Chapter  3—ENGINE  MAINTENANCE the cylinder. This allows the crown and ring lands to drag on the cylinder wall. The results of drag- ging can be determined by visually inspecting the parts of the piston in question. However, most of the pistons now in use in the Navy are free from this trouble, since the crown and ring lands are of smaller diameter than the skirt and do not con- tact the cylinder wall. Some piston wear is normal in any engine; the amount  and  rate  depends  on  several  controllable factors.  The  causes  of  excessive  piston  wear  are also the causes of other piston troubles. One of the factors controlling wear is lubrica- tion. An adequate supply of oil is essential to pro- vide the film necessary to cushion the piston and other parts within the cylinder and prevent metal- to-metal  contact.  Inadequate  lubrication  will  not only cause piston wear but the extra friction may also  cause  piston  seizure,  land  breakage,  and piston pin bushing wear. Lack of lubrication is caused either by a lack of lube oil pressure or by restricted oil passages. The  pressure-recording  instruments  usually  give warning of low oil pressure before any great harm occurs. However, clogged passages offer no such warnings. Only by inspecting and cleaning the piston  and  connecting  rod  assembly  may  you insure adequate lubrication. Another   controllable   factor   that   may   be directly or indirectly responsible for many piston troubles  is  improper  cooling  water  temperatures. If  an  engine  is  operated  at  higher  than  the specified  temperature  limits,  lubrication  troubles will develop. High cylinder surface temperatures will reduce the viscosity of the oil. As the cylinder lubricant thins, it will run off the surfaces. The resulting lack of lubrication leads to excessive piston and liner wear. On the other hand, if the engine is operated at temperatures that are below those specified, viscosity will be increased, and the oil will not readily reach the parts requiring lubrication. Oil plays an important part in the cooling of the piston crown. If the oil flow to the underside of  the  crown  is  restricted,  deposits  caused  by oxidation of the oil will accumulate and lower the rate of heat transfer. For this reason, the under- side of each piston crown should be thoroughly cleaned whenever pistons are removed. While  insufficient  lubrication  and  uneven cooling may cause ring land failure, excessive oil temperatures   may   cause   piston   seizure.   An increase in the rate of oxidation of the oil may result in clogged oil passages or damage to piston pin  bushings. Seizure and excessive wear of pistons may be caused by improper fit. New pistons or liners must be  installed  with  the  piston-to-cylinder  clearances specified in the manufacturer’s technical manual. If clearance is insufficient, a piston will NOT wear in and will probably bind. The resulting excess surface  temperatures  may  lead  to  seizure  or breakage. Binding increases wear and shortens piston life by scuffing the liner or galling the piston skirt. Scuffing roughens the liner so that an abrasive action  takes  place  on  the  piston  skirt,  thus generating additional heat which may distort or crack the piston or liner. Galling, especially on aluminum  pistons,  causes  the  metal  to  be  wiped in  such  a  manner  that  the  rings  bind  in  the grooves. A loose fitting piston may be just as destruc- tive as one which is too tight. A loose piston may cause dragging and cocking of the piston, which in turn may cause broken or cracked ring groove lands. Excessive wear on the piston and piston pin bushing  may  be  caused  by  either  an  overload  or by an unbalanced load. Overloading an engine increases the forces on the pistons and subjects them to higher temperatures, thus increasing their rate of wear. There should be a load balance on all pistons at all times. Balance of an engine is determined   by   checking   the   exhaust   gas temperature at each cylinder, the rack settings, and the firing and compression pressures. Cracking of the lands of a piston is caused by insufficient  ring  groove  clearance.  For  correct piston ring operation, proper clearance must be maintained between the ring and the land, and also between the ends of the ring. This is necessary in order that the ring may be free to flex at all temperatures of operation. The clearance depends upon the ring and the materials involved. After installing a ring, check the clearance be- tween the ring and the land. This check is made 3-27