Believed to be the world's largest single-cylinder two-wheeler, NSU Bison 2000 was built by a gentleman called Fritz Langer, and it’s based on an NSU Konsul 500.
Fritz started with an NSU Konsul, which displaces 500cc then tossed the original cylinder in favor of something from a radial airplane engine. The new cylinder was still far below what he was looking for so he raised the displacement to a full 2 liters. After much modification of the original engine to accommodate the huge new cylinder plus a new lighter Mahle piston and 75mm diameter exhaust, the engine was done. One needs to have really strong legs as it comes with a kickstart and no silencer.
Article source : BikeExif