AOPA Pilot Magazine - July 2018 - 90

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My twin Cessna has a dozen URHB32E spark plugs installed in the
left engine and a dozen RHB32Es in
the right engine. These cryptic part
numbers tell a lot about the characteristics of the spark plugs.
The "U" prefix simply means
my left-engine plugs come from
Tempest Plus, one of two U.S. firms
that manufacture aviation spark
plugs. The absence of the prefix
on my right-engine plugs denotes
they're from Champion Aerospace.
The "R" denotes a resistor plug
(as almost all aviation plugs are
now). The resistor serves as a current limiter that results in a longerduration, lower-current spark that
reduces electrode erosion and
prolongs plug life.
The "H" means the plug has
3/4-20 threads to mate with an
all-weather ignition harness; an
"E" would denote 5/8-24 threads.
The "B" says the end of the plug
that screws into the cylinder
head has 13/16-inch-long (long
reach) threads; an "M" would
denote 1/2-inch-long (short-reach)
threads.

your engine on a regular basis-think of
it as an advanced preflight inspection.
Doing your own oil changes and spark
plug maintenance is a great way to get
involved with the maintenance of your
aircraft.
If you choose to do it yourself, there
are various rookie mistakes to avoid.
I've already mentioned several: overapplying thread lube, overtightening
ignition lead caps, allowing a lead to
twist while tightening the cap, and reusing a work-hardened copper gasket.
Another common mistake is dropping
a spark plug during removal or installation. Never reuse a spark plug that
has been dropped. Dropping an aviation spark plug just a few inches onto a
hard surface can fracture its fragile nose
core insulator. Even an invisible hairline
insulator crack can lead to destructive
90 | AOPA PILOT July 2018

The "32" is the heat rating, a
measure of the plug's ability to
transfer the heat of combustion
from the nose core to the cylinder
head. Hotter plugs (36-40) have a
long insulator nose that creates a
long heat dissipation path; colder
plugs (29-32) have a shorter
insulator nose and heat dissipation path. The optimum heat
rating will cause the nose core to
operate between 1,000 and 1,300
degrees Fahrenheit-hot enough
to minimize plug-fouling deposits but cool enough to prevent
preignition.
Finally, the "E" suffix denotes
a massive-electrode plug; an "S"
would mean a fine-wire iridium
plug. A "BY" suffix denotes a
special kind of massive-electrode
plug with a projected-core design
to reduce fouling.
Common plugs include the
short-reach REM40E, RHM38E
and REM37BY; the long-reach
RHB32E, RHB37E and RHB38E;
their fine-wire brethren (with an
S-suffix instead of E); and their
Tempest equivalents (with a U
prefix). If you don't know what's in
your engine, you should. -MB

preignition that could trash your engine
and ruin your day.
Aircraft Spruce and Specialty Co. sells
a nice spark plug maintenance kit for $120
that contains almost everything you need
to maintain your spark plugs: spark plug
tray, spark plug cleaner, feeler gauge, gap
setting tool for massive-electrode plugs,
aircraft spark plug socket, some copper
gaskets, and a bottle of thread lubricant/
anti-seize. The only additional thing
you'll need is a suitable torque wrench,
which you can pick up for about $20 from
Harbor Freight. The total investment of
$140 will more than pay for itself the first
time you do your own spark plug maintenance instead of paying your shop $100 an
hour to do it.
AOPA
MIKE BUSCH is an A&P/IA.
EMAIL mike.busch@savvyaviator.com

https://www.magnusaircraftinc.com/ http://trade-a-plane.com

AOPA Pilot Magazine - July 2018

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