technical counselor
diameter for the aluminum cylinder
head. These baffles are designed by
the airframe manufacturer (you, as a
homebuilder). Between the cylinders
are “intercylinder” baffles, which are
provided by the engine manufacturer.
The “connecting device” for the
unique airframe baffles varies by
airframe manufacturer.
For instance, on older Mooneys,
the baffles are flanged, and a threaded
rod goes between the front and back
and is tightened with high-temp 6/32
nuts on each end. The outer rod on
the aluminum head portion must be
joggled so that it does not rub on the
cylinder head oil-drain tubes. On later
Mooneys, a long hooked wire with a
spring on one end is used. The hook
is laced through a hole in both baffles
until it compresses the spring enough
to hold the baffles closed.
Other installations simply use
safety wire. (Again, the wire on the
outer baffle, the one around the head,
must be held away from the oil-drain
tubes. I use a split piece of 1/4-inch
rubber hose). If these “connecting
devices” are missing, you can expect
significantly higher CHTs, particularly
at higher speeds. At high speed, the
high-velocity pressurized air from the
upper cooling air plenum can blow
these baffles away from the cylinder
fins, causing the cooling air to exit
the slots between the cooling fins and
dump straight down to the area below
the engine and out the cowling. If
you have higher-than-expected CHTs,
I suggest you check your plane for
these connecting devices. This is
what happened to the engine I was
inspecting. The engine had been
overhauled and reinstalled without
them.
If you are missing the “clamps,”
you should find the proper parts for your plane and
install them, which is often quite complicated because of
everything else running through the area. You may have
to remove the intake tubes and exhaust system. On a
homebuilt, you could use .032 safety wire, but be careful
that it does not rub on the oil-drain tubes.
You will probably have to drop the lower cowl to
inspect for this discrepancy, which could be found on a
production aircraft as well as a homebuilt.
Between the cylinders are “intercylinder” baffles, which are provided by the engine
manufacturers. The “connecting device” for the unique airframe baffles varies by
airframe manufacturer.
Baffling is typically made from a corrosion- and crack-resistant alloy of aluminum. Don’t
use the 2024 aluminum you probably used for the aircraft structure as the heat and
vibration of the engine compartment will rapidly cause it to corrode and crack.
Richard Koehler is a former associate professor of aerospace
sciences at the University of the District of Columbia and has
been an EAA member since 1980. He is an active airframe
and powerplant mechanic with inspection authorization, a
commercial pilot with instrument and multi-engine ratings,
and a technical counselor and flight advisor.
