The Menasco D4 Pirate was a popular 4-cylinder, air-cooled, in-line, inverted, aero-engine, for light general and sport aircraft in the 1930's and 1940's. The 6-cylinder Menasco engines had the name Buccaneer, while the 4-cylinder engines had the name Pirate. The Menasco engines were interesting in that they came in both supercharged and non-supercharged models.
The supercharged models had the S suffix added to their designation to show supercharging. The supercharged models had superior performance at higher altitudes with a relatively small increase in dimensions and weight.
The engine shown here is a 125 hp version and was used to power the Ryan ST-A aircraft. The Monasco D4S (150 hp supercharged version) was also used in the ST-A