be burned in the cylinder. This results in theaccumulation of excessive carbon deposits.Crown and Land DraggingPistons and liners may become sufficiently worn topermit the piston to cock over in the cylinder. Thisallows the crown and ring lands to drag on the cylinderwall. The results of dragging can be determined byvisually inspecting the parts of the piston in question.Piston WearAlthough piston wear is normal in all engines, theamount and rate of piston wear depend on severalcontrollable factors. (The causes of excessive pistonwear, and crown and land dragging, are also the causesof other piston troubles.)One of the controllable factors is LUBRICATION.An adequate supply of oil is essential to provide the filmnecessary to cushion the piston and other parts withinthe cylinder and prevent metal-to-metal contact.Inadequate lubrication will not only cause piston wearand crown and land dragging, but also may cause pistonseizure, and piston pin busing wear.Lack of lubrication is caused either by a lack of lubeoil pressure or by restricted oil passages. Thepressure-recording instruments usually give warning oflow oil pressure before any great harm results. However,clogged passages offer no such warnings, and theirdiscovery depends on the care that is exercised ininspecting and cleaning the piston and connecting rodassembly.Another controllable factor that may be directly orindirectly responsible for many piston troubles isIMPROPER COOLING WATER TEMPERATURE.If an engine is not operated within the specifiedtemperature limits, lubrication troubles will develop.High cylinder surface temperatures will reduce theviscosity of the oil. As the cylinder lubricant thins, it willrun off the surfaces. The resulting lack of lubricationleads to excessive piston and liner wear. However, iftemperatures are below those specified for operation,viscosity will be increased, and the oil will not readilyreach the parts requiring lubrication.Oil plays an important role in the cooling of thepiston crown. If the oil flow to the underside of thecrown is restricted, deposits caused by oxidation of theoil will accumulate, lowering the rate of heat transfer.Therefore, the underside of the piston crown should bethoroughly cleaned whenever pistons are removedWhile insufficient and uneven cooling may causering land failure, excessive temperatures may causepiston seizure; an increase in the rates of oxidation ofthe oil, resulting in clogged oil passages; or damage topiston pin bushings.Seizure or excessive wear of pistons may be causedby IMPROPER PIT. New pistons or liners must beinstalled with the piston-to-cylinder clearancesspecified in the manufacturer’s instruction manual.PISTON PINS AND SLEEVE BEARINGS ORBUSHINGSEvery time you remove a piston assembly from anengine, inspect it for wear. Measure the piston pins andsleeve bearings or bushings with a micrometer, as shownin figure 3-25, to determine whether wear is excessive.Do NOT measure areas that do not make contact. Suchareas include those between the connecting rod andpiston bosses and areas under the oil holes and grooves.You can press bushings out of the rod with a mandreland an arbor press or with special tools, as shown infigure 3-26. You can also remove bushings by firtshrinking them with dry ice. Dry ice will also make iteasier to insert the new bushing.Figure 3-25.—Measuring a piston pin and piston bushing forwear.3-17
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