breakage of the ring and perhaps of the land isalmost certain if the ridge is not removed.PISTON PINS AND PIN BEARINGSPiston pins are made of hardened steel alloy,and their surfaces are precision finished. Pistonsleeve bearings or bushings are made of bronzeor a similar material. These pins and pin bear-ings require very little service and total failureseldom occurs.Wear, pitting, and scoring are the usualtroubles encountered with piston pins and pistonpin bearings.Wear of a pin or bearing is normal, but therate of wear can be unnecessarily increased bysuch factors as inadequate and improper lubrica-tion, overloading, misalignment of parts, orfailure of adjacent parts.Every time a piston assembly is removed froman engine, the complete assembly should bechecked for wear. Piston pins and bushings shouldbe measured with a micrometer to determine ifwear is excessive. Do NOT measure areas that donot make contact, such as those between the con-necting rod and piston bosses, and the areas underthe oil holes and grooves. The correct and limitingvalues for measurements may be found in themanufacturer’s technical manual for the par-ticular engine.Excessive wear of pins, bushings, or bearingsis often the result of insufficient or improperlubrication. (These parts are usually pressurelubricated.) The failure of a pressure lubricatingsystem is usually detected before piston pins,bushings, or bearings are seriously damaged.Insufficient lubrication of these parts is usuallycaused by obstructions blocking the oil passagesof the connecting rods. If the bushings have beeninstalled so that the oil holes do not line up,lubrication may be restricted. Such misalignmentof oil holes may also be caused by a bushing com-ing loose and revolving slightly out of position.Also interchanging the upper and lower connect-ing rod bearings ON SOME ENGINES mayobstruct the flow of oil to the upper end of therod. Always check the manufacturer’s technicalmanual for information on interchangeability ofparts.If there is misalignment of the connectingrods, uneven loading on piston pins and bearingswill result. The fact that a rod is misaligned isusually indicated by uneven wear of the piston pinand bushing and by piston skirt wear. Misalign-ment may be caused by improper reaming of thebushing for proper clearance.CONNECTING RODSConnecting rod troubles usually involve eitherthe connecting rod bearing or the piston pin bear-ing. Some of these troubles, such as misalignment,defective bolts, cracks, or plugged oil passages,can be avoided by performing proper maintenanceand by following instructions in the manufac-turer’s technical manual.Misalignment causes binding of the piston,piston pin, and the connecting rod journal bear-ing. This binding is likely to result in breakageand in increased wear of the parts, leading to totalfailure and possible damage to the entire enginestructure. Connecting rods must be checked forproper alignment before being installed in anengine, and after any derangement involving thepiston, cylinder, or crankshaft.Defective bolts are often the result of over-tightening. Connecting rod bolts should betightened by using a torque wrench, or anelongated gage to ensure that a predeterminedturning force is applied to the nut. Defectivethreads can cause considerable trouble by allow-ing the connecting rod to be loosened and causeserious damage to the engine. Whenever rod boltsare removed they should be carefully inspectedfor stripped or damaged threads and elongation.Cracked rods are usually the result ofoverstressing caused by overloading oroverspeeding or because defective material wasused at the time of manufacture. It is of primeimportance to discover the cracks before they havedeveloped to the point where the failure of therod will take place. No attempts should be madeto repair cracked rods. They should be replaced;serious damage may result if breakage occursduring operation.Restricted oil passages are often the result ofimproper assembly of the bushing and the con-necting rod bearing inserts. They may also be dueto foreign matter lodging in the oil passages.ENGINEMAN 1 & C3-30
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