<VV> MORE INFO ON CYLINDER HEADS

[BobHelt at aol.com]

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.