This is an O.S. MAX 91 VR-DF “Round Head” model airplane engine. It was designed specifically to operate at the high rpm necessary to run a model airplane ducted fan, hence the “DF” in the name. Ducted fan engines are used in model RC aircraft to simulate the look of a jet engine without actually running a turbine engine. This engine turns a large fan inside a section of tubing to create thrust.
Engine specifications—
Displacement: 14.76 cc or 0.9 cu. in.
Bore: 27.7 mm or 1.09″
Stroke: 24.5 mm or 0.965″
Rpm range: 2,500-22,000
Weight: 662 g