Symptom and causes of cracked head?

[Vipergrün]

In a recent conversation with the PPO of my 80SC it was leaked that there is/was a hairline crack in one of the heads of my car. The PO neglected to tell me this, nor was it caught in the rather extensive PPI performed. All compression/leakdown figures were fine.

What are some basic symptoms of a cracked head?


What can cause it?
Possible that someone really over-revved it at one time?
Can excessive heat do this?
This is a 70K original mile car verified thru receipts.

Finally, what's it gonna cost to get it fixed?

Thanks
-Brad
80SC

[stlrj]

The metallurgical properties of aluminium begin to rapidly deteriorate at temperatures in the low to mid-400° range, so it is quite possible that excessive heat is to blame.

Joe

[stlrj]

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.

http://www.liquidcooledairpower.com/lc-topendheat.shtml

Joe

[island911]

hmmm . . .So to summarize;

The Hot-Headed become the Hard-headed until they get Cracked in the Head at which point they find it time to buy a 996 or a Boxster.

. . .is that right?

[Doug Zielke]

A really insightful essay.

But this......

"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. "

....has me wondering how this information relates to the longevity of our flat-6 motors? Are cracked heads another reality of air-cooled motors?

[island911]

Originally posted by Doug Zielke
....has me wondering how this information relates to the longevity of our flat-6 motors? Are cracked heads another reality of air-cooled motors?

No - the article is in support of a sales pitch for water cooling.