The Chevrolet Corvair engine was a flat-6 (or boxer engine) piston engine used exclusively in the 1960s Chevrolet Corvair automobile. It was a highly unusual engine for General Motors: It was air-cooled, used a flat design, with aluminum heads (incorporating integral intake manifolds) and crankcase, with individual iron cylinder barrels. The heads were modeled after the standard Chevrolet overhead valve design, with large valves operated by rocker arms, actuated by pushrods run off a nine lobe camshaft (exhaust lobes did double duty for two opposing cylinders) running directly in the crankcase bore without an inserted bearing, operating hydraulic valve lifters (which eliminated low temperature valve clatter that would otherwise be heard at cold startup with that much aluminum in the engine, due to its high degree of thermal expansion). The flat horizontally opposed ("flat engine") air-cooled engine design, previously used by Volkswagen and Porsche as well as Lycoming aircraft engines, offered many advantages.