So you suspect that your engine heads are cracked, you can’t actually see a crack, but you take them into the shop just to be safe. They take them and disappear into a back room only to return and tell you they’re cracked and you need to buy new ones. Surprise, surprise.
What exactly did they do back there? Honestly, maybe nothing, a quick smoke behind the building and came back out front to sell you the heads they knew they were going to sell you the second you walked in. If they actually checked them, how do they know the heads are really cracked? How do you know you’re not being scammed?
First off, the process being used is correctly referred to as Magnetic Particle Inspection, MPI or MT. MPI is part of a group of testing techniques collectively referred to as Nondestructive Testing or NDT. Magnaflux is simply a product brand, sort of like saying you want to buy a Chevy when you really mean a car.
Persons “certified” to perform MPI are certified by the American Society for Nondestructive Testing (ASNT) where there is enough information on the technique to make the average persons head spin. Just because someone doesn’t have the piece of paper from ASNT doesn’t mean they don’t know what their doing, but the paper just means they've passed a written and practical test and adds to you confidence level. If there is any chance you'll want to ask someone else for money, say you bought a set of "good heads" from someone and they turned out to be bad and you're going to court, you definitely need the test done by someone certified to ASNT standards.
MPI creates a magnetic field in the part, either by passing electrical current directly through the part or by creating it in something like a bar that’s passed through a hole in the part. The shop should either have a machine they clamp the head in or they may use a smaller, u-shaped hand held device called a yoke. AC and DC are both used, AC current locates only cracks open to the surface, DC can also detect flaws within a specific distance of the surface. An AC Yoke would be more than adequate and suitable for testing engine heads. This test done with DC may have happened before all the time and energy was put into machining the heads.
When a magnetic field is created in you head, any cracks will cause a break in the surface; think of it as a small canyon. When the magnetic field reaches the edge of the canyon, it will literally jump out of the surface and across the gap to the other side of the canyon. While this magnetic field is present, the person doing the test will either spray, wash or sprinkle very fine iron oxide on the surface. The iron oxide will be attracted to that spot where the magnetic field jumps across the canyon. It’s also attracted to seams because if you look closely, a seam is exactly like a crack, a little canyon that makes the magnetic field jump into the air.
So they take your heads and they should clean them or at least do their best to get oil and grease off them. You can NOT properly test greasy or oily heads. The iron oxide needs to be able to jump to any cracks in the head. Think about it, if you sprinkled flour over a greasy or oily surface, can it move, no, it sticks! If they throw them in the machine all dirty and say they’re bad, scam alert, scam alert!!
We’ll assume the heads have been cleaned by either you or them. There are two ways to perform the test, wet and dry, I’ll discuss each briefly.
Wet – In this method either water or oil are used as a carrier for very small iron oxide particles, probably in an aerosol can, and sprayed on the head while power is applied. The liquid helps move the iron oxide around and over the contours of the head. This liquid and the iron oxide can either be seen with regular light or may need a black light if the particles are fluorescent, fluorescent is more sensitive. Wet, especially and oil based formula also tends to deal with very small amounts of oil on your head better. The absolute best method of testing is with an oil based fluorescent solution, it's the gold standard.
Dry – Dry powder is also a valid if less sensitive way to test the heads and is more susceptible to any grease or oil on the heads. Under a black light grease and oil will glow white or have a milky appearance. The powder used can be any color, gray and red are the most common colors used. Usually you want a color that shows up good against the surface, I’ve even used yellow and royal blue! To apply the powder they will use something like a little puffer ball. As the power is applied, they puff small amounts of the dry powder over the surface. Just like the wet method, the dry powder will leap into any gaps caused by cracks or seams.
So what will it look like...like a crack! If it’s the nice fluorescent liquid, it will be a glowing green crack. If dry red powder, well, you guessed it. Remember, seams, inserts and other “non-relevant indications” (official NDT term for other stuff we’re not worried about) can show up any may not mean your head is cracked. Even something like a screwdriver dragged across the surface of the metal can cause a change in the surface big enough to show up. If they drag a magnet across, it will show up big time but as a thicker fuzzy line, not as a sharp, clear crack. It's sharp, clear cracks that you're worried about, especially very near or down and holes in the head.
Maybe you’re saying, what should I do, where do I find a place with trained and certified people? Open Google or you search engine of choice and look for “nondestructive testing” and the name of your home town or a bigger nearby city. These people will know the correct way to test your head and guess what, it’s not in their financial interest for your head to fail! Who are you going to trust more, the guy selling heads or the one that only tests parts for a living?
What will it cost? This varies but if you bring the heads in nice and clean or de-greased, it’ll cost less. Call them and ask as some may refuse dirty heads altogether (it contaminates the fluorescent liquid solution), or only accept them completely disassembled.
In a nutshell, a clean head has a magnetic field created in it. The inspector adds iron oxide that jumps into the magnetic field created by any cracks. You either see the crack and hand over the money or the heads okay. If the shop that wants to sell you new heads says it’s cracked, ask to see. If they can’t show you a crack to your satisfaction, walk away and find someone else, you’re probably being scammed.