since 1940

Collapsed valve spring. Weak valve spring

Lycoming exhaust valve showing damage caused by flying airplane with a leaking exhaust valve. Notice missing pieces of valve.

Excessive heat exposure can also be noted by oxidized oil and flakes of carbon on the springs or in the spring recess. Problem is more often seen on the exhaust valve spring. If the exhaust valve leaks hot combustion gas past the valve seat then the exhaust valve overheats and this exposes the valve springs to excessive heat. More often seen on Lycoming engines because the sodium cooled stem of the Lycoming exhaust valve transfers more heat up into the stem than on the Continental valve. Lycoming engines that use sodium cooled valves have the rocker arm push against the rotator cap whereas on Lycoming engines using solid stem valves the rocker arm pushes directly against the valve stem.

I have also seen two cases where the valve springs collapsed from excessive heat where the heat was caused by an absence of oil flow to the rocker box. This is more likely to occur in engines that have a separate rocker box for the intake and the exhaust such as the Continental GTSIO series engines, the Continental TSIO-520BE engine used on the Piper Malibu and the Continental 

See also Broken valve spring

Valve spring strength - the trade-off's

From a performance standpoint the perfect camshaft profile is a square - instant opening, long open duration, and instant closing. This maximizes the engine's ability to pump air - volumetric efficiency. Importunely we have to round off the lobes to lower the loading on the camshaft lobes and the valve train components. The lighter we can make the valve train components the quicker we can make them open. Conversely, using the same camshaft and valve spring tension but increasing valve train weight increases camshaft lobe loading. Some roller rocker arms are significantly heavier than the traditional rocker arms. How will the camshaft lobes hold up with this increased valve train mass? 

 Less mass means we can have lower valve spring tension. However if the valve springs were too weak they could not provide sufficient force upon closing, to accelerate the valve quickly enough to maintain contact between the cam follower and the cam. To overcome this situation stronger valve springs are needed.

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