The Continental O-520 is a six-cylinder, horizontally-opposed aircraft engine produced by Teledyne Continental Motors. First run in 1963 as a development of the IO-346, it has been produced in versions incorporating fuel injection (IO-520), turbo-charging (TSIO-520), and gearing (GTSIO-520).
The IO-520 series of engines normally produced 285-310 HP and were used in numerous aircraft such as certain models of the Bellanca Viking 300 and Super Viking, the Beech Bonanza and Baron, and the Cessna 185, 206, 210, 310, and 400 series of aircraft. It has largely been superseded by Continental's newer IO-550 engine. The IO-520 series of engine is considered to be a reliable engine, but some of the early versions of this engine had a weaker crankcase than subsequent versions. Versions featuring more substantial crankcases are often referred to as having "heavy crankcases."
The GTSIO-520 was not as common and in its most common application (the Cessna 404 and Cessna 421 twin-engine aircraft) it produces 375 horsepower (280 kW). The engine has a reputation of being expensive to maintain and is intolerant of improper pilot technique. The intolerance is in the pilot going from cruise power to idle, reducing the temperature of a working engine to an idle engine and causing cracks to develop on the cylinder head from the fast cooling. More pilot planning is involved with this engine than the basic entry level engine because of the high temperatures of the engine and the environment (low temperature) that it is in. When operated properly, most pilots consider the engine to be reliable. Gearing reduces the RPM of the propeller making the engine quieter running than many other engines, although the gearboxes on early versions of this engine were notoriously unreliable. The gearing also adds measurably to the overhauling costs of these engines.