is less and the overload test is neither required nor desirable. Another important point to remember is that if you cannot overhaul an engine due to lack of space,  manpower,  or  expertise,  you  may  request outside help by using an OPNAV Form 4790.2K. This form, when used as a work request, will be sent to a Ship Intermediate Maintenance Activ- ity (SIMA). The SIMA will then accept or reject the work request. If the work request is accepted, the SIMA will order all repair parts, overhaul the engine, and perform an operational test in accord- ance  with  manufacturers’  technical  manuals  and NAVSHIPS  Technical  Manual,  chapter  233. As   stated   earlier   in   this   section,   since maintenance  cards,  manufacturers’  maintenance manuals, and various other instructions discuss repair procedures in detail, this chapter will be limited to general information on some of the troubles  encountered  during  overhaul,  the  causes of such troubles, and the methods of repair. PISTON ASSEMBLIES AND RODS Piston assemblies may have the trunk-type or the   crosshead-type   pistons.   The   majority   of engines  in  use  by  the  Navy  have  trunk-type pistons.  Since  the  troubles  encountered  with crosshead pistons are very similar to those en- countered with the trunk type, only the latter is discussed  here. PISTONS Trunk-type  pistons  are  subject  to  such  forces as  gas  pressure,  side  thrust,  inertia,  and  friction. These forces, together with overheating and the presence  of  foreign  matter,  may  cause  such troubles as piston wear, cracks, piston seizure, and piston pin bushing wear (see figure 3-17). Piston  wear  is  characterized  by  an  excessive clearance between the piston and the cylinder. Symptoms of excessive clearance between a piston and cylinder are piston slap and excessive oil con- sumption. Piston slap occurs just after top dead center and bottom dead center, as the piston shifts its  thrust  from  one  side  to  the  other.  As  the cylinder taper increases with wear, oil consump- tion increases. Since taper causes the rings to flex on each stroke of the piston, excessive ring wear 3-26 Troubles Undue   piston wear;   crown and  land dragging Cracks Crown Possible   Causes Insufficient   lubrication Improper   cooling   water temperatures Overload Unbalanced   load Improper   fit Dirty   intake   air   cleaner Dirty   oil Improper  starting  procedures Faulty  cooling Loose  piston Obstruction   in   cylinder Faulty   nozzle   spray Lands Insufficient    lubrication Cocked  piston Insufficient   ring   groove clearance Excessive  wear  of  piston ring   grooves Broken   ring Improper   installation   or removal Piston   seizure Inadequate   lubrication Excessive    temperatures Improper  cleaning Piston   pin bushing  wear Insufficient   lubrication Excessive    temperatures Overload Unbalanced  load ENGINEMAN  1  &  C 121.1 Figure  3-17.-Piston  troubles  and  their  causes. occurs,  allowing  lubricating  oil  to  pass  and  be burned  in  the  cylinder.  This  results  in  the accumulation  of  excessive  carbon  deposits  on  the piston,  the  combustion  chamber,  and  the  engine exhaust valves or ports. This accumulation of car- bon   deposits   will   cause   erratic   operation   and greatly  reduce  engine  efficiency. Occasionally  pistons  and  liners  become  suffi- ciently  worn  to  permit  the  piston  to  cock  over  in