Mack M100SX Restored: Giant 29.3L V12 Now Roars Again

Historic Mack M100SX Comes Back to Life

The largest truck ever built by Mack — a hulking M100SX that had been sidelined for more than five decades — has been fully restored and can run again. Purchased years ago by the Mahan Collection foundation in New Jersey, this one-of-three behemoth has been returned to operational condition after an exhaustive restoration that took roughly two and a half years.

Powertrain and performance

At the heart of the M100SX is a Detroit Diesel 12V149T two-stroke V12 — a 29.3-liter powerhouse that makes around 1,000 horsepower at 1,900 rpm. The engine’s deep, mechanical roar echoes through the center exhaust stacks and is a reminder of an era when brute force and simplicity defined heavy-duty truck design. Despite its tremendous output, the truck’s gearing is extremely low; top speed is limited to about 45 km/h, reflecting the M100SX’s original role in mining and heavy construction rather than highway freight.

Specifications at a glance:

• Engine: Detroit Diesel 12V149T, 29.3L V12, two-stroke
• Power: approximately 1,000 hp at 1,900 rpm
• Length (bumper to chassis end): 9.78 m
• Height: 4.11 m
• Curb weight: ~43.3 tonnes
• Gross vehicle weight rating: up to 150 tonnes (payload capacity exceeding 100 tonnes)

From tipper to tractor: a working life

Originally built in the 1960s as a heavy-duty dump truck for earthmoving and off-road sites, the M100SX later received a custom trailer and was converted into a tractor unit. Only three M100SX units were ever produced; this example is the last surviving unit and the third built. Historical records show it was first delivered on January 21, 1970, to Hoffman Rigging in New Jersey. It subsequently spent time in Texas with Mashburn Trucking before making its way back to New Jersey — a journey that, at the truck’s modest cruising speed, took roughly two weeks.

One of the truck’s most impressive feats was supporting the expansion of Newark Liberty International Airport. Coupled with its bespoke eight-axle trailer, the M100SX reportedly moved up to 1,000 tons of rock and aggregate during airport construction projects.

Restoration story and significance

The Mahan Collection found the truck abandoned on the grounds of Pile Foundation in Long Island, New York. Restoring such an enormous, uncommon vehicle required sourcing rare parts, overhauling the colossal two-stroke V12, and refurbishing heavy chassis and suspension components. The restoration team focused on maintaining authenticity: original engine architecture, drivetrain configuration, and the truck’s distinctive proportions were preserved while worn components were rebuilt or replaced with period-appropriate parts.

"Hearing the Detroit diesel fire up after all those years was indescribable," said one restoration technician. The sound, described by onlookers as a rolling, guttural thunder, underscores why historic heavy trucks captivate both engineers and enthusiasts.

Why this matters to collectors and industry fans

This restoration is more than nostalgia. It preserves a tangible piece of industrial and transportation history — a rare example of extreme heavy-duty engineering from a time when manufacturers built bespoke machines for massive earthmoving tasks. For museums, private collections, and trucking events, the running M100SX is a headline attraction that illustrates how vehicle design priorities have shifted: modern heavy trucks emphasize fuel efficiency, emissions compliance, and modularity, while the M100SX centers on raw capacity and durability.

Highlights:

• One of only three ever built; last known survivor
• Restored to operational condition with original-style 29.3L V12
• Demonstrates historical heavy-haul capabilities and engineering philosophy

Where it goes from here

With the restoration complete, the M100SX is expected to appear at special exhibitions and preservation events. For car and truck enthusiasts, seeing the truck under its own power offers rare insight into mid-20th-century heavy-equipment design and an opportunity to appreciate the scale and sound of classic diesel engineering.

Whether viewed as a museum piece, a technical milestone, or a collector’s trophy, the revived Mack M100SX reconnects us with a chapter of trucking history defined by sheer size and uncompromising capability.