The Chevrolet "big block" engine is a term for a series of large-displacement, naturally-aspirated, 90°, overhead valve, gasoline-powered, V-8 engines; that were developed and produced by the Chevrolet Division of General Motors, from the 1950s until present. Chevrolet had introduced its popular small block V8 in 1955, but needed something larger to power its medium duty trucks and the heavier cars that were on the drawing board. The big block, which debuted in 1958 at , was built in standard displacements up to , with aftermarket crate engines sold by Chevrolet exceeding . W-series (Mark I) The first version of the "Big Block" V8 Chevrolet engine, known as the W-series, was introduced in 1958. Chevrolet designed this engine for use in passenger cars and light trucks. This engine had an overhead valve design with offset valves and uniquely scalloped rocker covers, giving it a distinctive appearance. The W-series was produced from 1958 to 1965, and had three displacement options: , available from 1958 to 1961 in cars, and in light trucks through 1964; , available from 1961 to 1965; and , available in 1962 and 1963. The W-series engine was made of cast iron. The engine block had bore centers, two-bolt main bearing caps, a "side oiling" lubrication system (the main oil gallery located low on the driver's side of the crankcase), with full-flow oil filter, and interchangeable cylinder heads. Heads used on the high performance 409 and 427 engines had larger ports and valves than those used on the 348 and the base 409 passenger car and truck engines, but externally were identical to the standard units - but for the location of the engine oil dipstick, on the driver's side on the 348 and the passenger's on the 409/427. No satisfactory explanation was ever offered for why this change was made, but it did provide a reliable means of distinguishing a 348 from the larger engines. As with the "Small Block" engines, the W-series valve gear consisted of tubular steel pushrods operating stud-mounted, stamped-steel rocker arms. The push rods also acted as conduits for oil flow to the valve gear. Due to the relatively low mass of the valve train, mechanical lifter versions of the W-series engine were capable of operating at speeds well beyond 6000 RPM. The combustion chamber of the W-series engine was in the upper part of the cylinder, not the head, the head having