Chapter  7—AUXILIARY  MACHINERY (MRC) for the proper methods and tools required for removing the heads. The following is general information  on  removing  a  piston,  fitting  new piston rings, and checking piston end clearance for a vertical compressor. To remove trunk pistons from vertical com- pressors, it is first necessary to remove the cylinder heads. In the case of three- or four-stage com- pressors, the third and fourth stage cylinders will also have to be removed. Next, you must turn the compressor by hand to top center and remove the lower half of the crank pin bearing, or the entire crank pin bearing box, if so directed. Pull up the piston and connecting rod through the cylinder. In removing pistons from compressors fitted with cross heads and piston rods, the general procedure is to loosen the piston rod locknuts adjacent to the cross head, then unscrew the piston rod from the cross head and lift the piston and the rod out of the cylinder. If  the  piston  rings  are  worn  or  broken  and require replacement, take accurate measurements of the cylinder or liner to determine the exact diameter. Standard size rings may be used in over- size cylinders if the oversize does not exceed 0.003 inch  per  inch  of  the  cylinder  diameter.  If  the cylinder or liner is worn beyond the manufac- turer’s recommended limits, it should be replaced. On compressors not equipped with liners, such as  air-cooled  or  large  pneumatic  service  com- pressors, one reboring is allowed. When piston rings are being replaced, they should first be fitted to the cylinder to check for proper  end  clearance.   If  necessary,  the  ends should be filed until the manufacturer’s recom- mended  clearance  is  obtained.  Rings  should  then be fitted to the piston grooves, making sure that the side clearance of each ring is such as to allow it to fall into the groove by its own weight. The thickness of each ring should be checked to make sure that the groove is deep enough for the ring thickness. In replacing the rings, care must be taken to ensure that they are staggered so that the ring  splits  are  not  in  line.  After  the  piston  is assembled, it is advisable to wire the rings tightly with a soft copper wire so that they will enter the bore without difficulty. As each ring enters the bore, the binding wire can be removed through the  valve  ports.  Most  cylinder  liners  are  beveled at the top to permit the rings to compress and enter   the   bore   easily.   In   certain   designs   of differential pistons, it is necessary to loop a wire around the top of the lower liner, using valve ports for  access,  and  squeeze  each  ring  closed  as  it approaches the top of the liner. One technique is to slip the rings over a thin piece of shim stock which, in turn, is slipped over the piston. This prevents overstretching and other possible ring damage. After a piston has been replaced in a com- pressor  and  the  head  has  been  drawn  down evenly, it will be necessary for you to check piston end clearance. NOTE: The piston end clearance must  always  be  measured  and  adjusted  after replacing the pistons, or after any adjustment or replacement of the main, crank pin, wrist pin, or cross head bearing has been made. This is ac- complished by inserting a lead wire through a valve port or indicator connection, and then jack- ing the compressor over so that when the piston has moved to the end of its stroke (top dead center (TDC)), the lead will be flattened to the exact amount of clearance at the end of the stroke. The lead  wire  should  be  long  enough  so  that  the reading can be taken at a point near the center of the piston. In differential piston compressors in which compression takes place in two stages during the same stroke, the clearance must be measured for both  stages.  In  double-acting  compressors,  or  in those compressors in which compression in any stage takes place on the reverse stroke, a second reading  of  the  clearance  at  the  bottom  of  the stroke  (bottom  dead  center  (BDC))  of  that cylinder must be taken. After taking these readings, you can adjust the piston  end  clearances.  Proper  clearances  for  a specific  compressor  are  usually  found  in  the manufacturer’s   instruction   book   or   on   the blueprints.   If   neither   of   these   documents   is available, then you should adjust the clearance so that the TDC stroke is approximately 1/64 inch greater than the BDC stroke. This is necessary to allow for the expansion of the running parts when the compressor is in operation. The  method  of  adjusting  the  piston  clearance varies  with  different  compressors  or  designs. Adjustments may be accomplished by one of the following methods: (1) by adding or removing shims in the connecting rod between the lower end 7-3