An engine designed at the Hirth Motoren company in Stuttgart. In 1934-35, an economically and technically advantageous concept emerged at this manufacturer of producing engines of different power outputs and displacements based on standardised cylinder, crankshaft, and valve train components.
In this way, the “500” engine family was created — types 504, 506, and 508. In the 504A engine, the crankcase is an elektron (magnesium alloy) casting, which reduces the structural weight. Approximately 6,000 examples were produced. They were used in the Bucker Bu 131 Jungmann, Bucker Bu 181 Bestmann, and Arado Ar 79 aircraft.
| Configuration | 4-cylinder inverted inline |
| Cooling | air-cooled |
| Compression ratio | 6.2 |
| Displacement | 4 l |
| Weight | 104 kg |
| Maximum power | 105 hp at 2530 rpm |
| Rated power | 85 hp at 2360 rpm |
