The Hot Head Pressure Testing Myth

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. I’m amazed at how often I am asked about this. There’s a belief that some cracks only leak at operating temperatures, when the casting expands. The concern is that a machine shop 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.

I’m not talking about pressure tests conducted on a coolant system in the vehicle at normal system operating pressures. In that circumstance very small cracks like the one in the linked video below can leak so slowly at ambient temps that warming the engine may help it leak faster and register a system pressure drop quicker, allowing the mechanic to make speedier diagnosis. But at ambient temps even at normal operating pressures the crack will leak, though it may take a while to bleed off system pressure, given the sheer volume of most vehicle coolant systems these days. In the machine shop setting a crack under pressure will always be visible at room temp if you know where to look. It may be small and difficult to see, but soapy water will expose it every time, even at factory pressures. But to get a really good look at these tiny cracks we pressure test at much higher pressures, up to 100psi. Click here to see a short video of a tiny crack under high pressure. This test was conducted at room temperature.

The myth of the need to hot pressure test in the machine shop is just that. A myth. Perhaps we should refer to it as an old-mechanic’s tale.


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