generally leads to valve hot spots (burned
valves) and ultimately to valve failure
(swallowed valves).
In most Lycoming and some TCM
engines that use relatively hard valve guides,
the deposit buildup on the valve stem makes
it difficult for the valve to close fully. This
can also cause leakage past the valve,
resulting in hot-spotting and ultimately in
valve failure. If the situation gets bad
enough, the result is a stuck valve that won’t
close. (The same problem can be caused by
valve guide corrosion in engines that are not
flown for long stretches of time.) The first
symptom of this condition is usually
“morning sickness,” where the engine runs
very rough when first started but smoothes
out as the cylinder head temperatures come
up to operating temperature. If the problem
isn’t addressed promptly, it can lead to an
in-flight stuck valve that can have serious
consequences: bent pushrod, damaged cam,
or even snapping the head right off the valve
if the piston strikes the head of the stuck-
open valve. Stuck valves are quite common
in Lycomings and TCM O-200/O-300
engines, but they’re quite rare in big-bore
TCM engines.
Mike Busch, EAA 740170, is a certificated flight instructor
and an airframe and powerplant mechanic with inspection
authorization. Honored by the FAA as the 2008 National
Aviation Maintenance Technician of the Year, Mike has been a
pilot for more than 45 years, logging more than 7,000 hours.
E-mail questions to Mike at mike.busch@savvyaviator.com.
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