since 1940

Exhaust valve leakage in Continental or Lycoming aircraft engine, effects

Lycoming exhaust valve showing damage caused by flying with a leaking exhaust valve.

Valve Face Breakage The face of an exhaust valve is a ring. Exhaust gas leakage past the valve face heats one spot on the ring, expanding it into an oval shape. This uneven ring expansion creates Hoop stress. After a few cycles of uneven expansion and contraction, stress cracks form at the outside edge of the face. These cracks progress inward toward the center of the valve. When the cracks progress about 1/4 of an inch, they turn 90 degrees and grow around the valve face. When two cracks come together, a piece of exhaust valve breaks-off, the engine loses power, and the piece of broken valve damages the cylinder and most likely the turbocharger.

Valve Sticking  Exhaust gas leaving the cylinder flows past the valve face causing heat loading to the valve. Most of the heat the valve absorbs conducts to the valve seat and from there to the cylinder and into the atmosphere. A leaking valve reduces the amount of heat conducted to the seat causing the valve to overheat. The hot valve causes oil to carburize and form carbon deposits on the valve stem, that stick the valve in the guide.

If the valve sticks closed, combustion gas never leaves the cylinder and compresses during the next compression stroke. The pressure against the intake valve is so high that it cannot open. Either the push rod bends or the rocker arm support breaks. If the valve sticks open, the push rod is no longer held in the hydraulic lifter's socket when the camshaft lobe rotates to the closed position. If the push rod comes out of the socket and does not find its way back in during the next revolution of the camshaft, the push rod damages the crankcase by jamming against it. If the valve is sticky, the high forces required to open the valve usually damages the camshaft lobe and the engine loses power. Although the engine will appear to operate normally from the pilots view, an exhaust valve leakage is a serious engine defect that has the potential to cause engine failure. For this reason an engine with a leaking exhaust valve should be considered un-airworthy.

Continental exhaust port showing exhaust valve, guide, and seat

	Broken exhaust valve head in Continental 520 engine
	Area of leakage indicated by black line. High temperatures burn deposits from the valve face therefore uneven deposits and missing deposits around the edge are tell-tale signs of leakage. Valve circumference will be hotter at leakage spots. Heat expands the metal so uneven heat expands the circumference unevenly resulting in Hoop (circumferal) stress. This stress creates cracks that radiate from the edge inward toward the center. These cracks grow until a semi-circular piece breaks off the valve face. Pictured in topmost valve picture.
	Valve face breakage caused by  valve overheating. Notice dark color of valve stem. Valve was abnormally hot from leakage past valve face.
	Leakage is much less visible when looking at the valve seat. Notice uneven coloration.

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