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Re: Question...

Posted by Christer on Thu Aug 22, 2002 06:59:31 AM

In reply top Re: Question... posted by Mick on Thu Aug 22, 2002 03:29:59 AM

I once listened in on a discussion about why cylinder heads of some Lycomings develop cracks between the exhaust valves and the spark plug.
The reason was said to be an alteration of the granular structure of the aluminium. This was triggered by the number of cycles of heating - cooling and not of over heating or normal high temperatures in itself.
A test was made, in an oven, on a separate cylinder head and after a certain number of cycles the head became so brittle that it could be snapped in halves by the force of hands.
This could be cured if the heat treatment process was renewed but, since the aluminium head is shrunk onto a steel cylinder and can?t be separated, it?s a no-go.

Of course, there must be a certain difference in temperature since it?s the product of this difference and the number of cycles that adds up. I believe that it would take a very long time in changes of "ambient" temperatures.

I?ve also been told that, regarding sheet metal, vibration can cause this kind of alteration to the granular structure.

Am I misinformed or does someone agree?

Christer

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