is less and the overload test is neither required nor
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
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
Improper cooling water
Dirty intake air cleaner
Improper starting procedures
Obstruction in cylinder
Faulty nozzle spray
Insufficient ring groove
Excessive wear of piston
Improper installation or
ENGINEMAN 1 & C
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