Cracked or warped Heads seem to be common to the Ace family.

Aluminium is a relatively soft metal that conducts heat much better than Iron. It is also much lighter, which is why car manufactures like it so much. Better heat transfer means better cooling and lighter engines mean better fuel economy.

One of the drawbacks with Aluminium is that it has a lower melting point than Iron, and therefore more prone to distort under excessive heat. The engine goes through many thousands of heat cycles during its life, and overheating will accelerate the ageing of the metal.

The head is not a solid block of metal. It has to allow coolant, oil, fuel, and exhaust gasses to flow around it or through it, without allowing them all to mix together. For this purpose, there are many channels and indentations within the head. Some are close together, and have a thin wall separating them, whist others are far apart with much thicker walls separating them. Thick walls are far more heat tolerant than thin walls, and each will expand and contract at different rates. Under normal heat cycles, the head and block should be robust enough to withstand these heat cycles. If the cooling system starts to fail, and the engine develops overheating problems, the head and block may not be able to withstand these much higher heat cycles.

As the head and Block are tightly bolted together, there is no movement between the mating surfaces. Ultimately, this puts varying amounts pressure on different parts of the head. If the head is removed, the varying tensions will pull against each other to try and balance themselves out, thus causing the head to warp slightly. If the head is not removed, the tensions will continue to build, and may eventually result in small fractures appearing within the inner walls. Once a fracture opens up, the coolant, oil, fuel, and exhaust gasses will no longer be entirely separated, and crossover will occur in some form or another.

If the head has been removed from the block, it is advisable to have it properly tested by a specialist that can skim the mating surface, so that when it is refitted, it is perfectly flat, and will seal against the head gasket and block correctly.

Dave Bright, 25 Sep 2003.