This photo shows the rod supported...
This photo shows the rod supported so longitudinal cracks will show up.
The Sunnen DCM has a 14-inch internal diameter. The advantage of having this large size allows cylinder heads, crankshafts, and other large parts to be fed through the machine. Smaller parts such as connecting rods can be hand-held. Holding a small part with your hand allows you to turn the part around and in several directions. This way flaws in any area of the part can be found.
MPI machines can be either an alternating current (AC) or a direct current (DC). AC will concentrate the current on the surface of a part. Furthermore, AC will let the magnetic particles (iron dust) spread evenly across a part, thus assisting in the search for flaws. The AC process is used when the primary goal is to find surface defects. It is the type commonly used in automotive parts inspection.
The use of DC electricity allows a skilled operator to actually look under the surface. This can expose hidden defects in castings, forgings, and even in welded parts. Use the DC process on critical parts. Operator skill and having the correct machine are important here as there are no clearly defined fault lines such as surface cracks.
The electrical current in the magnetic field should flow in the direction that a crack is suspected to lie. We will use the crankshaft as an example. With it supported in the machine, the magnetic field will be passed from end to end. This will reveal cracks that are parallel to the centerline of the shaft. Next, the crankshaft will be passed through a circular magnetic field. This will reveal cracks around the journals as well as any flaws perpendicular to the shaft.
This lower A-frame has been...
This lower A-frame has been subjected to a beaucoup of highway miles. It is being checked for cracks around the inner attachment end. One end at a time is inserted into the magnetic field.
This is the reason a connecting rod should be exposed to the magnetic field in several positions. Using only one pass with the magnetic coil could cause you to miss cracks in other directions.
After a part is magnetized for inspection, it must then be demagnetized. If not, the part will attract steel and iron dust. With a crankshaft or a rod, it would be virtually impossible to reassemble an engine while keeping these parts clean. On the other hand, it might make little difference on an A-frame.
For the most part, you can't perform MPI at home. Machines such as the Sunnen are a tad expensive for the home-shop engine builder. However, if you call around, you should be able to find a shop with this type of equipment. The fee for checking some of your parts is reasonable considering the costs incurred if one of the parts were to break.
You can usually obtain one of the U-shaped permanent magnet yokes locally. With this you can check cylinder heads for cracks on their outer surfaces, deck, combustion chamber, and so on. Your nearest auto machine shop should be willing to part with some of the cast-iron dust gathered under its machinery. While a little bit coarse, this dust will get you started and you'll see how the process works.
MPI testing can find the beginnings of part failures before they actually fail. Iron dust will cling to an otherwise invisible surface flaw on a magnetized steel or iron part. Be sure to demagnetize an internal engine part. AC magnetics are used to detect surface flaws. DC magnetics can find internal flaws. MPI is a form of insurance. If it is important, then protect it.
 A crankshaft is being fed...  A crankshaft is being fed through the Sunnen DCM. Note how it is supported. Nonconductive materials hold it in place so as not to disturb the magnetic flow. Going through in this direction, cracks in the journals will show up. If longitudinal cracks are suspected, the shaft will have to be checked in another direction. |  Valves take a lot of abuse...  Valves take a lot of abuse in high-revving engines. Checking this one, supported on each end, will show up any damage around the outer edge of the valve head. |  The flange end of a rear axle...  The flange end of a rear axle is supported on a phenolic block. Cracks or flaws near the flange would indicate a potential failure point. If this failure came at the wrong point...well, think about it the next time you are exiting Turn 4. |