As an update on repairing cast iron cylinder heads - I only ever found one company (sadly, no longer in existence) that could do a reliable repair. They would 'V' out any cracks, then bring the cylinder head up to red heat, and use a special oxy-acetylene torch with a powder feed attachment to fill in the V'd out cracks. The cylinder heads would then be left to cool down slowly, with the oven, over a period of about 48 hours. The machined faces would then be cleaned up, rendering the whole repair invisible. Back then (about 40 years ago), the process was expensive (about £100 per head) but we never suffered any more cracked heads (that was on Foden 2 stroke diesels that had a worse reputation for cracking than the Dragonfly heads).
I am of the opinion that most castings are pre-stressed because the surface solidifies first and the core then tries to shrink as it cools, but can't. It is these areas of stress that fail first, especially with repeated heating and cooling.
It would be an interesting experiment to stress relieve a new Dragonfly head by bringing it up to red heat and allowing it to cool very slowly - then see if that still suffered the dreaded cracks!
Regards,
Eddie.
