The Whys and Hows of Demagnetizing Engine Components

By Jim Tapp
Retired Goodson Tech Services Manager

We get a lot of questions on the Tech Help Line about demagnetizing engine components. If you do a web search, there’s a lot of differing information available. Some sources say it’s not all that important, some say it’s essential. Since we’re getting these questions frequently, I thought I’d weigh in on the subject too.

In my mind, not demagnetizing components you’ve worked on is kind of like taking a bath, then putting your dirty clothes back on. Doesn’t make a lot of sense and undoes all of the good work you’ve just done. So here goes. This is my view on whether or not to demag components:

Rebuilding engine components without removing the residual magnetism is flat out dangerous.

Magnetism is produced in engine components in a couple of ways.

  • Deliberately introducing a magnetic field into the part for the purpose of magnetic particle inspection (Magnafluxing).
  • Magnetizing parts with heat and/or friction such as when a connecting rod has bearing failure. This is the most common way that magnetism is introduced into engine parts.

The down side to this, and the point I’m trying to make, is that if the magnetism is not removed from the effected parts, engine failure is more likely to occur. Imagine a magnetic connecting rod in your engine. It’s going to collect all of the metal particulates and use them to abrade your machine work.

So how do you tell that magnetism is present? Measure it. Magnetic fields are measured in gauss and the gauge used to measure it is a magnetic field indicator (Goodson #MFI-10010). To see if you have any residual magnetism in your components, simply hold the field indicator close to the part. If it doesn’t read “0”, you have magnetism.

Demagnetizing is accomplished by breaking up the D.C. (direct current) field with A.C. (alternating current). This is possible with the use of an A.C. demagnetizing table (Goodson #SPD-46). Place your part on the table top and turn the machine on. You will notice severe vibration. This is the A.C. attracting and repelling the part. While the machine is operating, slowly move the part away from the table to a distance of about two feet. Turn off the demag table and recheck the part with the magnetic field indicator. If the reading is not “0”, repeat the procedure until it does.

Remember if you have questions, the Goodson Techxperts are as close as a phone call (1-800-533-8010) Monday – Friday, 8am to 5pm (central). You can also reach the techs by email or visit the Goodson Tech Library.

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