Exhaust Valve
Failures
Part 2: Preventing failures by early detection
BY MIKE BUSCH
LAST MONTH WE DISCUSSED how exhaust valves fail and why they
sometimes fail prematurely. This month, we’ll shift our focus to how
we can monitor exhaust valve condition, detect incipient valve problems, and deal with them before in-flight failure occurs.
I started last month’s column with a description and photos of an
in-flight exhaust valve failure that occurred in my airplane nearly 20
years ago. That failure occurred before we had the sophisticated
engine monitoring tools that we have today—specifically spectrographic oil analysis, borescope inspections, and digital engine
monitors. Nowadays, there’s no excuse for such an in-flight failure
because we have the technology to detect these problems early.
Anyone who experiences an in-flight exhaust valve failure today just
wasn’t paying attention.
BORESCOPE INSPECTIONS
In my opinion, regular borescope inspections should be the first line
of defense against exhaust valve failure. The borescope is an optical
probe (see Figure 1, next page) or a subminiature digital camera,
depending on which model is used, that can be inserted through a
spark plug boss (usually the top one). It is used to perform a direct
visual inspection of the combustion chamber, including the valves,
cylinder head, cylinder barrel, and piston crown. The borescope permits a quick, inexpensive, unambiguous determination of whether
PHOTOGRAPHY BY JIM KOEPNICK
MIKE BUSCH
A close-up view of what a
borescope inspection reveals.
the exhaust valve is operating normally or
whether it is starting to burn or stick.
Figure 2 shows the appearance of normal
valves in a TCM cylinder. The smaller valve
on the left is the exhaust valve. Note the reddish pattern of exhaust deposits on the face
of the valve. The deposits are quite minimal,
indicating that this cylinder has been running a nice, lean, clean-burning mixture that
produces relatively few deposits. More
importantly, the deposit pattern is almost
perfectly symmetrical, similar to a bull’s-eye,
showing that this valve is operating at the
same temperature all the way around the
circumference of its face with no hot spots—
exactly the way a healthy exhaust valve
should look.
Compare that with the exhaust valve in
Figure 3. Look at the highly asymmetrical
pattern of exhaust deposits on the face of
this valve. The cylinder has been running
rather rich, causing thick deposits to form
around most of the circumference of the
valve face. The valve has an extreme hot spot
in the 8 to 10 o’clock position, so hot that it
has burned off almost all the exhaust deposits in this area. This wouldn’t have survived
many more hours without failing.
The borescope inspection is the gold
standard for evaluating exhaust valve condition. Unlike the differential compression
test, which has proven to be inconsistent
and unreliable, the borescope provides a
