Cylinder Head Warpage
A distortion of the cylinder head's mating surface, typically caused by severe engine overheating, which ruins the head gasket seal.
Cylinder head warpage is a physical distortion of the flat mating surface of a cylinder head where it bolts to the engine block. It is almost always caused by severe engine overheating, which causes the aluminum cylinder head to expand at a faster rate than the cast-iron engine block or the steel head bolts. This uneven expansion warps the head, lifting it slightly off the block.
Once the cylinder head is warped, the flat mating surface is lost, preventing the head gasket from establishing a tight seal. This leads to a blown head gasket, allowing combustion gases to leak into the coolant passages (causing cooling system over-pressurization) or coolant to enter the oil galleries (causing oil dilution). If warpage is severe, the cylinder head must be machined (decked) flat at an engine machine shop, or replaced entirely.
Symptoms of cylinder head warpage include white exhaust smoke, rapid coolant loss, engine overheating, oil that resembles chocolate milk (coolant mixing with oil), and bubbles in the radiator neck. Mechanics diagnose warpage by removing the head, cleaning the surface, and placing a precision machinist's straightedge across the face, using feeler gauges to measure any gaps beneath the edge. Warpage exceeding 0.002 to 0.004 inches typically requires machining.