To achieve the highest level of precision engine building, you must perform diligent detective work and pay close attention to detail. When using flaw detection equipment, you are searching for damage that’s more and less apparent. And many kinds of damage are not easily recognizable to the naked eye. Flaw detection equipment and technology help you discover potential problem areas in your engine components. A variety of testing methods are available for inspecting individual components for cracks or porosity, as well as prejudging cylinder wall thickness prior to block cylinder boring.
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This specialty gauge is used to measure material thickness, and is especially useful for measuring cylinder wall thickness in an engine block. This hand-held tester has a corded probe that emits a sonar signal that travels through the material. When the signal reaches the opposite side of the material, the signal bounces back to the tester, which displays the thickness measurement.
Probe measurement checks are made from the top of the cylinder wall to the bottom, in all radius directions. It’s especially important to check areas where water jackets exist behind the cylinder walls. Being able to determine existing cylinder wall thickness tells you how much material can be safely removed during an overbore. Walls that are too thin at critical areas deflect more during engine operation and degrade piston ring sealing. In addition, these walls can crack and/or blow out.
Ideally, you want to maintain at least .125-inch wall thickness at the thinnest areas. This requirement can vary depending on the specific block, but this is a reasonable ballpark. Any cylinder wall thickness concern increases if the engine is to be force-inducted with supercharging, turbocharging, or nitrous injection. The higher the cylinder pressures, the more critical wall thickness becomes.
Before using a sonic checker, the unit must be calibrated. Commonly, sonic testers include a machined calibration sample of steel that is milled at specific thicknesses of .200, .300, .400, and .500 inch. The calibration procedure is outlined with the specific brand of tester, but it’s a simple process. Once the unit has been calibrated, apply a dab of lithium grease to the probe face. Contacting the probe against a clean cylinder wall (using moderate pressure on the probe) reveals material thickness.
Don’t just measure one cylinder bore. It’s very common for cast-iron blocks to have variations in wall thickness, so be sure to check each cylinder. If the wall thickness in any given cylinder is marginal, you have the choice of scrapping the block or (depending on the specific block) installing a sleeve/cylinder liner. Checking your block with a sonic tester helps you avoid investing in a block that doesn’t hold up.
Hand-Held Magnetic Particle Testers
Applicable to cast-iron and steel materials only, this process involves placing magnetic poles at opposite sides of a suspected crack area and introducing a ferrous powder onto the inspection area.
The workpiece must be clean before testing. Only clean with solvents, and do not clean with abrasives or anything that might disturb the metal surface. In other words, don’t use glass bead, steel blast, etc. because they may peen crack edges, which makes inspection more difficult.
A magnetic (or “mag”) tester is a hand-held electromagnet used to check for cracks in ferrous materials, such as cast iron and steel. The surface to be checked must be clean and dry. Spray a light dusting of special iron powder onto the inspection area, and place the twinpoled magnet onto the surface. Press the momentary ON button to turn on the magnet. When using a twin-post magnetic tester, the best results are obtained if the bases are placed at a 45-degree angle to the suspected crack. Repeat the test 90 degrees from the first test (test in a crisscross manner). This pulls the powder into both walls of a crack for easier viewing.
Wet Mag Bench Testers
Also referred to as a magnetic particle inspection station, a dedicated “wet mag” bench has a large-diameter magnetic ring and a black-light setup. Passing the tested part through the ring magnetizes it. A hand-held magnetic field tester verifies the integrity of the part. A special penetrating liquid (mineral spirits mixed with florescent particles) is rinsed over the part and then it’s viewed under the black light. Any cracks show as a brightly colored line. Once the inspection is complete, the switch is flipped to de-magnetize mode to remove the magnetic field. If left magnetized, metal particles continue to cling to the part due to a potential residual field, even during engine assembly, which can obviously cause contamination and wear problems. The handheld field tester is used to verify that it has been demagnetized. The part is passed through the mag ring again. A wet mag testing unit is applicable to any ferrous (iron or steel) component, and is most commonly used for checking crankshafts and connecting rods.
This photochemical inspection process is used with or without the need for ultraviolet light. It can be used to test ferrous (steel/iron) and non-ferrous (aluminum and titanium) materials. Dye penetrant testing uses special chemistry to reveal surface cracks. Because no magnetic field is involved, this process can be used on any steel, cast-iron, aluminum, or plastic part.
The process typically involves three chemicals: a cleaner, a dye, and a developer, but it also often uses a cleaning solvent to remove any grease. Spraying the special cleaner prepares the surface. A dye penetrant is then sprayed onto the part. This dye seeps into any cracks, pits, or other surface irregularities.
Once the dye is given a few minutes to soak and dry, a special developer is then sprayed onto the part. The developer reacts with any concentrations of the dye, such as in a crack line. A readily visible, colored line reveals the crack. There are two types of these kits: One allows cracks to be seen under ambient room lighting; the other provides really bright-colored crack visibility under ultraviolet light. Once the crack is located and marked, a cleaning solvent is used to wash off the dye and developer.
Dye penetrant kits are available in affordable spray-can kit form from a number of sources, such as Goodson Tools. Additional spray cans are available individually when replacement is needed. Magnaflux Corporation also offers a similar Zyglo dye penetrant kit in a variety of sizes. It has a florescent dye that’s viewed under UV light (some kits include the black light).
For example, the procedure for using Goodson’s Glo Kit is simple. Clean the surface using a fast-drying solvent or hot water and soap. Rinse and dry. Spray the Goodson Cleaner onto the surface. This is a fast-drying solvent that prepares the surface. Allow this to completely dry. Then spray a liberal amount of penetrant. Allow this to soak for about 5 minutes. Using a clean rag, wipe off any excess. Next, spray several light coats of developer onto the area until you see a white chalky appearance. Place the UV light over the surface and inspect for cracks, which are revealed as contrasting lines.
Any of these kits are easy to use and come in handy for a variety of crackchecking jobs, including not only engine parts (blocks, cranks, rods, intake and exhaust manifolds, heads, etc.). They also work on socket wrenches, race car suspension parts, welded brackets, brake rotors, car frames, etc.
Pressure testing is indispensable for inspecting a cylinder head or other engine components for cracks, pinholes, porosity, or erosion. Two types of pressure testers are available. A positive pressure tester involves sealing all passages with special plugs. A gasketed sealing plate is mounted to the head deck. The head is then secured to a handling fixture and immersed in a water bath. Compressed air is introduced into the head revealing any leaks (cracks, faults, etc.) as bubbles escaping from the head.
A negative pressure tester (vacuum) can be used to check valve seating. With valves closed, special plugs are installed into the ports and vacuum is applied. Any drop in vacuum pressure confirms that a specific valve(s) is not sealed against its seat.
Written by Mike Mavrigian and Posted with Permission of CarTechBooks