AOPA Pilot Magazine - January 2022 - 97

Low-time cylinder (right engine #3) with
excellent 70/80 compression.
Right engine cylinder number 3, compression
is 31/80.
the valve didn't appear burned under the
borescope, then Continental says to fly the
aircraft for at least 45 minutes and then recheck
the compression hot.
" My A&P doesn't have a decent borescope...but
I do! " Sam replied. " After
attending some of Mike's talks at EAA
AirVenture in Oshkosh, I bought a
ViVidia Ablescope VA-400 for less than
$250, and I am doing the borescope
inspection myself with my A&P looking
over my shoulder. "
I was starting to really like Sam. I wish
all aircraft owners were that proactive
with their maintenance.
What the scope saw
Not long afterward, Sam sent a veritable
gallery of excellent digital borescope
images. On cylinder number 5 of the right
engine-one of the good ones, with compression
of 70/80-the exhaust valve
looked perfectly symmetrical, and the cylinder
wall had a beautiful crosshatch. It
almost looked brand new.
" Well it certainly should, " said Sam.
" The shop replaced that cylinder last year,
and it has less than 300 hours on it.
" Now, here's cylinder number 3 of the
right engine, one of the ones my mechanic
says needs to be replaced because the
compression is only 31/80 with air audibly
leaking out the tailpipe. "
" That exhaust valve is definitely in
trouble, " I told Sam, " but it's still in the
early stages and I believe it can be saved
without removing the cylinder. The barrel
looks in good shape with plenty of
crosshatch left. We recommend lapping
that valve in place using some valve
grinding compound. We've had good
luck rescuing low-compression cylinders
using this procedure. " A nice write-up
of the lapping procedure by my colleague
Paul New, an A&P/IA, is available at
bit.ly/exhaustvalvelapping.
We told him there was a good chance
the exhaust valve rotator cap was worn,
too, which could cause the valve not to
rotate properly. This might be the reason
it was starting to burn. After the valve
lapping was complete, we recommended
installing a new rotator cap while reassembling
the valve train. Then repeat the
compression test. " If it is up in the passing
range-at least 45/80-then fly it and
re-test it hot and we bet it comes up even
more, " I told him. " We're quite optimistic
about recuing this cylinder without having
to remove it. "
Then Sam showed us the other cylinder
his mechanic said needed to be pulled:
cylinder number 4 of the left engine. " The
compression reading was 35/80 and it
seemed to be leaking past the rings. "
" That exhaust valve looks good, " I
told Sam, " but the barrel of this one looks
quite worn with very little crosshatch
and some shiny spots. We're less optimistic
about saving this one, but before we
give up on it we'd suggest performing an
oil control ring solvent flush procedure
to wash the accumulated gunk out of the
piston ring grooves and hopefully allow
the rings to seal better. " We provided
Sam the details of our recommended
Left engine cylinder number 4, compression is
35/80.
solvent flush procedure, which you can
find at bit.ly/solventflush.
Unfamiliar and skittish
" My A&P has never done the lapping of
the exhaust valve in place, " Sam said. " He
says he's heard about the procedure but
is somewhat skittish to try it. He says
he's also never done the ring flush procedure
and never heard of it before. I told
him I thought it couldn't hurt to try these
things, and that if he botches it then we
just replace the cylinder. He still seems
reluctant. With the known low compressions
on both engines, I guess the airplane
is no longer airworthy so I can't fly it to
another A&P. "
" Lapping of the valve is a finishing process
whenever valves or seats are worked
on, " I explained. " It's completely routine
so far as processes go, although most A&Ps
have never done it unless they do cylinder
repairs or work in an engine shop. As you
say, if your mechanic attempts the lapping
and the compression does not improve,
then cylinder removal is the next step. It's
certainly worth a shot. "
The solvent flush procedure is simple:
remove the top spark plug, rotate the
propeller to bring the piston to bottomdead-center
on the compression stroke,
pour the solvent mixture through the spark
plug hole to fill the combustion chamber,
replace the top spark plug, and then
pull the prop to force the solvent past the
rings. Lather, rinse and repeat a few times.
Then drain the solvent-contaminated oil
and service the engine with fresh oil. " This
AOPA PILOT / January 2022
97
http://www.bit.ly/solventflush http://www.bit.ly/exhaustvalvelapping
AOPA Pilot Magazine - January 2022

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