The Hirth 3502 and 3503 are a family of liquid-cooled, in-line twin-cylinder, two-stroke, carbureted aircraft engines, with optional fuel injection, designed for use on ultralight aircraft and small homebuilts.
[1][2] The engines were developed as competitors to the 64 hp (48 kW) Rotax 582 with an identical mounting bolt pattern to the 582, allowing either to be a drop-in replacement.
The engines are similar to the Rotax powerplant in being liquid-cooled, two-cylinder in-line engines, with dual capacitor discharge ignition.
[1][2][3][4] Both the 3502 and 3503 are equipped with dual Bing 34 mm slide carburetors or optionally electronic fuel injection.
The cylinder walls are electrochemically coated with Nikasil and the crankshaft is forged from 4130 steel.