so any trailer towed by any M123 had to have an interchangeable pin.

A standardized REO designed cab, also used in the 2 1/2 and 5‑ton trucks, was used. It had hinged doors with roll-up windows, a folding windshield, and a removable canvas roof. A hard roof could be fitted. With the same cabs and similarly designed fenders and hoods, the main visual difference between these three models was the size of the vehicle and the scale of the cab to the vehicle.

M123
The M123 was used to tow semi-trailers carrying MBT's (in conjunction with the M15A1 and later the M747 semi-trailers) superseding the M26(A1) Pacific "Dragon Wagon" and the semi-trailers carrying engineering equipment (in conjunction with the M127 low bed semi-trailers).

Variants:
 * M123 had dual winches and high mounted fifth wheel, gasoline engine
 * M123C had a single mid-ship winch and low mounted fifth wheel, gasoline engine
 * M123D had dual mid-ship winches and low mounted fifth wheel, gasoline engine
 * M123A1 had a single rear winch and a V‑8 diesel engine (300 hp).
 * M123A1C, built from 1965 to 1969 by CONDEC, had a diesel engine and single rear winch
 * M123E1 were M123's that were upgraded with the diesel engine.
 * M123E2 with dual rear winches, diesel engine

M125
The M125 succeeded the Mack NO as a heavy cargo transport vehicle; it also served as a prime mover towing the 155‑mm gun and 8‑in. howitzer. The chassis was designated M121. It had a 14 ft steel cargo body with folding troop seats, bows and canvas cover. A single front-mounted Garwood DSA716 45,000 lb capacity winch was fitted, and a chain hoist at rear for handling ammunition.