La t´ecnica del An´ alisis de Correspodencias

The specific procedures to overhaul an engine are listed in its maintenance and overhaul manuals. The following discussion is a general overview of the practices and procedures in a major overhaul of a typical horizontally opposed engine.

Using the engine manufacturer’s manuals, service bulletins, and other service information is critical for an overhaul. One of your first tasks should be researching airworthiness directives and manufacturer’s service bulletins. Additionally, you need to gather the necessary inspection forms and tools for the job.

When an airplane arrives for an engine overhaul, inventory all of the accessories. These include the alternator, vacuum pump, hydraulic pump, and propeller governor.

Accessories will be removed and sent to appropriate repair facilities for overhaul.

Other components, including intercylinder baffles, carburetor or fuel injection components, magnetos, ignition leads, and the induction system, are considered engine parts and remain with the engine.

DISASSEMBLY

Before disassembling an engine, install it on a sturdy stand and inspect its general condition. Look for signs of oil leakage, overheating, or impact damage, and make notes of any discrepancies you find. Clean the engine with an appropriate solvent to remove dirt and oil. However, be sure to rinse all of the solvent from the engine.

After the engine is clean, remove the accessories and send them out for overhaul.

Remove and set aside the exhaust system, intercylinder baffles, ignition leads, spark plugs, and induction system components. When disassembling the engine, label all removed parts by attaching a tag to each part as you remove it. Place nuts, bolts, and other small parts in suitable containers during disassembly to prevent loss and damage.

Discard safety wire, cotter pins, and some other safety devices that are used only once and replace them with new parts. [Figure 2-35]

Figure 2-35. After being mounted on an engine stand, an engine is disassembled for a thorough visual inspection and cleaning.

Use the proper procedures for each task to avoid damaging engine parts or hardware.

For example, manufacturers recommend that before removing any cylinder, the associated piston should be placed at top dead center on the compression stroke. Doing so enables you to remove the cylinder without applying any pressure to the valve mechanism and provides support to the piston and connecting rod as you extract the cylinder. If the connecting rod and piston are not properly supported, they can fall and strike the crankcase, which can lead to failure in subsequent operations.

Crankshaft bearings, oil seals, gaskets, and stressed bolts and nuts are typically replaced at overhaul and should be discarded. In every case, consult the applicable manufacturer’s maintenance and overhaul manuals and service bulletins for a complete listing of required replacement items. Note any loose or damaged studs or fittings and examine them closely for evidence of more serious wear or damage.

Completely disassemble the engine and lay out the parts on a clean workbench for a preliminary inspection. Before you clean the internal engine parts, perform a visual inspection to look for residual deposits of metallic particles in oil residue and sludge, which might indicate an impending failure. Cleaning the parts before inspection could flush away the particles and make identifying damage more difficult. [Figure 2-36]

Figure 2-36. Engine parts are laid out in an orderly fashion on a clean workbench after cleaning to facilitate inspection. Keeping parts arranged in an orderly manner minimizes loss or accidental damage.

CLEANING

After you disassemble and visually inspect an engine, clean all of the parts. Soak or spray each part with a commercial safety solvent approved by the engine manufacturer.

Water-soluble degreasing solutions with soap or caustic compounds are normally not recommended because water-soluble solutions can be corrosive to aluminum and magnesium. Furthermore, residual soap can become trapped in the pores of engine components and cause oil contamination and foaming after the engine is returned to service. [Figure 2-37]

Figure 2-37. Several manufacturers produce fixtures that circulate an approved solvent for cleaning engine parts. These fixtures limit solvent splashes and provide containment for small parts that could otherwise be misplaced or lost.

Solvent is not always strong enough to remove hard carbon deposits that can build up on interior engine surfaces. To remove these deposits, follow the manufacturer’s recommendations and use an approved decarbonizing solution. For personal safety, exercise caution and follow the manufacturer’s recommendations. Always wear protective clothing and eye protection when you are working with these chemicals.

Decarbonizing solutions are available in a water-soluble or a hydrocarbon base.

Hydrocarbon-based solution is preferable because the water-soluble solutions have the same corrosion-inducing properties as water-soluble degreasers.

Avoid leaving engine parts in a decarbonizing solution longer than the prescribed time periods. Be sure the solution is safe for magnesium parts before immersing them. Do not mix steel and magnesium parts in the same container because the cleaning solution might cause a chemical reaction between the dissimilar metals and corrode the magnesium. Be especially careful with accessory housings because they often have high magnesium content.

After the parts have soaked the prescribed time, clean them to remove all traces of the decarbonizing solution. You can steam-clean parts or use mineral spirits and a stiff-bristle parts cleaning brush. Except for bearings or polished surfaces, remove any remaining carbon deposits with a scraper or wire brush, or by grit-blasting with plastic pellets or organic media such as rice, baked wheat, or crushed walnut shells. However, before beginning any grit-blasting process, insert rubber or plastic plugs in all drilled oil passages and mask all machined surfaces with a recommended material. Bolts and screws can be screwed into threaded holes to prevent grit from getting into the screw threads; discard this hardware after the part is cleaned. Because valve seats are typically hardened, they can usually be left unprotected during grit-blasting. In fact, you can use grit-blasting to remove the glaze that forms on valve seats, as well as the enamel from cylinder cooling fins that does not come off in a decarbonizing solution. To avoid damage when using a grit blast, use the lowest air pressure practical and blast the part only long enough to remove the carbon.

Always use extra caution when using mechanical means to clean hard carbon deposits;

you can cause unintentional damage if you use the tools too aggressively. Pay extra attention when cleaning any part with abrasive tools.

To clean machined or polished bearing surfaces such as journals and crankpins, use crocus cloth moistened with mineral spirits. After all the carbon is removed, polish the surfaces with dry crocus cloth.

Because hollow crankpins in a crankshaft are used as sludge chambers, the plugs must be removed from the crankpin ends for cleaning and visual inspection. If the crankpins are not thoroughly cleaned, sludge loosened during cleaning can clog crankshaft oil passages and cause subsequent bearing failure.

After cleaning, coat steel parts with a film of protective oil to prevent corrosion. If the oil coating must be removed for subsequent inspections, it is imperative to reapply it when the inspections are complete.