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The Value and Benefit of Sonic Testing

Sonic testing is probably one of the most valuable tools of engine building – and often the most overlooked. It allows the engine builder to gain a basic perspective of the integrity of the parts that are going to be used for the engine build.

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Sonic testing is probably one of the most valuable tools of engine building – and often the most overlooked. It allows the engine builder to gain a basic perspective of the integrity of the parts that are going to be used for the engine build and it’s a way of seeing what limitations there may be along with the quality of the pieces you are working with.

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However, sonic testing is rarely considered before work begins. Unless you are dealing with new aftermarket parts or components, the sonic tester should be the first tool out of the toolbox. Of course, sonic testers are needed with aftermarket parts also, but their use should be a priority if OEM components are going to be used again.

I say this because of an experience I had several years ago. A customer of mine had brought me an engine block that he had picked up for a “real good deal.” He wanted the engine block to have some mild street performance and go back into a truck that he had found. Now, granted, he did pick up the block at a cheap price but after sonic testing the block was on its way to the scrap pile. After measuring the block with a bore gauge it was found to have been bored .040˝ over at some point of its life. Through sonic testing, we saw that some of the bores were as thin as .023˝ on one side and .190˝ on the other. The block had major core shift and someone had decided to bore the block .040” over to correct some issues but little did they know that they were creating even bigger ones. If a sonic tester had been used to begin with, the block would never have been machined that much.

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This type of core shift and other flaws can be more common when dealing with OEM components. You have to remember that back in the day blocks and other components were manufactured strong and heavy. But as manufacturers looked for cost savings and weight reduction, engine components became thin and disposable. Aftermarket components tend to be thick and rigid but it’s still important to use sonic testing to know your limitations on things such as maximum bore size. Remember, try not to only look at factors such as thickness of the object you are sonic testing but also to see what flaws or casting voids may be hidden under the surface. Sonic testing not only applies to components such as blocks but can also be applied to cylinder heads and intake manifolds.

Sonic testing goes back as far the early 1940s known as a “non-destructive testing technique.” This procedure was patented by Floyd Firestone as a way to detect flaws in castings. Sound waves or what is known as high frequency vibrations were placed into a cast steel or aluminum object to detect thickness and defects. By transmitting sound waves through an object you can determine the density of the object when they reflect back. If there are any casting defects, flaws, or inconsistencies the sound waves reflecting back will show a change in density.

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Sonic testing became very popular in the late 1970s in the aerospace industry. The aerospace industry was looking for a way to measure exact bolt torque, which can vary due to material used, fractures and different friction coefficients. Sonic testing was used because the sound waves could be used to determine load stress and strain. This was a more effective and accurate way to measure stretch instead of torque that is still practiced today.

Sonic testing done the proper way is not cheap but can still be affordable. There are proper procedures for sonic testing different alloys. Cast iron, for instance, tends to have a very coarse grain structure and is very sandy. Because of its structure, sound will scatter and reflect, making it hard to get penetration. The sonic tester being used will need to have a way of utilizing a lower frequency and probe that will penetrate and be forgiving of the grain structure. The game changes when trying to sonic test cast aluminum, especially when it is polished like an intake port of a cylinder head. When sonic testing, the polished surface will cause the sound wave to be very noisy and try to talk to the probe instead of penetration. This is where higher frequency can offer more resolution for the cast aluminum alloy.

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Here are five basic things to remember when purchasing and operating a sonic tester:

Sonic testing is the measure of density with sound

Sonic testing dictates the way sound travels

• Sonic testing different alloys requires different frequencies

• Sonic testing with high frequency offers resolution

• Sonic testing with lower frequency offers penetration

Proper sonic testing can only be achieved with the proper frequency and probe for the alloy being tested.

Most aftermarket blocks are made up of CGI (Compacted Graphite Iron). With CGI, the tensile strength is much higher and cylinder walls are usually very thick so there are not many worries about enough material. When building an engine the object is to get the proper bore size for the valve area of the cylinder head being used. Pay close attention to the thickness between the cylinders when trying to achieve the bore size desired. Most builders will want enough thickness there for proper head gasket seal which is usually between .200” to .240” thickness. This is very important when dealing with Siamese bores. When dealing with OEM blocks this range of thickness may be hard to achieve so keep the bores as thick as possible and use some sort of block filler which will help stabilize the bores.      

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In order to deliver a more uniform and flawless casting of components such as cast iron, stainless steel, and cast aluminum, many manufacturers are using a process known as “HIPPING” which stands for Hot Isostatic Pressing. In HIPPING, parts or components are loaded into a barrel-shaped high pressure vessel. The size of the vessel depends on how big the components are and how many of them will be treated at a given time. Once the components are loaded and the vessel is sealed, the vessel is filled with heated inert gas – usually argon – under extreme pressure. Pressures can range anywhere from 1500 psi to a maximum of 30,000 psi depending on the alloy being treated. The high pressure heated gas causes the alloy to become like “plastic” which will cause the voids that may lie under the surface to collapse. The surfaces where the voids were bond together to eliminate the defects which will achieve density without changing shape while improving the mechanical properties of the components.

This method of treatment offers total elimination of porosity in casting along with increased resistance to fatigue and temperature with a higher resistance to impact, wear, and abrasion along with improved ductility.

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Most OEM’s are incorporating the use of HIPPING on certain components along with some manufacturers of aftermarket parts. Some aftermarket manufacturers of aluminum cylinder heads now offer the HIPPING treatment process for an additional expense when purchasing. This treatment is also used for components such as connecting rods and compressor wheels on turbochargers. So depending on your application such as drag racing, tractor pulling, circle track or other extreme engine requirement, the expense may be well worth the additional integrity of the component.

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