Smooth, even gray band Sharply defined edges Indication  Fuzzy edge Cause  Rough grind Remedy  Regrind valve or continue lapping Too narrow contact Unintentional interference fit Grind both valve and set Too lap bands Improper narrowing of seat Renarrow seat Figure 10-29.  Incorrectly lapped valves.  Figure 10-28.  A correctly lapped valve.  After the lapping process is finished, be sure that all lapping  compound is removed from the valve face, seat, and adjacent  areas. The final step is to check the mating surface for leaks  to see if it is sealing properly. This is done by installing the  valve in the cylinder, holding the valve by the stem with the  fingers, and pouring kerosene or solvent into the valve port.  While holding finger pressure on the valve stem, check to see  if the kerosene is leaking past the valve into the combustion  chamber. If it is not, the valve re-seating operation is  finished. If kerosene is leaking past the valve, continue the  lapping operation until the leakage is stopped. The incorrect  indications are of value in diagnosing improper valve and  valve seat grinding. Incorrect indications, their cause, and  remedy are shown in  Figure 10-29 . Piston repairs are not required as often as cylinder repairs  since most of the wear is between the piston ring and cylinder  wall, valve stem and guide, and valve face and seat. A lesser  amount of wear is encountered between the piston skirt and  cylinder, ring and ring groove, or piston pin and bosses. The most common repair is the removal of scores. Usually,  these may be removed only on the piston skirt if they  are very light. On engines where the entire rotating and  reciprocating assembly is balanced, the pistons must weigh  within one-fourth ounce of each other. When a new piston is  installed, it must be within the same weight tolerance as the  one removed. It is not enough to have the pistons matched  alone; they must be matched to the crankshaft, connecting  rods, piston pins, etc. To make weight adjustments on new  pistons, the manufacturer provides a heavy section at the  base of the skirt. To decrease weight, file metal evenly off  the inside of this heavy section. The piston weight can be  decreased easily, but welding, metalizing, or plating cannot  be done to increase the piston weight. If ring grooves are worn or stepped, the pistons are normally  replaced. Small nicks on the edge of the piston pin boss may be  sanded down. Deep scores inside the boss, or anywhere around  the boss, are definite reasons for rejection. It has become more  economical to replace pistons rather than reconditioning and  reusing old ones, especially during overhaul. If a cylinder has excessive taper, out-of-roundness, step,  or its maximum size is beyond limits, it can be reground to  the next allowable oversize. If the cylinder walls are lightly  rusted, scored, or pitted, the damage may be removed by  honing or lapping. Regrinding a cylinder is a specialized job that the powerplant  mechanic is not usually expected to be able to do. However,  the mechanic must be able to recognize when a cylinder  needs regrinding, and he or she must know what constitutes  a good or bad job. Generally, standard aircraft cylinder oversizes are 0.010 inch,  0.015 inches, 0.020 inch, or 0.030 inch. Aircraft cylinders  have relatively thin walls and may have a nitrided surface,  that must not be ground away. Nitriding is a surface hardening  process that hardens the steel surface to a depth of several  thousandths of an inch. Any one manufacturer usually does  Cylinder Grinding and Honing Piston Repairs 10-18