Both maintenance and complete engine overhauls are performed normally at specified intervals. This interval is usually governed by the number of hours the powerplant has been in operation. The actual overhaul period for a specific engine is generally determined by the manufacturer’s recommendations. Each engine manufacturer sets a total time in service when the engine should be removed from service and overhauled. Depending upon how the engine is used in service, the overhaul time can be mandatory. The overhaul time is listed in hours and is referred to as time before overhaul (TBO). For example, if an engine had a life of 2,000 hours and had operated 500 hours, it would have a TBO of 1,500 hours. Tests and experience have shown that operation beyond this period of time could result in certain parts being worn beyond their safe limits. For an overhauled engine to be as airworthy as a new one, worn parts, as well as damaged parts, must be detected and replaced during overhaul. The only way to detect all unairworthy parts is to perform a thorough and complete overhaul process while the engine is disassembled. The major purpose of overhaul is to inspect, repair, and replace worn engine parts.
A complete overhaul process includes the following ten steps: receiving inspection; disassembly; visual inspection; cleaning; structural inspection; non-destructive testing (NDT) inspection; dimensional inspection; repair and replacement; reassembly; and testing and break in. The inspection phases are the most precise and the most important phases of the overhaul. Inspection cannot be slighted or performed in a careless or incomplete manner. It is always recommended that complete records be made of the inspection process and kept with the engine records.
Each engine manufacturer provides very specific tolerances to which the engine parts must conform and provides general instructions to aid in determining the airworthiness of the part. However, in many cases, the final determination must be made by the technician. Although the determination must be made if the part is serviceable, repairable, or should be rejected, the technician should follow the manufacturer’s manuals and information. When dimensional tolerances are concerned, the manufacturer publishes a new minimum and serviceable dimension for all critical component parts. Knowledge of the operating principles, strength, and stresses applied to a part is essential in making decisions regarding visible wear. When the powerplant technician signs the release for the return to service for an overhauled engine, this certifies that the complete overhaul process has been performed using methods, techniques, and practices acceptable to the Federal Aviation Administration (FAA) Administrator.
Top Overhaul
Reciprocating piston aircraft engines can be repaired by a top overhaul. This means an overhaul of those parts on top of the crankcase, without completely dismantling the engine. It includes removal of the units (i.e., exhaust collectors, ignition harness, intake pipes) necessary to remove the cylinders. The actual top overhaul consists of reconditioning the engine’s cylinders by replacing or reconditioning the piston and piston rings, and reconditioning or plating the cylinder wall and valve-operating mechanism, including valve guides if needed. A top overhaul is a little misleading, because it is really an engine repair procedure and not a real overhaul as described earlier. Usually at this time, the accessories require no attention other than that normally required during ordinary maintenance functions. This repair is generally due to valves or piston rings wearing prematurely. Many stress that if an engine requires this much dismantling, it should be completely disassembled and receive a major overhaul.
Major Overhaul and Major Repairs
Major overhaul consists of the complete reconditioning of the powerplant. A reciprocating engine would require that the crankcase be disassembled per the FAA; a major overhaul is not generally a major repair. A certified powerplant-rated technician can perform or supervise a major overhaul of an engine if it is not equipped with an internal supercharger or has a propeller reduction system other than spur-type gears. At regular intervals, an engine should be completely dismantled, thoroughly cleaned, and inspected. Each part should be overhauled in accordance with the manufacturer’s instructions and tolerances for the engine involved. At this time all accessories are removed, overhauled, and tested. Again, instructions from the manufacturer of the accessory concerned should be followed.
General Overhaul Procedures
Because of the continued changes and the many different types of engines in use, it is not possible to treat the specific overhaul of each engine in this text. However, there are various overhaul practices and instructions of a nonspecific nature that apply to all makes and models of engines.
Any engine to be overhauled completely should receive a runout check of its crankshaft or propeller shaft as a first step. Any question concerning crankshaft or propeller shaft replacement is resolved at this time, since a shaft whose runout is beyond limits must be replaced.
Receiving Inspection
The receiving inspection consists of determining the general condition of the total engine as received, along with an inventory of the engine’s components. The accessory information should be recorded, such as model and serial numbers, and the accessories should be sent to overhaul if needed. The overhaul records should be organized, and the appropriate manuals obtained and reviewed along with a review of the engine’s history (log books). The engine’s service bulletins, airworthiness directives, and type certificate compliance should be checked. The exterior of the engine should be cleaned after mounting it on an overhaul stand.
Disassembly
As visual inspection immediately follows disassembly, all individual parts should be laid out in an orderly manner on a workbench as they are removed. To guard against damage and to prevent loss, suitable containers should be available in which to place small parts (nuts, bolts, etc.) during the disassembly operation.
Other practices to observe during disassembly include: 1. Drain the engine oil sumps and remove the oil filter. Drain the oil into a suitable container; strain it through a clean cloth. Check the oil and the cloth for metal particles. 2. Dispose of all safety devices (safety wire, cotter pins, etc.) as they are removed. Never use them a second time. Always replace with new safety devices. 3. All loose studs, and loose or damaged fittings, should be carefully tagged to prevent being overlooked during inspection. 4. Always use the proper tool for the job. Use sockets and box end wrenches wherever possible. If special tools are required, use them rather than improvising.
Inspection Process
The inspection of engine parts during overhaul is divided into three categories: 1. Visual 2. Structural NDT 3. 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:
1. Abrasion: an area of roughened scratches or marks usually caused by foreign matter between moving parts or surfaces.
2. 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.
3. Burning: surface damage due to excessive heat. It is usually caused by improper fit, defective lubrication, or over-temperature operation.
4. Burnishing: polishing of one surface by sliding contact with a smooth, harder surface. Usually no displacement nor removal of metal.
5. Burr: a sharp or roughened projection of metal usually resulting from machine processing.
6. Chafing: a condition caused by a rubbing action between two parts under light pressure that results in wear.
7. Chipping: breaking away of pieces of material, that is usually caused by excessive stress concentration or careless handling.
8. 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.
9. 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.
10. 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.
11. Dent: a small, rounded depression in a surface usually caused by the part being struck with a rounded object.
12. 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.
13. Flaking: the breaking loose of small pieces of metal or coated surfaces, that is usually caused by defective plating or excessive loading.
14. Fretting: a condition of surface erosion caused by minute movement between two parts usually clamped together with considerable unit pressure.
15. 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.
16. 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.
17. Grooving: a recess, or channel, with rounded and smooth edges usually caused by faulty alignment of parts.
18. 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.
19. Nick: a sharp-sided gouge or depression with a V-shaped bottom, that is generally the result of careless handling of tools and parts.
20. Peening: a series of blunt depressions in a surface.
21. 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.
22. Pitting: small hollows of irregular shape in the surface, usually caused by corrosion or minute mechanical chipping of surfaces.
23. Scoring: a series of deep scratches caused by foreign particles between moving parts or careless assembly or disassembly techniques.
24. 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.
25. Stain: a change in color, locally, causing a noticeably different appearance from the surrounding area.
26. 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: 1. Interior surfaces of piston pins 2. The fillets at the edges of crankshaft main and crankpin journal surfaces 3. 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. 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 over-speed 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.
