Extreme caution should be exercised when using a decar-bonizing solution on magnesium castings. It is recom-mended that the use of heated solutions be avoided unless the operator is thoroughly familiar with the particularsolution being used In addition, the operator is strongly advised against immersing steel and magnesium parts in the same decarbonizing tank, because this practice often results in damage to the magnesium parts from corrosion.
3-11. Decarbonizing will usually loosen most of the hard carbon deposits remaining after degreasing; the complete removal of all hard carbon, however, generally requiresbrushing, scraping or grit-blasting. All of these operationsdemand care on the part of the mechanic to avoid damage to machined surfaces. In particular, wire brushes and
metal scrapers must never be used on any bearing or con-tact surface.
3-12. When grit-blasting parts do not use sand or any metallic abrasives. It is recommended instead that mildlyabrasive organic substances such as rice baked wheat,plastic pellets, or crushed walnut shells be used. All machines surfaces must, of course, be adequately masked and all openings tightly plugged before blasting. The one exception to this is the valve seats, which may be left un-protected when blasting the cylinder head combustion chamber. It is often advantageous to grit blast the seats,since this will cut the glaze which tends to form (particular-ly on the exhaust valve seat) thus facilitating subsequentvalve seat reconditioning. Under no circumstances
Section 3 OVERHAUL MANUAL
LYCOMING 76 SERIES AIRCRAFT ENGINES
General Overhaul
should the piston ring grooves be grit blasted. If necessary, soak the piston in petroleum solvent and scrape with a wooden scraper. When grit-blasting hous-ings, plug all drilled oil passages with rubber plugs or other suitable material to prevent the entrances of foreign matter.
3-13. The decarbonizing solution will generally remove most of the enamel from exterior surfaces. All remaining enamel should be removed by grit-blasting particularly in the crevices between the cylinder cooling fins.
3-14. At the conclusion of cleaning operations, rinse the parts in petroleum solvent, dry and remove any loose par-ticles by air-blasting. Apply a liberal coating of preser-vative oil to all surfaces.
INSPECTION
3-15. The inspection of engine parts during overhaul is divided into three categories, visual, structural and dimensional. The first two deal with the structural defects in parts while the third is concerned with the size, shape and fit.
3-16. Visual inspection should precede all other inspection procedures. Do not clean any parts prior to visual inspec-tion, since indications of dangerous operating conditions can often be detected from the residual deposits found in some particular recess of the engine.
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