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. One minor difference between the 348 and 409/427 was the location of the engine oil dipstick: it was on the driver's side on the former and the passenger's side on the latter. No satisfactory explanation was ever offered for why this change was made. However, it did provide a fairly reliable way to differentiate between the smaller and larger versions of the engine.