I am a confirmed subscriber to the Reliability Centered Maintenance process. A part of that is to not
euthanize an engine until proper testing has been completed and confirms the need to take drastic
action. There are many articles of harrowing images of metal in the oil filter with an immediate
reaction of “I need a new engine”. Not every filter with metal in it spells impending disaster, but
occasionally it does. A measured discovery approach to determine the appropriate level of financial
panic is called for.
When an oil filter inspection shows first signs of significant metal, we tend to follow guidance found
from Lycoming (in the absence of usable guidance from Continental), and some common sense, that
suggests the sudden appearance of metal may be from recently dislodged sludge. The guidance is
to continue to fly for a much shorter time, maybe 10 hours, and recheck. If there is still significant
metal, then further investigation is warranted. If the 10 hour inspection looks good, then extend the
next interval to 20 hours and check again.
Sending the oil filter elements out for spectral analysis is a great idea at this point. Even if the next
inspections are clean, we’d still like to know what materials the metals are. With that information we
can also determine what components likely shed that metal. We might even be able to have a look in
the engine with a borescope to see how it looks.
If the 10 hour filter inspection still shows significant metal, then we’ll remove the propeller governor
and inspect the oil screen that is part of the governor’s gasket. ANY metal found in this screen would
have had to travel through the crankshaft main bearings and rod bearings to get there. This finding
would be cause to expect damage to the lower end of the engine and sufficient reason to pull the
engine from service.
If the 10 hour filter inspection shows significant metal, but the prop governor screen is clean, we’d
recommend another 10 hour interval and check again. If the second 10 hour inspection is clean, then
continue to fly and monitor at longer intervals. If the second 10 hour inspection still shows significant
metal, then it’s time to consider more drastic measures.
In our current situation, a new customer reported finding a little metal in the filter on their Continental
IO-360 in an SR20 during a routine oil change at their home airport. The plane was brought to us for
a borescope look in the cylinders and another oil filter inspection at 10 hours flight time. We noted
minimal nonferrous flakes in the filter and nothing of note during the cylinder borescope inspection.
We pulled the prop governor and found that screen completely clean. We recommended flying
another 10 hours and recheck.
The last oil filter inspection showing the increased level of metallic flakes
After the second 10 hours the oil filter showed an increase in metal, not a decrease. Most
interestingly, when we removed the oil drain plug, a couple of small metal chunks came out as well.
At that point, we realized something bad was indeed happening, so continued flight could not be
recommended and further investigation was in order.
The ferrous metal captured by the magnet after swiping through all the filter pleats
We ran a borescope into the oil sump through the drain hole but couldn’t see anything obvious. The
oil sump was removed to see what else might be there. The sump seemed very clean overall. With
the sump removed, we could see the crankcase parting areas which are just large enough to get our
borescope into. The decision to pull the engine had mostly already been made, but curiosity to find
the source of the metal was too strong to ignore.
While looking around the engine core, we noticed a lot of rust on the big steel parts. Not immediately
related to the problem at hand but very concerning from a structural integrity standpoint. At this point
we were just looking around to see how much we could see inside this IO-360. Almost by
happenstance, we noticed a divot in the #2 cylinder skirt as well as a matching divot in the #2 piston
skirt. This engine investigation suddenly became very interesting. We shifted our search from what
would make small metal flakes to something with enough mass to do damage on the scale of a sledge hammer.
Rust in the connecting rod
The divot is deep but only appears to have a few strikes
More digging around and the culprit was discovered. One of the crankshaft counterweights was out of
place at one end. The pin was missing, as well as the clip ring that holds it in place. Given the mass
of the counterweight and the speed at which the crankshaft turns, we could only suspect that the
failure must have occurred on the last flight and possibly during the taxi to the shop. There was no
report from the pilot of any strange noises from the engine, which was interesting considering the
extent of the damage found.
The holes should be aligned and have a steel pin in place, retained by a snap ring. This engine was too old to be covered by the recent FAA AD on counterweight pin snap ring installation.
It isn’t likely that the metal flakes initially found in the oil filter were directly related to the
counterweight pin loss. The metal flakes were nonferrous, and everything related to the
counterweights is steel. The metal chunks found in the oil sump very likely are from the counterweight
impact. The massive amount of rust in the engine should have presented in the oil analysis as higher
than normal iron. Unfortunately, we didn’t have any historical oil analysis data that might have given
some heads up on the rust.
Even though the nonferrous metal flakes may not have been part of the counterweight issue, at least
that finding got the ball rolling in the right direction. The measured approach led to an investigation
just in time to prevent a catastrophic failure in flight. We haven’t found the counterweight pin or the
snap ring yet, as they are too large to fall through the crankcase parting area. We know they’re there,
but the damage is done, the saboteurs were exposed, and the engine is coming off. No one was hurt
and the pain is limited to the wallet.
© Paul New 2025
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