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Non-Destructive Magnetic Particle Inspection... You know it as...


wet testing unitWe live in a world of generic products having taking on brand names. For example, when you grab for a tissue, you are reaching for a Kleenex. Grab a soft drink and it’s a Coke. Kleenex® is a registered name for the tissue product manufactured by Kimberly-Clark. Coke® is a registered trademark of the Coca Cola Company, but those products, and others like them, have become so ingrained into our society that the branded name takes over and serve as blanket identification of all products of that type.

Magnaflux® is a brand name of the Illinois Tool Works, and is one of many types of magnetic particle inspection systems. But, like Kleenex® and Coke®, the name Magnaflux® has come to mean any type of non-destructive examination of ferrous materials. Okay, now that you know where I’m going with this, let’s discuss magnafluxing (sic).

pole magnetsOften, whether to solve a specific problem, or as preventative maintenance, it is necessary to examine a car’s engine, or engine components, to locate and identify “invisible” cracks. Obviously, a visual examination is not going to be enough if the crack is tiny or sub-surface. A special technique has to be employed that will get below the surface to identify problems. If you were building an aircraft or space shuttle, you might use radio waves, radar, or other highly sophisticated (and very expensive) systems. But for us, magnetic waves are the most practical method of locating hidden problems.

Before we go into the “how” of magnetic particle inspection, let’s look at where it will work and where it will not. First of all, being magnetic, it requires a ferrous metal base. For the most part, with our older cars and trucks, that’s not a problem. Most engines and engine parts are iron or iron-alloy based. It won’t work on aluminum engine blocks, pistons or other aluminum parts; pot metal (not used for an engine block, but frequently for components), copper or brass, and most stainless steel. The MPI has to set up a magnetic field within the metal so that the identification powder – usually an iron oxide – will react to the magnetic waves.

That means understanding what a magnet does, and how the waves from (and to) a magnet act. A magnet has poles generally referred to as the north pole and the south pole. The magnetic waves go from the north pole in a complete loop back to the south pole or to the closest south pole. As a kid you used to play with magnets, and found that when two magnets were brought into contact with each other they sometimes attracted and grabbed each other and other times repelled each other and seemed to push each other away. Remember the expression, “opposites attract”? Well, that’s your magnet. The north pole attracts the south pole and repels another north pole. So if two north poles (or south poles) are brought together, they push away. No complete loop of magnetic waves is created. But if a north pole and south pole are brought together, a loop of magnetic field goes from the north pole to the south, creating a complete loop. Something else happens, too.

magnetic fieldsPicture two bar magnets. Bring the north pole of one and the south pole of the other together. They will snap together, and lose the polarity at the joint. The poles will be at the open ends of the magnet. Essentially the two have formed a single magnetic bar. There is no magnetic current flowing from or to the center of the magnet.

Now we can discuss magnetic particle inspection. The ferrous material to be tested is magnetized, forming north and south poles, and then dusted or coated with very fine ferrous particles – iron oxide dust. The dust is attracted to the pole ends of the magnet. If there is no interruption in the magnet from pole to pole, the metal dust will just lie in the center. But if there is a crack or fissure in the part that is being tested, the dust will be attracted to the new pole ends – each side of the crack. (Slightly separate the two bar magnets that we discussed previously, and you have four poles, a north at one end, a south at the other end of that bar, a north adjacent to it and a south at the far end.) That’s your crack. Simply stated, that is magnetic inspection.

crack in magnetic field

“Magnafluxing” an engine block or head or pistons merely means that the parts are exposed to a magnetic field. They are then dusted or sprayed with either a dry or wet solution of ferrous metal dust. If a crack exists, even if it is too small to be seen by the naked eye, the dust will accumulate on both sides of the crack (a north and south pole).

The magnet that is used may be either a permanent magnet or an electro-magnet (a magnetic field is created only when the current is turned on and supplied to the magnet. The field breaks down when the current is shut off). It may be a direct current magnet or an alternating current magnet. Normally the alternating current magnet will easily detect surface flaws due to the “skin effect” of the AC current, and the DC current will penetrate deeper into the metal identifying internal fissures. Often the iron dust used is treated with a special dye that makes it even more visible under black light.


magnaflux machineSeeing magnetic anomalies does not necessarily mean that the part is damaged. Any interruption of the magnetic field will show up, but it could identify a weld joint with a different magnetic affinity, a non-ferrous bolt or rivet, or a void in the metal – intentional or otherwise. MPI can also identify thin walls or corrosion damage not readily visible or other problems on or within the part being tested.

So sophisticated has non-destructive testing become that there now is a formal organization accrediting operators of this type of inspection: the American Society for Nondestructive Testing (ASNT). Certification as a member of ASNT requires much more than just “magnaflux” testing expertise. There are many other techniques and areas of discipline within the ASNT, but as stated earlier, most do not really concern us with our old car.

Many professional engine rebuilding shops have MPI capabilities. Some, the larger high volume shops, may have massive units that will examine an entire engine block. Others may have only a magnetic yoke that they manually pass over the part to locate failures. Some units are dry; the powder that is used is a dry powder that is puffed onto the surface and moves to locate the magnetic break within the part. Other units are wet testers. The part is wet down with water or oil and a solution containing the iron dust is sprayed onto the part. Again, the iron dust within the solution is attracted to the cracks in the part.

It is imperative that the part to be tested be clean and free of oil and grease. This is especially true of the dry testing method. The dust will find the oil or grease spots on the metal and stick to it, giving either a false indication of damage or failing to be able to move freely on the surface of the metal to locate the internal crack. Wet testing is a little more forgiving with oil or grease, but still, a clean part will test better and give better results.

Be wary of a local auto parts store that offers “free magnafluxing.” Odds are that the kid behind the counter has no idea what he is really doing and may give you incorrect information. Go to an experienced person - a large engine rebuilding shop, a large auto parts supplier, or do an Internet search for “non-destructive testing” in your area.

handheld hoke

What can you do if a crack or fissure is disclosed? Well, replacement of the part with a new one is one option. Obtaining a used replacement is another, but before installing the used part, get it tested. You may be buying the same problem that you are trying to fix. Sometimes welding or brazing will take care of the problem. A good welder can advise you about material, heat, stress, location, etc. Sometimes a small crack can be repaired by drilling a small hole at each end of the crack and installing a bolt or pin. If properly done, this will often prevent the crack from spreading. Then, of course, the crack itself has to be repaired.

This article originally appeared in the November 2010 issue of Skinned Knuckles. It is copyrighted by SK Publishing and may not be reproduced, in whole or in part, without written permission from SK Publishing. See Skinned Knuckles for more vintage and classic car tips. Also see vintage car repairs and these other articles from Skinned Knuckles:

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