<VV> MORE INFO ON CYLINDER HEADS

[Mark Durham]

yes, all true, and there are certain aviation cylinder heads more
susceptible than others.
But, in aluminum's defence there are still many cylinder heads from the
1930's - 1940's still flying today and still providing reliable service
to antique aircraft owners.
The thought to remember is our heads are old and we should do a good
job of inspecting them for obvious defects prior to use. Mark Durham

Sent from my Windows Phone
From: BobHelt at aol.com
Sent: 5/31/2012 11:04
To: virtualvairs at corvair.org
Subject: <VV> MORE INFO ON CYLINDER HEADS
Hello all,
Here is part of an aircraft aluminum cylinder head article that I found on
the Internet. I don't know who the author is but he does seem to have a
good  handle on the head failure situation. It would seem that all of this
information  definitely applies to all Corvair cylinder heads.
Regards,
Bob Helt


Aluminum has some great properties that  make it ideal for use in aircraft
both in terms of airframes and engines.  However, aluminum also has a couple
of significant drawbacks when compared to  say, steel alloys. These are,
(a) a relatively low melting point and (b) work  hardening. The first drawback
- a relatively low melting point is self  explanatory, however the 'work
hardening' problem needs to be better explained.
Work hardening is an effect that causes  metallurgical structure of
aluminum to break down and fracture. As an example,  take a piece of
aluminum sheet
metal and bend it at a moderate angle of 45 to 60  degrees back and forth
and after a few times it will harden up and then simply  snap in half. A
simpler more everyday test is to use a soda can. Bend the tab on  top back and
forth 20 to 30 degrees and in a matter of a few times - clink - it  will
break right off. This fracturing is known as work hardening. Aluminum  doesn't
like to be bent back and forth. Steel alloys however can handle such
stresses quite easily but their weight penalty limits their use to structural
areas that are absolutely necessary.
An aluminum air-cooled cylinder head is  subjected to an incredible range
of temperatures. Prior to startup a cold engine  may be anywhere between a
balmy 80F to as low as -40F depending on where the  engine is being used.
After startup in a matter of a few minutes the cylinder  head temperatures rise
to around 200F at idle. During runup head temperatures  rise to as high as
350F and at takeoff and climbout 400 to 450F is not unusual.  In the worst
case scenario an air-cooled engine's cylinder head temperature can  go from
-40F to 450F in a matter of 5 to 10 minutes! That's a change of 500F!  Then
there is the cool down cycle which as it turns out is more destructive  than
one might think since the rate at which aluminum cools down has a  direct
effect on its hardness. A slow cooling from a high (>325F) has the  effect of
weakening the metallurgical structure of an aluminum casting.
The repetitive heating and slow cooling  of an aluminum head both weakens
the metalurgical structure and serves to create  a form of work hardening in
much the same way that bending an aluminum metal  strip does. The structure
of the casting becomes brittle over time which when  combined with extreme
temperature changes or temperature variations across a  cylinder head leads
to stress fractures.
In the last 20 years the majority of  automotive engines have been designed
with aluminum cylinder heads yet when  compared to aircraft cylinder heads,
automotive heads rarely suffer from fatigue  cracks. Why is this? The
answer is simple - heat! Too much heat!
As it turns out most automotive cylinder  heads are hardened to what is
known as a T6 hardness. This hardening process is  done shortly after the part
is cast and serves to relieve casting stresses and  to create a more uniform
metallurgical structure. The T6 hardening process  involves heating the
casting to 1000F for about 6 hours and then quenching the  part in water for a
few seconds. Next the part is 'aged' in an oven at about  320F for around 5
hours and then allowed to cool to ambient temperature. The  result is a part
that has a Rockwell hardness on the 'B' scale of around 84-88  and a nice
dense and uniform metallurgical structure.
The key to note is the aging temperature  of 320F. If the part is kept at
or below 320F it will retain its hardness and  uniform metallurgical
structure however, if it is repeatedly heated above 320F  the uniform
metallurgical
structure starts to break down and the parts starts to  become brittle.
Air-cooled cylinder heads regularly see  temperatures over 320F and it is
these high temperatures that lead to cylinder  head problems which can run
the gamut of cracks, loss of valve seats,  loosening of valve guides and so
on. But there is more to this story.
It turns out that an air-cooled cylinder  head has a wide temperature
variation across the head during operation. The  intake side of the head is
seeing relatively frigid temperatures from the intake  mixture while
the exhaust
side of the head is exposed to blast furnace  temperatures. The result is a
huge temperature differential between the intake  and exhaust valve seats
and its no wonder that this is the area where the  majority of cylinder head
cracks are found.
In effect an air-cooled aluminum  cylinder head is destined to fail after a
relatively short lifespan of  service. It is considered acceptable practice
not to run cylinder heads more  than twice the TBO of the engine before
being replaced. There are even those  that recommend replacing the cylinder
heads at each overhaul and based on  service data it can be shown that the
second time around cylinder heads are more  likely to encounter cracking or
other fatigue failures.
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