莱康明发动机大修手册 OVERHAUL MANUAL LYCOMING 76 SERIES AIRCRAFT ENGINES(9)

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3-24. Magnetic Inspection. All ferro-magnetic steel parts should be inspected by the magnetic particle method. The successful detection of structural failure by magnetic in-spection demands skill and experience on the part of operating personnel. It must be remembered that almost any fabricated steel part will shown indications of some kind, and it is important that the operator exercise good judgement in evaluating the indications. Too rigid an in-terpretation may result in the rejection of a sound part, while on the other hand, a part showing a dangerous in-dication may be returned to service as a result of a too casual diagnosis. In general, areas of stress concentration must be watched closely for fatigue cracks. These areas include such locations as keyways, gear teeth, splines,
roots of threads, small holes and fillets.
3-25. Proper judgement must also be used in determining the amount of current (amperage) applied; too little cur-rent will not sufficiently magnetize the part, while too heavy an application will permanently damage the part by overheating and burning thin areas adjacent to the elec-trodes. Again, skill and experience on the part of the operator are of the utmost importance. Consult the latest edition of Service Instruction No. 1285 for proper amperage.
3-26. Corrosion-Prevention. Upon completion of inspec-tion, coat all steel parts with preservative oil.
REPAIR AND REPLA CEMENT

3-27. Damaged Parts. Abnormal damage such as burrs, nicks, scratches, scoring, or galling should be removed with a fine oil stone, crocus cloth, or any similiar abrasive substance. Following any repairs of this type, the part should be carefully cleaned in order to be certain that all abrasive has been removed and then checked with its mating part to assure that the clearances are not ex-
OVERHAUL MANUAL LYCOMING 76 SERIES AIRCRAFT ENGINES Section3
cessive. Flanged surfaces that are bent, warped, or nicked may be repaired by lapping to a true surface on a surface plate. Again the part should be cleaned to be certain that all abrasive has been removed. Defective threads can sometimes be repaired with a suitable die or tap. Small nicks can be removed satisfactorily with Swiss pattern files or small, edged stones, pipe tapped threads should not be tapped deeper in order to clean them up, because this practice will invariably result in an oversized tapped hole. If scratches or galling are removed from a bearing surface of a journal it should be buffed to a high finish. Generally it is impossible to repair cracks; however, welding operations may be performed in some parts of housings, providing the area is not a stressed section of the part. For example, almost any area of a rocker box may be welded, but no part of the cylinder head except the fins may be welded.
3-28. Painted Parts. Parts requiring use of paint for pro-tection or appearance should be painted in accordance with the following recommendations using material from the following list of approved material. Thinner -Toluene or equivalent (AMS3180 or equivalent Federal Spec. TT-T-548). Primer -Zinc chromate (AMS3110 or equivalent MIL-P-8585). Enamel -Phthalate resin type (AMS315C or equivalent MIL-E-7729).
　
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