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Looking For Leakers: Crack Detection Technology
Crack detection has always been an essential step in the engine building process for identifying parts that have to be replaced or repaired. It’s a way to verify the condition of critical parts like cylinder heads, engine blocks, crankshafts, camshafts and connecting rods, and a smart way to make money, save money and save yourself and your customers a lot of unnecessary grief.
By Larry Carley
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It doesn’t matter if you’re rebuilding a high mileage engine and reusing salvaged components or using brand new parts right out of the box any highly stressed component or casting that has to withstand high loads, temperatures and/or pressures should always be inspected to make sure it is free from cracks or other defects that could cause it to leak or fail. Making sure parts are free from cracks will help assure maximum engine reliability and minimize your risk of warranty problems because of component failures or leaks.
With today’s high labor costs, you can’t afford to waste a lot of time machining and reconditioning cores and used parts that may be flawed. Though some cracks may not be a concern (depending on their location), others can cause serious problems. And even if a crack isn’t very big or isn’t leaking now, there’s no guarantee it will stay that way.
As a rule, cracks usually spread and only get worse as the miles accumulate. Heat, thermal stress, heavy loads, repeated bending and flexing, metal fatigue, pounding and vibration are all factors that contribute to the formation of cracks and make them grow. So if metal is pulling away from metal, it means an area is experiencing more stress than it can handle. That’s why cracks form in the first place.
If critical parts are not inspected for cracks, there’s no way to know if they will stand up to normal use and abuse. In the case of cylinder heads and blocks, you won’t know if the castings can hold pressure until the engine has been assembled which means you may have wasted a lot of time and effort if a casting turns out to be a leaker.
Cracks and other flaws such as porosity leaks in castings need to be identified so a decision can be made as to whether repair or replacement is the best option. 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.
Additional payback comes from what you can charge your customers for crack inspection of their parts. Like cleaning, crack detection can be a profitable service in any shop when it is priced right. Some shops may charge $10 to inspect a crank while others will charge $50. Some shops may charge $25 to check a cylinder head while another charges $75. The price you charge should reflect the time invested in the job, the cost of your equipment and any consumable supplies that may be used. Don’t base your price on what the guy down the street charges.
Cracks are quite common in late model cylinder heads and are often found between valve seats, in exhaust ports, between the spark plug hole and valve seats, around valve guides, between combustion chambers, and even on top of the head.
Thinner castings and higher engine loads are contributing factors, but in many instances the underlying cause is engine overheating due to a cooling system failure (coolant leak, inoperative cooling fan, stuck thermostat, etc.), or a detonation problem (carbon buildup, inoperative EGR valve, too much ignition advance, etc.).
Some heads, such as the Ford 2.9L V6 and Escort 1.6L heads, are notorious for cracking. So, too, are Ford "HSC" (High Swirl Combustion) cast iron heads like those on the 2.3L and 2.5L OHC engines. Others include the General Motors 2.5L "Iron Duke" head, the GM 250 six-cylinder head with an integral exhaust manifold, and 1987 and later Chevy small block V8 "Vortex" heads.
When cracks form between the cooling jacket and combustion chamber, coolant will enter the combustion chamber. Steam is quite effective at decarbonizing the combustion chamber, but it also washes away the lubricant from the rings and cylinder wall, which accelerates wear. If coolant ends up in the crankcase through a crack in a cylinder wall, it can wipe out the bearings.
One "quick check" for coolant leaks in used cylinder heads and blocks is to simply note the appearance of the combustion chambers and pistons when the engine is torn down. If a combustion chamber or piston lacks the normal accumulation of carbon deposits, it probably has a coolant leak in the head or cylinder.
"Dry" cracks that do not leak coolant can also cause trouble. Cracks between or around valve seats in an aluminum head may allow the seats to come loose and fall out. Cracks around valve guides may lead to loosening of the valve guides, which can damage the valves.
Cracks in cranks, cams and connecting rods can lead to breakage of these parts, too. In such cases, you don’t have to look for the crack if the part has already failed. What you do have to look for are any underlying causes that may have contributed to the formation of the crack or caused the component to fail.
For example, most broken camshafts in an OHC cylinder head are not caused by flaws in the camshaft. They’re often caused by engine overheating. The head gets too hot, warps, seizes the cam and causes it to snap.
The same goes for broken crankshafts, except the list of possible causes is longer. Many factors can lead to a broken crank, including an incorrect radius when the crank journals were machined, a severe engine detonation problem, bent connecting rods, a loose main bearing cap, main bore misalignment, a bent crank, a severe engine imbalance or an imbalance in the vibration damper, flywheel, clutch or torque converter.
Though many cracks may be clearly seen once parts have been disassembled and cleaned, other cracks are nearly invisible or may only be seen under special conditions (such as porosity leaks in heads and blocks). Other cracks may be entirely hidden from view, such as those inside a casting.
Never assume a part or a casting is okay just because you can’t see any visible cracks. Always assume there may be cracks.
There are a variety of crack inspection techniques that can be used by themselves or in combination with other techniques to find cracks in castings and other components. These include magnetic particle inspection, various types of penetrating dyes, pressure testing, vacuum testing, ultrasonic (acoustic) testing and even x-rays. All except the last two are commonly used in the automotive industry.
Magnetic Particle Inspection
This technique can be used to find cracks in cast iron or steel alloys that are "ferromagnetic" and can be temporarily magnetized. The magnetic field is created by a permanent magnet, an electromagnet or by passing a part through a large magnetic coil. Tiny iron oxide particles are then 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. It’s a quick and easy-to-use test that can be employed to screen cores, machined parts and new parts.
Though some people call this technique "magnafluxing," the term "Magnaflux" is actually a trademark of the Magnaflux Corporation and applies to its magnetic particle detection equipment and process. Even so, there are a number of other companies that make similar equipment for magnetic particle detection.
The iron particles used to detect cracks are finely divided in sizes varying between .125˝ and 60 microns, are easily magnetized but don’t stay that way for long (so they don’t stick together or to the metal being tested). The particles may be in a dry powder or a wet solution. Particles are dyed yellow, white, red, gray, black or with a 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.
Dry particles may be applied with a squeeze bulb or spray gun. Wet particles may be mixed with an oil- or water-based liquid or paste. 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).
One of the limitations of this technique is that neither the wet nor dry method can find cracks that are more than about .100˝ to .250˝ below the surface of the metal. Magnetic particle inspection also won’t work on nonferrous metals such as aluminum, magnesium, titanium, nonmagnetic alloys of stainless steel or plastic.
Magnetic particle inspection is most often used to inspect cast iron cylinder heads for surface cracks in and around the 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.
When checking for cracks, the stronger the magnetic field the more easily it will reveal cracks. For this reason, more powerful electromagnets or magnetic coils may be better than less expensive permanent magnets.
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