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I'm surprised that it's such a mystery what aluminium alloy the heads were made of. I looked it up in General Data in the '69 manual.
CYLINDER HEAD
Material - - - - DTD 424

I looked up the composition, on the internet.
3% copper 0.5% Manganese 5% silicon
That designation DTD 424 is now obsolete, but the modern equivalent is LM4

"Mechanical Properties of Aluminium Castings (obtained on separately cast test samples)
Aluminium Alloy LM4

Mechanical Properties (Sand Cast)
Aluminium Alloy (Heat Treatment) . . LM4M . . . . LM4 TF
Tensile Strength (N/mm2) . . . . . 140 - 170 . . 230 – 290
Elongation (%) . . . . . . . . . . . . . . . 2 – 3 . . . . . . 0 - 2
Brinell Hardness Number . . . . . . 65 - 80 . . . . . 90 – 120 . . . ."

M= as cast. TF= heat treated
The final part of heat treatment involves a temperature of 160 C. for 6-18 hours, followed by air cooling.


LM4 has different designations or equivalents, in different countries.
http://www.cast-alloys.com/products/lm_chart.htm
 
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