The Yakovlev M‑501 diesel engine was one of the most unconventional and ambitious piston engines ever designed — a 42‑cylinder, water‑cooled, diesel aircraft powerplant developed by Soviet engineers in the late 1940s and early 1950s. With its inline radial layout consisting of seven banks of six cylinders each, the M‑501 was enormously complex and powerful by the standards of its time. It was intended to power large Soviet aircraft, including strategic bomber projects, and in testing achieved over 6,000 horsepower. However, the rapid rise of gas turbine technology and the impracticality of such extreme piston engines for aviation meant the M‑501 never entered widespread airborne service.
Though the M‑501 never took to the skies in production aircraft, its innovative design found a new life at sea. The basic architecture was adapted by the Soviet firm Zvezda into the Zvezda M503 marine diesel engine — a 42‑cylinder radial diesel that became a reliable powerplant for fast attack missile boats and patrol craft. Built in St. Petersburg in the 1960s and 1970s, the M503 delivered nearly 4,000 horsepower and was prized for its endurance and compact power density in naval applications.
The evolution didn’t stop there: Zvezda later developed larger derivatives such as the 56‑cylinder M504 and even joined engines into twin arrangements like the M507, effectively creating propulsion systems with over 100 cylinders and extraordinary power output for their size.
While these massive diesel engines never revolutionised aviation as originally envisioned, their development highlights the Soviet Union’s bold engineering ambitions — pushing piston engine design into realms few others bothered to explore. Today, the M‑501 and its marine successors are celebrated among enthusiasts and engineers as some of the most extreme and fascinating piston engines ever constructed.
