Just a quick word about a common misconception that comes my way from time to time. I recently fielded an oft repeated question regarding the nature of cracks in water cooled aluminum heads. There’s a belief that some cracks only leak at operating temperatures, when the casting expands. The concern is that pressure testing at room temperature will not catch these cracks.
In my thirty plus years of pressure testing and repairing cracked aluminum cylinder heads I have never once dealt with a casting that exhibited this characteristic, nor have I ever heard any other experienced cylinder head tech say they have.
When a crack ranges from the water jacket to the outside of the casting it creates a path that will leak at room temperature. It may be small and difficult to see, but it will leak under pressure.
The myth of the need to hot pressure test is just that. A myth. Perhaps we should refer to it as an old-mechanic’s tale.