By Jim Tapp, Tech Services Manager (retired)
First, let’s talk about why we check for cracks in heads and blocks. The obvious reason is that you can’t fix what you don’t know is there. Cracks and pinholes in heads and blocks can lead to a loss of power in the engine, resulting in poor performance. That, unfortunately, is the best-case scenario. Cracks or pinholes can also lead to overheating and complete engine failure, not to mention dissatisfied customers returning again and again in search of that elusive leak.
My recommendation is when a customer brings in a head or a block for you to work on, before you even take the piece, ask him a few questions. Ask if he’s had any coolant leaks. If he has, and the deck checks flat you’re probably going to be dealing with cracks and/or pinholes. Ask him where he got the head. If it came from a “core specialist” (a.k.a. the junkyard), you have to think the worst. You have no history so you’ll need to check every inch of the core. Okay, you get my drift. If your customer wants you to diagnose a problem with the head or block, you’ll probably want to use one or more methods of crack detection to determine where the problems are.
That brings us to the inspection methods. There are four inspection methods that you can use, each of which has its applications. These methods are:
- Dry magnetic particle inspection (use for cast iron)
- Wet magnetic particle inspection (rarely used for heads or blocks; very messy)
- Dye penetrant inspection (ideal for aluminum castings)
- Pressure (or vacuum) testing (use to find pin holes in aluminum or cast iron)
You may find yourself using more than one inspection method on a single work piece to diagnose and fix some problems.
Let’s take a closer look at each method.
Dry Magnetic Particle Inspection
As the name implies, this inspection method uses a magnetic field and dry magnetic crack detection powder. Both the dry and wet magnetic particle inspection methods will work only on ferrous materials (cast iron, for instance). With this method, a magnetic field is applied to the piece being tested. Where there is a crack in the surface (or near the surface) a break in the magnetic field will show when crack detection powder is applied. The break or crack sets up a secondary magnetic field, attracting the powder to the casting flaw.
Dry magnetic powder is a fine cast iron dust with dye added to make visual identification easier. Commonly available colors are red, white and yellow. There’s no difference in the colors; they don’t work differently. But you’ll want a powder color that will provide contrast with the casting so you can see it easily.
Wet Magnetic Particle Inspection
This inspection method is VERY closely related to the Dry Particle Inspection. The only difference is that in this method, the magnetic powder is mixed with a carrier that you can spray onto the piece you’re testing. This method uses fluorescent powder that glows under a black light. The biggest advantage that Wet Particle Inspection has over Dry Particle Inspection is that the carrier will flow into very small cracks, making them easier to see.
This leads me to the biggest disadvantage of wet magnetic testing. It is MESSY. Not only will you need to clean the casting thoroughly before testing it, once you’re done testing, you’ll have another clean-up job ahead of you. The carrier is usually an oil-based product and you will need to get that off of the casting before you start on your repairs.
In both inspection methods, the magnetic field is applied in a variety of ways – through a permanent magnet, an electromagnet or hand-held “yokes” or coils. Most machine shops that I’ve had experience with use a hand-held device like the Y-6 and Y-7 Yokes from Magnaflux®. These are pretty versatile dry units that you can take to the work piece. I, for one, would much rather take a tool that weighs about 7-1/2 pounds to a head or block than the other way around as in wet testing.
One final tip on magnetic particle inspection – always test an area from two directions. If the crack lines up with the magnetic field it won’t show. To be sure that you’ve tested the area fully, turn the field generator 90º and test again.
Dye Penetrant Inspection
Again, the name says it all. Dye Penetrant Inspection (or DPI) uses a penetrating dye that shows where cracks and pinholes exist. Some of these penetrating dyes are visible under a black (or ultraviolet) light, while others don’t need a light source to be seen. Fluorescent penetrants are more sensitive than the visual dyes, but the down side is that they have to be viewed in a dark area, which may not be entirely practical in the shop.
The biggest advantage that this inspection method has over the magnetic methods is that it will also find pinholes – something magnetic particle inspection cannot do. Dye Penetrant Inspection can also be used on aluminum heads and non-metallic materials. Another advantage of DPI is the lower initial cost. You can get a complete kit for DPI for anywhere from about $45 up to $750 while the starting point for the Magnetic Inspection units is $700.
DPI is basically a three-step process:
- Clean the surface to be tested thoroughly to remove all traces of dirt, grease and carbon-based material.
- Apply the dye penetrant liberally to the surface and allow it to soak in for at least 20 minutes. One of the most common mistakes is not letting the dye set long enough.
- Remove the excess dye and apply the developer. The developer helps to draw the dye out of the crack allowing you to see it, either with the naked eye or with a black light, depending on the type of dye used.
A few words to the wise; these dyes will stain skin, cloth and other porous surfaces. So don’t grab your wife’s new white towels to clean off the excess dye. You might find yourself sleeping in the shop for a while if you make that mistake, along with a few others we won’t talk about here.
The last method of crack detection we’ll talk about here is pressure testing. This method is often used along with one of those listed above as a final check that all of the cracks or pinholes have been repaired. Of course, it can’t be totally simple; there are two ways to perform pressure testing – wet or dry. Procedures are essentially the same regardless of which method you choose.
First of all, the head being tested needs to be completely clean. You will attach a special block-off plate to the head to seal off the water passages, then pump pressurized air into the head through an air line inserted into a water port. Some sources will tell you to use about 60 psi, but in my experience, 20 to 25 psi is adequate. Some heads have core plugs pressed into them and these will blow out at 60 psi. It’s not only an inconvenience, it’s a safety hazard.
Here’s where the methods differ. With the wet method, you’ll lower the head into a water tank until it’s completely submerged. If you have holes or cracks, the escaping air bubbles will show you where. The dry method is similar. Instead of taking the head to the water, you’re bringing the water to the head. Once the head is pressurized, you’ll spray it with a soapy solution (bubble fluid or a little dish soap in water). If there are cracks or holes, the solution will bubble up and you’ll know where you need to repair.
Pressure testing is one of the easiest of the crack detection methods available A drawback is that pressure testing can’t identify all cracks. Surface cracks that don’t connect to a water passage won’t show any leakage so you could miss those if you just use pressure testing.
One last testing method we’ll talk about is vacuum testing. It’s most often used to test valve-to-seat seals on cylinder heads. Vacuum testing is easier than pressure testing, but it has one major down side. Vacuum testing can tell you that there is a leak, but it won’t locate it for you.
To perform vacuum testing, you’ll need a vacuum pump, a work surface with a rubber top and plugs that will close off the water passages. Once the holes are plugged, pull about 25″ Hg vacuum and watch what happens. If there are leaks, the vacuum will drop or not hold steady.
Whew! That was a lot of information in a small space. Probably more than you ever wanted to know about crack detection, but this is really just scratching the surface on the topic. For more information, we recommend the book, Engine Service Automotive Machining and Engine Repair by Gary Lewis or Sunnen’s Complete Cylinder Head and Engine Rebuilding Handbook by John Edwards. Unfortunately the Sunnen book is no longer in print, but you can probably find a used copy on e-bay or Amazon. There are also a lot of resources out there on the web. Just enter the search terms “magnetic particle inspection” and “dye penetrant inspection” and you’ll get lots of pages to check out.