The inspection of engine parts during overhaul is divided into three categories:
- Visual
- Structural NDT
- Dimensional
Many defects on the engine components can be detected visually, and a determination of airworthiness can be made at this time. If, by visual inspection, the component is determined to be unairworthy, the part is rejected and no further inspection or repair is required. Structural failures can be determined by several different methods. Magnetic parts can readily be examined by the magnetic particle method. Other methods, such as dye penetrate, eddy current, ultra sound, and X-ray, can also be used. The first two methods are aimed at determining structural failures in the parts, while the last method deals with the size and shape of each part. By using very accurate measuring equipment, each engine component can be dimensionally evaluated and compared to service limits and standards (tolerances) set by the manufacturer.
Visual Inspection
Visual inspection should precede all other inspection procedures. Parts should not be cleaned before a preliminary visual inspection, since indications of a failure may often be detected from the residual deposits of metallic particles in some recesses in the engine.
Several terms are used to describe defects detected in engine parts during inspection. Some of the more common terms and definitions are:
- Abrasion—an area of roughened scratches or marks usually caused by foreign matter between moving parts or surfaces.
- Brinelling—one or more indentations on bearing races, usually caused by high static loads or application of force during installation or removal. Indentations are rounded or spherical due to the impression left by the contacting balls or rollers of the bearing.
- Burning—surface damage due to excessive heat. It is usually caused by improper fit, defective lubrication, or over-temperature operation.
- Burnishing—polishing of one surface by sliding contact with a smooth, harder surface. Usually no displacement nor removal of metal.
- Burr—a sharp or roughened projection of metal usually resulting from machine processing.
- Chafing—a condition caused by a rubbing action between two parts under light pressure that results in wear.
- Chipping—breaking away of pieces of material, that is usually caused by excessive stress concentration or careless handling.
- Corrosion—loss of metal by a chemical or electrochemical action. The corrosion products are easily removed by mechanical means. Iron rust is an example of corrosion.
- Crack—a partial separation of material usually caused by vibration, overloading, internal stresses, defective assembly, or fatigue. Depth may be a few thousandths, to the full thickness of the piece.
- Cut—loss of metal, usually to an appreciable depth over a relatively long and narrow area, by mechanical means, as would occur with the use of a saw blade, chisel, or sharp-edged stone striking a glancing blow.
- Dent—a small, rounded depression in a surface usually caused by the part being struck with a rounded object.
- Erosion—loss of metal from the surface by mechanical action of foreign objects, such as grit or fine sand. The eroded area is rough and may be lined in the direction that the foreign material moved relative to the surface.
- Flaking—the breaking loose of small pieces of metal or coated surfaces, that is usually caused by defective plating or excessive loading.
- Fretting—a condition of surface erosion caused by minute movement between two parts usually clamped together with considerable unit pressure.
- Galling—a severe condition of chafing or fretting in which a transfer of metal from one part to another occurs. It is usually caused by a slight movement of mated parts having limited relative motion and under high loads.
- Gouging—a furrowing condition in which a displacement of metal has occurred (a torn effect). It is usually caused by a piece of metal, or foreign material, between close moving parts.
- Grooving—a recess, or channel, with rounded and smooth edges usually caused by faulty alignment of parts.
- Inclusion—presence of foreign or extraneous material entirely within a portion of metal. Such material is introduced during the manufacture of rod, bar, or tubing by rolling or forging.
- Nick—a sharp-sided gouge or depression with a V-shaped bottom, that is generally the result of careless handling of tools and parts.
- Peening—a series of blunt depressions in a surface.
- Pick up or scuffing—a buildup or rolling of metal from one area to another, that is usually caused by insufficient lubrication, clearances, or foreign matter.
- Pitting—small hollows of irregular shape in the surface, usually caused by corrosion or minute mechanical chipping of surfaces.
- Scoring—a series of deep scratches caused by foreign particles between moving parts or careless assembly or disassembly techniques.
- Scratches—shallow, thin lines or marks, varying in degree of depth and width, caused by presence of fine foreign particles during operation or contact with other parts during handling.
- Stain—a change in color, locally, causing a noticeably different appearance from the surrounding area.
- Upsetting—a displacement of material beyond the normal contour or surface (a local bulge or bump). Usually indicates no metal loss.
Examine all gears for evidence of pitting or excessive wear. These conditions are of particular importance when they occur on the teeth; deep pit marks in this area are sufficient cause to reject the gear. Bearing surfaces of all gears should be free from deep scratches. However, minor abrasions usually can be dressed out with a fine abrasive cloth.
All bearing surfaces should be examined for scores, galling, and wear. Considerable scratching and light scoring of aluminum bearing surfaces in the engine do no harm and should not be considered a reason for rejecting the part, provided it falls within the clearances set forth in the table of limits in the engine manufacturer’s overhaul manual. Even though the part comes within the specific clearance limits, it is not satisfactory for re-assembly in the engine unless inspection shows the part to be free from other serious defects.
Ball bearings should be inspected visually and by feel for roughness, flat spots on balls, flaking or pitting of races, or scoring on the outside of races. All journals should be checked for galling, scores, misalignment, or out-ofround condition. Shafts, pins, etc., should be checked for straightness. This may be done, in most cases, by using V-blocks and a dial indicator.
Pitted surfaces in highly stressed areas, resulting from corrosion, can cause ultimate failure of the part. The following areas should be examined carefully for evidence of such corrosion:
- Interior surfaces of piston pins
- The fillets at the edges of crankshaft main and crankpin journal surfaces
- Thrust bearing races
If pitting exists on any of the surfaces mentioned, to the extent that it cannot be removed by polishing with crocus cloth or other mild abrasive, the part usually must be rejected.
Parts, such as threaded fasteners or plugs, should be inspected to determine the condition of the threads. Badly worn or mutilated threads cannot be tolerated; the parts should be rejected. However, small defects, such as slight nicks or burrs, may be dressed out with a small file, fine abrasive cloth, or stone. If the part appears to be distorted, badly galled, mutilated by overtightening, or from the use of improper tools, replace it with a new one.
Cylinder Head
Inspect the cylinder head for internal and external cracks. Use a bright light to inspect for cracks, and investigate any suspicious areas with a magnifying glass or microscope. Carbon deposits must be cleaned from the inside of the head, and paint must be removed from the outside for this inspection. Exterior cracks show up on the head fins where they have been damaged by tools or contact with other parts because of careless handling. Cracks near the edge of the fins are not dangerous, if the portion of the fin is removed and contoured properly. Cracks at the base of the fin are a reason for rejecting the cylinder. Cracks may also occur on the rocker box or in the rocker bosses. Interior cracks almost always radiate from the valve seat bosses or the spark plug bushing boss. These cracks are usually caused by improper installation of the seats or bushings. They may extend completely from one boss to the other. Inspect the cylinder walls for rust, pitting, or scores. Mild damage of this sort can be removed when the cylinders are deglazed. With more extensive damage, the cylinder has to be reground or honed. If the damage is too deep to be removed by either of these methods, the cylinder usually will have to be rejected. Most engine manufacturers, or engine overhaul repair stations, have an exchange service on cylinders with damaged barrels.
Piston, Valve Train, and Piston Pin
When applicable, check for flatness of the piston head using a straightedge and thickness gauge. [Figure] If a depression is found, check for cracks on the inside of the piston. A depression in the top of the piston usually means that detonation has occurred within the cylinder.
Inspect the exterior of the piston for scores and scratches. Scores on the top ring land are not cause for rejection, unless they are excessively deep. Deep scores on the side of the piston are usually a reason for rejection. Examine the piston for cracked skirts, broken ring lands, and scored piston-pin holes. Do not mistake casting marks or laps for a crack. During major overhaul, most pistons are generally replaced, as it requires more labor to clean and inspect the piston than it costs to replace it.
Examine the valve visually for physical damage and damage from burning or corrosion. Do not re-use valves that indicate damage of this nature.
Using a magnifying glass, examine the valve in the stem area and the tip for evidence of cracks, nicks, or other indications of damage. This type of damage seriously weakens the valve, making it susceptible to failure. If superficial nicks and scratches on the valve indicate that it might be cracked, inspect it using a structural inspection method described later. Examine the valve springs for cracks, rust, broken ends, and compression. Cracks can be located by visual inspection or the magnetic particle method.
Inspect the rocker shaft bosses for scoring, cracks, oversize, or out-of-roundness. Scoring is generally caused by the rocker shaft turning in the bosses, which means either the shaft was too loose in the bosses or a rocker arm was too tight on the shaft. Inspect the rocker arm bushing for correct size by sliding the shaft into the bushings to check for proper clearance between the shaft and the bushing. This clearance is also dimensionally checked during the dimensional inspection to confirm the proper clearance. Often, the bushings are scored because of mishandling during disassembly. Check to see that the oil holes line up. At least 50 percent of the hole in the bushing should align with the hole in the rocker arm. On engines that use a bearing rather than a bushing, inspect the bearing to make certain it has not been turning in the rocker arm boss. Also, inspect the bearing to determine its serviceability. Inspect the valve rockers for cracks and worn, pitted, or scored tips. See that all oil passages are free from obstructions.
Inspect all the studs on the cylinder head for looseness, straightness, damaged threads, and proper length. Slightly damaged threads may be chased with the proper die. The length of the stud should be correct within ±1⁄32 (0.03125) inch to allow for proper installation of safety devices.
Crankshaft and Connecting Rods
Carefully inspect all surfaces of the crankshaft for cracks. Check the bearing surfaces for evidence of galling, scoring, or other damage. When a shaft is equipped with oil transfer tubes, check them for tightness.
Visual inspection of connecting rods should be done with the aid of a magnifying glass or bench microscope. A rod that is obviously bent or twisted should be rejected without further inspection. Inspect all surfaces of the connecting rods for cracks, corrosion, pitting, galling, or other damage. Galling is caused by a slight amount of movement between the surfaces of the bearing insert and the connecting rod during periods of high loading, such as that produced during overspeed or excessive manifold pressure operation. The visual evidence produced by galling appears as if particles from one contacting surface had welded to the other. Evidence of any galling is sufficient reason for rejecting the complete rod assembly. Galling is a distortion in the metal and is comparable to corrosion in the manner in which it weakens the metallic structure of the connecting rod.