Handling Stress Fractures - Engine Builder Magazine

Handling Stress Fractures

Salvaging Cracked or Flawed Engine Parts through Detection and Repairs

Cracks are often blamed as the cause of a head failure. In many instances, the cracks are not the cause of the failure, but a symptom of another underlying problem such as overheating, detonation or incorrect installation (wrong torque on head bolts, dirty bolt threads, etc.).

web crack1
Depending on their locations, crack severity will vary.

The good news is that cracks do not necessarily mean a cylinder head has to be replaced. In fact, many cracked heads that were once thought to be “unrepairable” are now being fixed. Repairing a cracked cylinder head always involves a certain amount of risk, but when done properly is usually much less expensive than replacing a cracked head with a new or used casting.

Today’s engine builders have available to them a number of state-of-the-art tools and techniques to locate, identify and repair cracks and other damage in a
variety of engine components. However, without excellent crack detection and repair methods, relying on good ol’ 20/20 eyesight may not be enough.

Finding a crack isn’t necessarily a bad thing – not looking for them definitely is.

Crack Happens
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.
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.

web crack 2
Without crack detection tools and techniques when building engines, what you can’t see can most definitely hurt you

Detecting Cracks
The following are some helpful ­procedures your builders can use to help detect cracks, flaws or other anomalies that can be repaired (at a price), and prevent engine damage down the road.

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.
Remember, for this method, the training of the operator is imperative, and so is part cleanliness.

web crack 3
There are a variety of techniques that can be used by themselves or in combination with other methods to find cracks in castings and other components, including crankshafts, camshafts, etc. These include magnetic particle inspection, various types of penetrating dyes, pressure testing, vacuum testing and ultrasonic (acoustic) testing.

Dye Penetrant Inspection:  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 penetrant 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.

Note: 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 will help you see what’s going wrong inside the engine.

Vacuum Testing: This 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.

Ultrasonic Testing: 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.

web crack 4
Cracking is an indication that an area is experiencing more stress than it can handle.

 

And the Survey Says…
According to Engine Builder’s 2013 Machine Shop Market Profile, “Repair before replace” is an increasingly common mantra in some segments of the cylinder head business. Though production numbers have shown some declines, cylinder heads continue to be profitable in gas and diesel rebuild facilities.

Our research found that a smaller percentage of diesel heads are being scrapped (although aluminum heads continue to be scrapped at a higher rate). When they are repaired, rebuilders continue to leave the work to the experts.

Our survey results indicate that 36 percent of respondents say they do aluminum cylinder head crack ­repairs themselves and 35 percent do their own diesel head repair.
Welding is used as a repair method nearly 75 percent of the time with aluminum cylinder heads and 36 percent of the time with diesel heads.

Pinning remains the most-often used method for repairing diesel cylinder heads (done 65 percent of the time) but is used in only one-quarter of the aluminum head repairs.

Pinning is also 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.

Tapered pins pull themselves into a crack as they are tightened to provide a tight seal along the entire length of the pin. This occurs because the threads on both the tapered pin and hole have an interference fit. Sealer really isn’t necessary, but may be used for added insurance. The holes for tapered pins must be carefully hand tapped with a tapered tap, and the pins hand tightened.

Straight pins, by comparison, can be installed with an ordinary straight tap and a power drill. Straight pins, however, must be sealed by a tapered shoulder on one end of the pin and/or with sealer.

Check out our Engine Builders ­Buyers Guides for pinning and ­welding tools and equipment.

You May Also Like

The Road to AAPEX Season 2, Ep 8

The innovation of the Lincoln Highway wasn’t just about building roads—it was about putting them on the map. The Wyoming Historical Society notes that the abandoned routes laid down by the Union Pacific railroad from 1867 to 1869 were often the best, and sometimes the only, east-west path available when developing the Lincoln Highway. In fact,

The innovation of the Lincoln Highway wasn’t just about building roads—it was about putting them on the map. The Wyoming Historical Society notes that the abandoned routes laid down by the Union Pacific railroad from 1867 to 1869 were often the best, and sometimes the only, east-west path available when developing the Lincoln Highway. In fact, staying so close to the rail line meant that the Lincoln Highway had about 100 crossings of train tracks throughout the state.

Shop Solutions – October 2023

A written warranty provides benefits for you and your customer. It sets expectations, protects both parties and is a great marketing tool that encourages repeat business.

Valve Springs

High-frequency fatigue, also known as harmonics, are a ubiquitous challenge in racing engines and can potentially wreak havoc on the valvetrain if left unchecked. Well-designed valve springs play a pivotal role in managing this, ensuring essential stability, and minimizing wear on valvetrain components.

The Latest on Lifters

For racing, a common trend to eliminate the problems associated with hydraulic lifters are the use of limited or short-travel hydraulic lifters. A reduction in plunger travel, which is usually about half that of a traditional full-travel lifter, reduces the amount of oil required to fill the lifter, which in turn reduces the compression of aerated oil.

What to Consider When Selecting Pushrods

Determining the correct pushrod length is often regarded as one of the most intricate aspects of the selection process due to the variability in valvetrain geometry and design.

Other Posts

The Road to AAPEX Season 2, Ep 7

Rolling through Nebraska, Babcox Media’s Joe Keene meets kindred spirits as he drives the rare Lincoln Blackwood he refurbished for his journey to AAPEX. Along the way, he meets a family traveling the Lincoln Highway section by section, taking in sights such as the Great Platte River Road Archway Monument and the Classic Car Collection,

Steve Morris Tackles a Tough Cylinder Head Repair

Cylinder head repairs like this don’t normally get done, but Steve Morris of @stevemorrisracing always seems to find a way! Related Articles – Your Brake Fluid Needs to be Changed! – Noonan’s 4.8 Bore Space Billet Hemi Engine – Take a Tour of PAR Racing Engines

The Road to AAPEX Season 2, Ep 6

It’s the unexpected moments that make a trip memorable. Babcox Media’s Joe Keene, an ASE-certified technician who fixed a rare Lincoln Blackwood, enters Iowa on his drive to AAPEX and discovers the charm of small-town America along the Lincoln Highway. A standout spot is the Lincoln Highway Hotel. Related Articles – The Road to AAPEX

Rocker Arm Update

Not only are customers asking for higher quality, they’re also becoming accustomed to having to wait a little while longer to get it. And, on the aluminum rocker side of things, the trend for customers has been a desire for lighter rocker designs.