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Crack Detection and Correction
Cracks don’t necessarily have to spell the end of an engine component’s life
By Doug Kaufman
First off, let’s get one thing perfectly clear there’s no such thing as “flawless.” Like those shocking tabloid photos of that Hollywood actress who gets blindsided beside a bistro, even the most conscientious engine builder sometimes has to deal with surprising surface imperfections.
And just as she has paparazzi to expose the damage and makeup artists to cover it up, today’s engine builders can call on a number of state-of-the-art tools and techniques to locate, identify and remediate cracks and other damage in a variety of engine components.
Without excellent crack detection and repair methods, what you can’t see can most definitely hurt you.
Depending on their locations, crack severity will vary. They tend to form, spread and get worse as heat, thermal stress, heavy loads, repeated bending and flexing, metal fatigue, pounding and vibration take their toll on a part. Cracking is an indication that an area is experiencing more stress than it can handle.
Finding those cracks will enable you to determine whether you should repair or replace those parts. You simply can’t afford to spend a lot of time machining or reconditioning cores or used parts that may be destined for failure.
With hard-to-find and high value cores and parts, the decision may hinge on the extent of the damage. If the part can be repaired economically and with a high degree of success, then it’s probably worth fixing. But if it can’t, you’ll have to factor in the cost to replace it.
Never assume a part or a casting is okay just because you can’t see any visible cracks. Always assume there may be cracks although, because engine parts are made of so many different materials these days, finding them may be a challenge.
Magnetic Particle Inspection
Magnetic particle inspection is most often used to inspect cast iron or steel alloys that are “ferromagnetic” and can be temporarily magnetized for such things as surface cracks in and around the cylinder head combustion chambers and for inspecting crankshafts, camshafts and connecting rods. But the technique can also be used to check gears, shafts, axles and steering and suspension components for cracks, too.
Magnetic particle inspection won’t work on nonferrous metals such as aluminum, magnesium, titanium, nonmagnetic alloys of stainless steel or plastic.
A magnetic field created in various ways causes tiny iron oxide particles that are sprayed or brushed on the part to reveal any cracks. If there are any cracks in the surface of the part, they will disrupt the magnetic field and act like a pole to attract the iron particles.
The iron particles (sized between .125” and 60 microns), may be applied in a dry powder or a wet solution. They can be dyed yellow, white, red, gray, black or other fluorescent color to improve their visibility against the metal background. With the fluorescent particles, an ultraviolet black light is required to make the particles stand out.
The wet particle detection method is more sensitive than the dry method for finding very small cracks, but dry particles are better for finding cracks that may be just under the surface (subsurface flaws).
The light, size of the particle and even the type of electrical current your equipment can produce can impact your ability to find cracks and other anomalies. The training of the operator is imperative, and so is part cleanliness. Sometimes, casting lines or a rough surface finish on the component that’s being inspected can hide cracks.
That’s one of the reasons parts should have be as clean as possible with no dirt, oil, grease or carbon on the surface. Parts can be chemically cleaned, spray washed or baked in an oven, but don’t shot blast them prior to inspection because blasting may peen shut small cracks that could reopen later.
Experts say one of the most important parts of magnetic particle inspection comes after testing has been completed: demagnetization. There are various ways to ensure the magnetism has been eliminated but don’t take it for granted. Parts should be checked with a Gauss meter to make sure there is no residual magnetism.
Dye Penetrant Inspection
Another method for finding surface cracks and flaws is to use a penetrating dye. Though used mostly on aluminum parts, this technique also works well on cast iron, steel, composite materials and even plastic.
The theory behind this technique is that a very light oil will wick into a crack. It’s the same idea as using penetrating oil to loosen a fastener except that the oil contains a dye. If the oil finds its way into a crack, the dye should then make the crack visible. Some penetrating dyes use fluorescent dyes and a black light to make the cracks stand out, while others use a chemical developer to make the dye more visible.
Several different styles of penetrants are available, depending on your needs. If you’re using a UV light and fluorescent dye, a shroud that blocks ambient light will make it easier to see the cracks. Cracks will glow green under the black light. With ordinary dyes, no special light is needed. Cracks usually stand out as a stark red line against the bright aluminum metal.
Multi-stage penetrating dyes typically use a three-step process to highlight cracks. The advantage of this process is that it is simple to do and can be used with non-ferrous metals. However, the drawbacks to the process are that it can only locate cracks or defects that break the surface of the part, it may be less sensitive than some other methods, it uses a relatively large amount of solution and may take extra time to complete testing.
While magnetic particle inspection and penetrating dyes can do a good job revealing surface cracks, neither technique can effectively look below the surface or find damage hidden inside a casting. In this case, pressure testing can help you see what’s going wrong inside the engine.
It is often used in conjunction with these other methods of crack detection to check the integrity of the cooling jackets in the cylinder head and block, to find leaks other techniques can’t (such as porosity leaks in aluminum castings) and to see if visible cracks are really leaking or not.
Vacuum testing is the same basic idea as pressure testing, except in reverse. Instead of using air pressure to test the cooling jackets for leaks, vacuum is used on a head or block after the water outlets have been plugged. If the casting holds vacuum, there are no leaks. But if it doesn’t, you’ve found a leaker.
Unfortunately, this technique does not use water or dye to pinpoint the leak so you still have to use one of the other techniques to find the leak. It’s mostly a quick check for verifying the integrity of a casting.
More commonly used in industrial and aviation applications, ultrasonics can also be used to find internal flaws in castings and other parts. The technology uses sound waves to find cracks. A transponder generates an acoustic signal (up to 25 MHz) that passes into and through the part. Cracks or flaws will reflect some of the sound waves back to the detector, which allows the information to be displayed on the tester.
The best applications for ultrasonic testing include heavy castings, large shafts and expensive parts that may be used for racing or extreme-duty service. Ultrasonics can also be used to check the integrity of welds and welded castings. They can also be used to check for the integrity of cylinder wall thicknesses before or after boring.
You can find a lot more detail about each of these crack detection methods in our archived stories or in the September 2006 issue of Engine Builder magazine.
According to Engine Builder’s 2012 Machine Shop Market Profile (published in the July issue and available online), cylinder head work remains one of, if not the biggest part of many machine shops’ production. Though production numbers have shown some declines, cylinder heads continue to be profitable in gas and diesel rebuild facilities.
Yet despite our industry’s traditional ability to get the most out of its components, we learned that fewer cylinder heads are being repaired. We found that nearly 26 percent of diesel heads and nearly 30 percent of aluminum heads are being scrapped, both numbers significantly higher than last year. Obviously, part of this can be attributed to the low cost alternatives available in the aftermarket. But in cases of value or scarcity, parts are repaired and many shops do the work themselves. How they do it depends on what it is.
Apparently, the mystery of aluminum welding is less frightening because an increasing number of respondents say they weld cracked aluminum cylinder heads. Welding is used nearly 83 percent of the time, up from 77 percent last year. For diesel heads, welding is performed 25 percent of the time. Pinning remains the most-often used method for repairing cast iron cylinder heads, and has opened up a huge lead over welding.
“With later model heads, we find many cracks are caused by design issues,” says Gary Reed, of Lock-N-Stitch. “These may be related to lighter castings, 3- and 4-valve heads with seats nearly touching each other and everything else from too few head bolts to induction hardened valve seats in cast iron heads. Compared to engines manufactured 30 or more years ago, the cracking rate is much higher now.
Reed classifies all cracked castings into two categories: those that cracked due to an accident or incident like impact, freezing or overheating and those that cracked during normal operating conditions, which indicates a design issue or usage beyond the design capabilities.
Within aluminum cylinder heads, Reed finds stripped sparkplug threads and other threaded bolt holes, cracks between valve seats and corrosion due to lack of coolant maintenance are frequent complaints. With cast iron heads, he says induction hardened seats will eventually result in cracks across the seats but cracks in the combustion chambers due to poor cooling caused by design or maintenance issues is still the biggest problem.
Small diesel heads that frequently crack in areas with little or no coolant exposure have become common. Valve seat insert bores that are too close together often don’t have enough strength to support the press fit of the seat inserts.
Most of the cracking issues with engine blocks are related to cylinder wall thickness…or to be more precise, thinness. “Core shift has always been an issue but in today’s blocks the dramatic thinning of the walls often leads to strain cracks,” Reed says.
Other problems are seen with main and head bolt hole cracks due to the damaging radial forces exerted form the threads when torqued. Freezing still occurs but not as often as earlier blocks. Light marine blocks always provide a great source of revenue and profit due to the increased value over automotive blocks of the same design.
“Diesels seem to crack more than gas engines mostly due to their life cycle,” Reed says. “Also, more diesels are made from cast iron now while gas engines see a higher number of aluminum components. In diesel blocks, connecting rod failures are fairly common and remain some of the easiest and most lucrative to repair.”
Pinning is the most commonly used technique for repairing cracks in cast iron heads because it’s fast, reliable and cheap. It can also be used to repair aluminum castings, too. Pinning is a relatively easy technique to learn and use, doesn’t require any special tools other than a drill, guide fixture and tap, and uses no heat.
The technique involves drilling holes in both ends of the crack to keep it from spreading, then drilling holes at various intervals along the length of the crack, installing overlapping pins to fill the crack, then peening over the pins with an air hammer to seal and blend the surface. Either tapered pins or straight pins may be used.
If a crack is along an outside edge or corner that requires support to hold the sides of the crack together, or if the crack is in an area that would open up or pull apart when the casting is under load or gets hot, ordinary pins won’t work.
One solution is to use “locks” to hold the two sides of the crack together, and/or to use special pins that have a “spiral hook” or “reverse pitch” thread pattern. These pins can actually hold a crack together rather than just fill it.
Cracks in thin areas of a casting (thinner than 1/8˝) can be difficult to repair because the metal isn’t thick enough to support the threads on a standard pin. For these applications, very small pins must be used to fill the crack.
On some applications, the crack between the valve seats can often be repaired with a single soft steel pin that has a countersunk shoulder. A steel pin works best in this application because it can withstand heat better than a cast iron pin. After the crack has been fixed, the seats can be remachined. There should be no need to cut the head to accept valve seat inserts.
Welding is another exceptional method of repairing damaged components, says Karl Hoes, instructor at Lincoln Electric’s Motorsports Welding School. Welding techniques vary, but the basic idea is to melt the surrounding metal and fill the crack with molten metal and filler rod.
An experienced welder can even “recast” a badly damaged area, saving a head that would otherwise be junk. The strongest welds are achieved by using a filler rod that’s the same alloy as the head, or very close to it.
Hoes says there are many ways to repair aluminum and cast iron components with welding. While none of them are easy, exactly, experience and skill can allow you to repair many parts you might have trashed before.
“With aluminum cylinder heads, for example, TIG welding is commonly used. But heads are a really thick mass, and can be hard to weld on. Typically, we would weld aluminum on AC, alternating current. What we’ve been seeing is guys going to DC on aluminum heads,” explains Hoes. “They use straight helium gas, which allows them to get the heat into it very quickly. The helium adds a lot of heat to the arc.”
Aluminum is a superhighway for heat, says Hoes, conducting heat rapidly away during the welding process. “To get the aluminum to melt, you’ve got to get it up to 1,200 degrees F. You may have a 10,000 degree F arc but that huge chunk of aluminum is pulling it away as fast as you’re putting it in. And it’s expensive to use helium instead of argon to TIG weld. But sometimes you have to add helium to the gas just to get more heat into the work faster.”
Hoes points out that size does matter, especially when it comes to welding equipment. “These guys are using large machines. A lot of hobbyists have TIG machines at home but they don’t have the equipment needed to do this kind of work. It’s definitely not hobby equipment for a professional engine shop. You just won’t get good results. You may be able to fool yourself into thinking you’re doing the work, but you won’t fool the metal.”
It is true for aluminum, that you have to have advanced welding skills. You have to know how to TIG or MIG weld to repair aluminum blocks. For cast iron, you have to understand a little bit about metallurgy. There’s more involved than just running a nice bead. It’s understanding what happens to that steel when you weld on it, that you’re likely going to harden it up and make it brittle.
You need to remember that filler material makes a huge difference, says Hoes. Depending on the materials you’re trying to weld, their application, the desired sheer strength or ductility and many other factors, filler materials will be very different case-to-case.
“But while welding does take a certain skill set, as far as it being a profit center, yeah, it is,” says Hoes. “Because when it takes more skill or special equipment to do something, there’s not as many people willing to do it.”