Very few depictions of the land battlefields that made up the Western, North African, and Pacific fronts of the Second World War feel complete without the presence of the venerable M4 Sherman. Produced in greater numbers than any other western tank of its era, it became the backbone of Allied armored forces. Yet among the Sherman’s seven hull variants, one stood apart for a remarkably unusual reason. Stretched by almost 11 inches to fit its one-of-a-kind powerplant, the M4A4 did not carry a conventional single-bank engine. Instead, its engine deck housed five 251-cubic-inch inline-six Chrysler automobile engines grafted together into one 30-cylinder amalgamation.

The M4A4 existed because of one of America’s greatest wartime challenges: building enough tanks, fast enough. As U.S. production expanded after 1941, demand for tank engines quickly exceeded supply. Aircraft manufacturers had priority for radial engines, while other suitable powerplants were open to wide competition. To enable the necessary scale of tank production demanded by the U.S. military, Chrysler devised an ingenious workaround that would generate an entirely new Sherman variant.

Chrysler’s solution was the A57 Multibank. Instead of designing a completely new engine, Chrysler combined five of its proven 251-cubic-inch flathead inline-six automobile engines around a common driveshaft and central gearbox. The result was a two-ton, 30-cylinder engine producing around 370 horsepower and packing five carburetors, five distributors, and thirty spark plugs into one of the most mechanically unconventional engines ever fitted to a production tank.

Chrysler’s creative powerplant solution had a direct impact on the tank itself. To accommodate the longer A57 engine, the M4 hull was lengthened, which demanded a greater distance between the suspension bogies than other Sherman variants and gave the M4A4 its distinctive silhouette. However, beyond this, the M4A4 retained the qualities that made the Sherman design so successful: good mechanical reliability, ease of manufacture, and a well-balanced combination of mobility, firepower, and protection.

The M4A4 also reflected the broader philosophy behind the Sherman program. Rather than relying on a single engine supplier, the United States deliberately built a number of Sherman variants using several different powerplants. These included the Continental R975 radial, Ford GAA V8, the General Motors twin diesel, and Chrysler’s Multibank. This diversity prevented production bottlenecks from disrupting one of the largest armored manufacturing programs in history.

Although built in the United States, the M4A4 saw relatively little American service. The U.S. Army preferred the Ford V8-driven M4A3 and issued most M4A4s to Britain and other Allied nations through the Lend-Lease program, where they became known as the Sherman V. A total of 7,499 M4A4s were reportedly produced, with 7,443 being given to Allied forces who used them extensively throughout Italy and Northwest Europe, including alongside American forces during the Normandy campaign.

For all its unusual engineering, the M4A4 proved far more successful than many initially expected. Although early production was hampered by teething problems, mostly centered on the complexity of its engine, improvements to cooling, piston rings, and exhaust valves steadily increased reliability. By 1944, the engine had successfully passed the U.S. Army’s demanding 400-hour endurance test, and the M4A4 had established itself as one of the most dependable Sherman variants, behind only the conventionally powered M4A3.

While its unique powerplant suffered from poor fuel efficiency and made maintenance more time-consuming than on other Sherman variants, these disadvantages were generally outweighed by the tank’s overall performance. Many foreign mechanics already had a good understanding of small-engine maintenance and repair, so once crews and maintenance personnel became familiar with the unconventional layout, the M4A4 earned a reputation for reliability and ease of field serviceability that exceeded many other Sherman models. 

The tank crews themselves also lauded the survivability of the M4A4. As the engine consisted of five independent six-cylinder banks, damage to one bank did not necessarily immobilize the vehicle, with Chrysler claiming that a tank could continue moving after losing up to 12 of its 30 cylinders. This was of enormous value to tank crews who recognized that redundancy in mobility was directly linked to their survival. 

Ultimately, the success of the M4A4 represents one of the war’s more inventive industrial solutions. Although inelegant, its Multibank engine provided an excellent option in addressing urgent production problems, and beyond the unusual nature of its engineering roots, the M4A4 Sherman succeeded admirably in its role as one piece of the mass production and mechanization of land warfare that contributed greatly to the Allied victory.