Cracks in blocks can be illusive and difficult to detect when they occur in areas that can't be seen or are so small they give no outward signs of poor engine performance. Used blocks from dismantling yards can be even more questionable when the buyer has no history of the part. Even the slightest cracks or pinholes can lead to engine overheating which, left untreated, can lead to engine failure.
Dry Magnetic Particle Test
Dry magnetic particle testing uses a process where crack detection powder is spread over the block surface during the application of a magnetic field. A hand-held portable electromagnet generates the field required to run the current through the metal being tested. Porous metals, such as cast iron, work best for the magnetic particle test. As the magnetic field comes in contact with the metal and the powder is distributed over the metal surface, any crack or deformity will show itself as an irregular pattern, be it a "shadow" line that follows the crack, or a smudge or glob that indicates a small hole. The dry powder (cast iron dust) comes in different colours, which contrast with the block's darker appearance, making the flaw stand out.
Wet Magnetic Test
Wet magnetic testing uses a magnetic powder that combines with a carrier agent. The combination of these two ingredients forms a liquid that can be sprayed from a bottle. The chemicals allow it to fluoresce under a black light, showing small details in the metal's surface. With the wet method, the liquid flows into cracks and most small holes. This process can be very messy and requires a complete block wash, before reassembly, since the carrier spray agent has oil in it. With either the dry or wet magnetic testing methods, passing the hand-held electromagnetic device several times over the suspect crack can verify that a crack exists rather than making one quick pass over it.
Dye Penetrant Test
As its name implies, the dye penetrant test uses a dye with special penetrating abilities that can seep into cracks and the tiniest of pinholes, unlike the magnetic process that may miss the smallest pinholes. Some dyes can be seen under a black light, which heightens their sensitivity, but the surrounding area must be dark to do so. Dye penetrant can also be used on aluminium blocks and non-metallic materials. Once the dye has been spread over the block surface and allowed to dry then wiped off, a developer has to be applied to the surface to draw the dye out of hidden cracks and pinholes. Whether under a black light or with the naked eye, the crack becomes apparent, even when looking for the smallest flaws.
With pressure testing, the block has to be sealed off with numerous plugs and cover plates for a leak-tight fit. An air hose supplies pressure to the inside of the block passages. The test can be performed in a tank, where bubbles can be seen, or it can be performed in dry conditions with a spray bottle filled with a mild soapy water solution. Pressure testing can find internal cracks that are otherwise hidden from view or undetected by other testing methods. It can also verify the integrity of crack repairs that have been made in heads and blocks to make sure they hold water.