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Piston Rings, Continued

 
  
 
Chapter  3—ENGINE  MAINTENANCE other  ring  and  piston  troubles.  In  addition  to symptoms  and  causes  of  piston  ring  troubles, there are other factors that may also be responsi- ble either for low compression or for excessive oil consumption. When  a  cylinder  with  a  low  compression pressure is located, the possibility of the cause be- ing some factor other than excessive wear should be  eliminated  before  the  pistons  rings  are disassembled or replaced. Look at figure 3-18. Of the causes listed under “Other factors which may cause low compression pressure” are a, b, c, d, and there are causes that would affect the pressure in only one cylinder assembly of a multicylinder engine. Causes f, g, and h may affect a group of cylinders,  or  possibly  all  cylinders.  Therefore, when  symptoms  indicate  compression  ring  wear consider first other possibilities. Excessive oil con- sumption is generally associated with worn oil rings, but there are other factors which may cause abnormal oil usage, and these should be checked before replacement of oil rings is undertaken. Oxidation   of   the   lube   oil   leaves   carbon deposits  on  the  rings  and  in  the  grooves.  It  is caused  by  excessive  operating  temperatures.  The carbon buildup limits movement and expansion of the rings, prevents the rings from following the cylinder contour and sealing the cylinder, and may cause sticking, excessive wear, or breakage. Proper  clearance  must  exist  between  the  ring and land as well as behind the ring, since insuffi- cient ring groove clearance can cause the rings to stick. It is not the function of the rings to sup- port or position the piston in the cylinder bore, but if the proper clearance does not exist, the rings are likely to become loaded by inertia forces and by side thrust on the piston—forces which should be borne solely by the skirt of trunk-type pistons. Two   factors   that   cause   improper   ring clearance  are: 1. Abnormal amount of carbon deposits on rings and in grooves. 2. Improper dimensions. New rings must have the  proper  thickness,  width,  diameter,  and gap. One cause of undue loads on a ring could be insufficient  gap  clearance.  This  condition  would cause the ring to be forced out and into a port of  a  ported  cylinder,  and  possibly  result  in breakage. A bright spot found on each end of a broken ring indicates insufficient gap clearance. Sufficient gap clearance must exist at both the top and the bottom  of  the  cylinder  bore  when  rings  are installed. Sticking and binding of the ring may result from insufficient ring pressure. The tendency of the ring to return to its original shape pushes it against the cylinder wall, and makes the initial seal. The pressure of the combustion gases behind the rings reinforces this seal. Pressures (compres- sion and combination) within the cylinder force the combustion rings down and cause a seal be- tween the bottom side of the rings and the upper side of the lands; therefore, properly wearing rings will appear shiny on the outer face and bottom side.  Any  discoloration  (usually  appearing  as black lines) indicates the leakage of gases past the rings. Extended use and overheating may weaken rings  to  the  point  where  they  do  not  seat properly,  and  the  rings  are  then  likely  to  bind  in the grooves. A check of the free gap for a piston ring will indicate the ring’s condition with respect to sealing qualities. If the instruction manual does not give a prescribed dimension for free gap, com- pare the gap with that of a new ring. Conditions which cause piston rings to stick in the grooves, wear excessively, or break are often the result of using improper lube oil. Some lube oils cause a resinous gumlike deposit to form on engine  parts.   Trouble  of  this  nature  can  be avoided  by  using  Navy-approved  oils,  or  oil recommended by the manufacturer. Probably  the  greatest  factor  affecting  the wearing of piston rings is a worn cylinder liner. Therefore,  when  new  rings  are  installed,  surface condition,  amount  of  taper,  and  out-of-roundness of the liner must all be considered. The ring is in the  best  position  to  make  allowance  for  cylinder wear if the ring gaps are in line with the piston bosses. Gaps of adjacent rings should be staggered 180° to reduce gas leakage. With the wearing away of material near the top of a cylinder liner, a ridge will gradually be formed. When a piston is removed, this ridge must also be removed, even though it has caused no damage to the old set of rings. The new rings will travel higher in the bore by an amount equal to the wear of the old rings, and the replacement of the connecting rod bearing inserts will also in- crease piston travel. As the top piston ring will strike the ridge because of this increase in travel, 3-29


   


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