Through Transmission Dimensional Inspection Cylinder Barrel Resonance Magnetic Particle Inspection X-ray Through transmission inspection uses two transducers, one to  generate the pulse and another placed on the opposite surface to  receive it. A disruption in the sound path indicates a flaw and  is displayed on the instrument screen. Through transmission  is less sensitive to small defects than the pulse-echo method.  This system differs from the pulse-echo method, in that  the frequency of transmission may be continuously varied.  The resonance method is principally used for thickness  measurements when the two sides of the material being tested  are smooth and parallel, and the backside is inaccessible. The  point at which the frequency matches the resonance point of  the material being tested is the thickness determining factor. Magnetic particle inspection is a method of detecting  invisible cracks and other defects in ferromagnetic materials,  such as iron and steel. It is not applicable to nonmagnetic  materials. The inspection process consists of magnetizing  the part, and then applying ferromagnetic particles to the  surface area to be inspected. The ferromagnetic particles  (indicating medium) may be held in suspension in a liquid  that is flushed over the part; the part may be immersed in  the suspension liquid; or the particles, in dry powder form,  may be dusted over the surface of the part. The wet process  is more commonly used in the inspection of aircraft parts. If a discontinuity is present, the magnetic lines of force  are disturbed, and opposite poles exist on either side of the  discontinuity. The magnetized particles form a pattern in  the magnetic field between the opposite poles. This pattern,  known as an indication, assumes the approximate shape of  the surface projection of the discontinuity. A discontinuity  may be defined as an interruption in the normal physical  structure or configuration of a part.  X-rays can penetrate material and disclose discontinuities  through the metal or non-metal components, making it an  excellent inspection process when needed to determine the  structural integrity of an engine component. The penetrating  radiation is projected through the part to be inspected and  produces an invisible or latent image in the film. When  processed, the film becomes a radiograph, or shadow picture,  of the object. This inspection medium, as a portable unit,  provides a fast and reliable means for checking the integrity  of engine components. Additional and more thorough information on NDT  inspection is covered in detail in the General Aviation  Maintenance Technician Handbook FAA-H-8083-30.  The dimensional inspection is used to assure that the engine’s  component parts and clearances meet the manufacturer’s  specifications. These specs are listed in a table of limits,  which lists serviceable limits and the manufacturer’s new part  maximum and minimum dimensions. Many measuring tools  are used to perform the dimensional inspection of the engine.  Some examples of these devices are discussed as the procedure  for measuring the engine’s components for dimensional  inspection is explained in the following paragraphs.  Inspect the cylinder barrel for wear, using a cylinder bore  gauge  [Figure 10-5] , a telescopic gauge, and micrometer or  an inside micrometer. Dimensional inspection of the barrel  consists of the following measurements: All measurements involving cylinder barrel diameters must  be taken at a minimum of two positions 90° apart in the  particular plane being measured. It may be necessary to take  more than two measurements to determine the maximum  wear. Taper of the cylinder walls is the difference between the  diameter of the cylinder barrel at the bottom and the diameter  at the top. The cylinder is usually worn larger at the top than  at the bottom. This taper is caused by the natural wear pattern.  At the top of the stroke, the piston is subjected to greater heat  and pressure and more erosive environment than at the bottom  of the stroke. Also, there is greater freedom of movement  at the top of the stroke. Under these conditions, the piston  1.  Maximum taper of cylinder walls 2.  Maximum out-of-roundness 3.  Bore diameter 4. Step 5.  Fit between piston skirt and cylinder Figure 10-5.  A cylinder bore gauge.  10-8