HANDS ON / SAVVY AVIATOR
How and why exhaust valves fail
BY MIKE BUSCH
I EXPERIENCED MY FIRST in-flight exhaust valve failure about 20 years
ago. The engine started running roughly, as you might expect of a
six-cylinder engine that was only running on five. After I landed, I
noticed the manifold pressure at idle was several inches higher than
normal, confirming that something was wrong with the engine.
In the hangar, I removed the top cowling and the top spark plugs
and performed a differential compression test. Five of the cylinders
measured just fine, but one measured 0/80 with a hurricane of air
blowing out the exhaust pipe. The jug had to come off.
When I wrestled the cylinder off the engine and looked at the
exhaust valve, something was missing (see Figure 1). A fragment of
the exhaust valve face had broken off and departed the premises.
Luckily, it departed through the wastegate and spared the turbocharger turbine wheel from destruction.
I sent the jug out for repair. It came back with a new exhaust
valve and guide and with some dressing to the valve seat. I installed
the cylinder back on the engine, where it’s happily operating to this
day, about 20 years and 3,000 hours later.
HOT, HOT, HOT!
Exhaust valves are the most heat-stressed
components in your engine. They are
exposed to high temperatures while oscillating back and forth through a valve guide
largely without benefit of lubrication (since
they’re too hot for engine oil to tolerate
without coking). Frankly, it’s astonishing
that they last as long as they do.
During the peak pressure and temperature portion of each combustion event, gas
temperatures in the combustion chamber
approach 4,000°F, far hotter than the
exhaust valve could withstand. Fortunately,
the valve is closed during this time, so the
heat energy absorbed by the valve face is
quickly transferred through the valve seat to
the cylinder head, where it’s absorbed by the