Showed it to a mechanic mate. When I told him how old it is and showed him the state of the metal, he just laughed. "Metal fatigue mate!" came the reply. "Same reason old plane wings drop off."
He reckons all the years of heating and cooling, things being under pressure, then not under pressure, swimming in mineral oil for the past 50 years, it all takes its toll. And considering this is only a very light, porous alloy material ("sh$$ metal" he called it), eventually it got soaked through, possibly following a flaw in the casting that had been there ever since day one, and you see the result. Basically it had to happen sooner or later, and I guess with a reconditioned engine, all sealed up with new gaskets and oil seals and new oil, the extra stresses were just enough to cause it to break.
