One of the most peculiar engines ever used in a production motorcycle is the radial engine. Radial engines are typically used in aircraft, but there have been a few motorcycles throughout history that featured this unique engine configuration.
One notable example is the Megola motorcycle, which was produced in Germany between 1921 and 1925. The Megola had a 5-cylinder rotary engine built directly into the front wheel. The engine was a radial design, with the cylinders arranged radially around the central crankshaft. The engine displaced 640cc and produced 14 horsepower. The rotation of the wheel also provided cooling for the engine.
The Megola's design was certainly unusual, and it offered some advantages such as centralized mass and low center of gravity. However, it also had some drawbacks, including complexity and the need for a specialized starting procedure. Ultimately, the Megola's production ceased after a few years, and today it remains a rare and peculiar example of a motorcycle with a radial engine.
It's worth noting that there have been other unconventional engines used in motorcycles over the years, such as the Wankel rotary engine used in some models by the Japanese manufacturer Suzuki. However, the Megola's radial engine stands out as one of the most peculiar and distinctive choices for a production motorcycle.