is less and the overload test is neither required nordesirable.Another important point to remember is thatif you cannot overhaul an engine due to lack ofspace, manpower, or expertise, you may requestoutside help by using an OPNAV Form 4790.2K.This form, when used as a work request, will besent to a Ship Intermediate Maintenance Activ-ity (SIMA). The SIMA will then accept or rejectthe work request. If the work request is accepted,the SIMA will order all repair parts, overhaul theengine, and perform an operational test in accord-ance with manufacturers’ technical manuals andNAVSHIPS Technical Manual, chapter 233.As stated earlier in this section, sincemaintenance cards, manufacturers’ maintenancemanuals, and various other instructions discussrepair procedures in detail, this chapter will belimited to general information on some of thetroubles encountered during overhaul, the causesof such troubles, and the methods of repair.PISTON ASSEMBLIES AND RODSPiston assemblies may have the trunk-type orthe crosshead-type pistons. The majority ofengines in use by the Navy have trunk-typepistons. Since the troubles encountered withcrosshead pistons are very similar to those en-countered with the trunk type, only the latter isdiscussed here.PISTONSTrunk-type pistons are subject to such forcesas gas pressure, side thrust, inertia, and friction.These forces, together with overheating and thepresence of foreign matter, may cause suchtroubles as piston wear, cracks, piston seizure, andpiston pin bushing wear (see figure 3-17).Piston wear is characterized by an excessiveclearance between the piston and the cylinder.Symptoms of excessive clearance between a pistonand cylinder are piston slap and excessive oil con-sumption. Piston slap occurs just after top deadcenter and bottom dead center, as the piston shiftsits thrust from one side to the other. As thecylinder taper increases with wear, oil consump-tion increases. Since taper causes the rings to flexon each stroke of the piston, excessive ring wear3-26TroublesUndue pistonwear; crownand landdraggingCracksCrownPossible CausesInsufficient lubricationImproper cooling watertemperaturesOverloadUnbalanced loadImproper fitDirty intake air cleanerDirty oilImproper starting proceduresFaulty coolingLoose pistonObstruction in cylinderFaulty nozzle sprayLandsInsufficient lubricationCocked pistonInsufficient ring grooveclearanceExcessive wear of pistonring groovesBroken ringImproper installation orremovalPiston seizureInadequate lubricationExcessive temperaturesImproper cleaningPiston pinbushing wearInsufficient lubricationExcessive temperaturesOverloadUnbalanced loadENGINEMAN 1 & C121.1Figure 3-17.-Piston troubles and their causes.occurs, allowing lubricating oil to pass and beburned in the cylinder. This results in theaccumulation of excessive carbon deposits on thepiston, the combustion chamber, and the engineexhaust valves or ports. This accumulation of car-bon deposits will cause erratic operation andgreatly reduce engine efficiency.Occasionally pistons and liners become suffi-ciently worn to permit the piston to cock over in
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business