Figure 3-3.—Improperly seated cylinder liner. improper fillet, grind it down until the lower surface of the flange seats properly on the mating surface of the cylinder  deck. An  oversized  sealing  ring  may  cause  improper positioning  of  the  liner.  As  the  sealing  ring  is overcompressed,   the   rubber   loses   its   elasticity   and becomes  hard,  which  may  cause  the  liner  to  become distorted. Use feeler gauges to check the clearance between the  mating  surfaces.  If  the  manufacturer’s  technical manual specifies the distance from the cylinder deck to the upper surface of the liner flange, use this dimension to check on the seating of the liner. Obstructions  in  the  combustion  chamber  may  be destructive not only to the liner but also to the cylinder head  and  other  parts. Erosion  and  corrosion  may  take  place  in  a  few isolated spots and weaken a liner sufficiently to cause cracks. Repairs Replacement   is   the   only   satisfactory   means   of correcting cracked, broken, or badly distorted cylinder liners. SCORED CYLINDER LINERS Scored  cylinder  liners  may  become  scored (scratched) by several means. These scratches degrade the  engine’s  performance  and  require  some  type  of repair. Scored  cylinder  liners  may  be  caused  by  broken piston   rings,   a   defective   piston,   improper   cooling, improper lubrication, or the presence of foreign particles or  objects.  Dust  particles  drawn  into  an  engine  cylinder will  mix  with  the  oil  and  become  an  effective  but undesirable lapping compound that may cause extensive damage.  The  importance  of  keeping  the  intake  air  clean cannot be overemphasized. Another precaution you should take is to make sure that  when  you  replace  a  cylinder  head,  you  leave  no metal chips, nuts, bolts, screws, or tools in the cylinder. Causes Scoring  may  be  in  the  form  of  deep  or  shallow scratches in the liner surface. With most liner scoring, there will be corresponding scratches on the piston and piston rings. The symptoms of scoring may be low firing or  compression  pressure  and  rapid  wear  of  piston  rings. The   best   method   for   detecting   scoring   is   visual inspection  through  liner  ports,  through  the  crankcase housing with pistons in their top position, or when the engine  is  disassembled. Badly  worn  pistons  and  rings  may  cause  scoring because  blowby  of  combustion  gases  increases  the temperature of the liner and may reduce the oil film until metal-to-metal contact takes place. Inspect the pistons and rings carefully. A piston with a rough surface (such as one that has seized) will score the liner. Scoring  as  a  result  of  insufficient  lubrication  or  dirt in  the  lubricating  oil  can  be  prevented  if  lubricating equipment   (filters,   strainers,   and   centrifuges)   is maintained  properly.  Lube  oil  must  be  purified according to required procedures. Repairs Ship’s force personnel normally do not repair scored liners;   they   replace   them   with   spare   liners.   When necessary,  liners  with  minor  scoring  may  be  kept  in service,  if  the  cause  of  scoring  is  eliminated  and  the minor defects can be corrected. The surface of the liner must be inspected carefully, especially in the region next to the ports, for any burrs, projections, or sharp edges that  will  interfere  with  piston  and  ring  travel.  Most projections can be removed by handstoning, using a fine stone. Figure 3-4 shows a liner before and after the ports were  stoned. EXCESSIVELY WORN LINERS Over a period of time, cylinder liners become worn simply because of engine operation. The best method of finding excessive wear is to take measurements of the cylinder liner with an inside micrometer caliper. Two 3-4