flush with the case must be drilled and removed with suitable  stud remover. Be careful not to damage any threads. When  replacing studs, coat the coarse threads of the stud with an  anti-seize compound. Minor damage to engine parts, such as burrs, nicks, scratches,  scoring, or galling, should be removed with a fine oil stone,  crocus cloth, or any similar abrasive substance. Following  any repairs of this type, the part should be cleaned carefully  to be certain that all abrasive has been removed, and then  checked with its mating part to assure that the clearances  are not excessive. Flanged surfaces that are bent, warped,  or nicked can be repaired by lapping to a true surface on a  surface plate. Again, the part should be cleaned to be certain  that all abrasive has been removed. Defective threads can  sometimes be repaired with a suitable die or tap. Small  nicks can be satisfactorily removed with Swiss pattern files  or small, edged stones. Pipe threads should not be tapped  deeper to clean them, because this practice results in an  oversized tapped hole. If galling or scratches are removed  from a bearing surface of a journal, it should be buffed to a  high polished finish. Cylinder and piston assemblies are inspected according  to the procedures contained in the engine manufacturer’s  manuals, charts, and service bulletins. A general procedure  for inspecting and reconditioning cylinders is discussed in  the following section to provide an understanding of the  operations involved. Visually inspect the head fins for other damage besides cracks.  Dents or bends in the fins should be left alone unless there  is danger of cracking. Where pieces of fin are missing, the  sharp edges should be filed to a smooth contour. Fin breakage  in a concentrated area causes dangerous local hot spots. Fin  breakage near the spark plug bushings or on the exhaust side  of the cylinder is obviously more dangerous than in other  areas. When removing or re-profiling a cylinder fin, follow  the instructions and the limits in the manufacturer’s manual. In general, welding of highly-stressed engine parts can be  accomplished only when approved by the manufacturer.  However, welding may be accomplished using methods that  are approved by the engine manufacturer, and if it can be  reasonably expected that the welded repair will not adversely  affect the airworthiness of the engine.  Inspect spark plug inserts for the condition of the threads  and for looseness. Run a tap of the proper size through the  bushing. Very often, the inside threads of the bushing are  burned. If more than one thread is missing, the bushing  should be rejected. Tighten a plug in the bushing to check  for looseness. Many minor parts not subjected to high stresses may be  safely repaired by welding. Mounting lugs, cowl lugs,  cylinder fins, rocker box covers, and many parts originally  fabricated by welding are in this category. The welded part  should be suitably stress-relieved after welding. However,  before welding any engine part, consult the manufacturer’s  instructions for the engine concerned to see if it is approved  for repair by welding. If the old piston is to be reused, or a new piston is to be used,  measure the outside of the piston by means of a micrometer.  Measurements must be taken in several directions and on  the skirt, as well as on the lands section. Check these sizes  against the cylinder size. Most engines use cam ground  pistons to compensate for the greater expansion parallel to  the pin during engine operation. The diameter of these pistons  measures several thousandths of an inch larger at an angle to  the piston pin hole, than parallel to the pin hole. Inspect the  ring grooves for evidence of wear. The groove needs to be  checked for side clearance with a feeler gauge to determine  the amount of wear in the grooves. Examine the piston pin  for scoring, cracks, excessive wear, and pitting. Check the  clearance between the piston pin and the bore of the piston  pin bosses using a telescopic gauge and a micrometer. Use  the magnetic particle method to inspect the pin for cracks.  Since the pins are often case hardened, cracks show up inside  the pin more often than they on the outside. Check the pin  for bends using V-blocks and a dial indicator on a surface  plate.  [Figure 10-14]  Measure the fit of the plugs in the  pin. In many cases, the pistons and piston pins are routinely  replaced at overhaul. Any studs that are bent, broken, damaged, or loose must be  replaced. After a stud has been removed, the tapped stud  hole should be examined for size and condition of threads.  If it is necessary to re-tap the stud hole, it also is necessary to  use a suitable oversize stud. Studs that have been broken off  Parts requiring use of paint for protection or appearance  should be repainted according to the engine manufacturer’s  recommendations. Aluminum alloy parts should have  original, exterior painted surfaces rubbed smooth to provide  a proper paint base. See that surfaces to be painted are  thoroughly cleaned. Care must be taken to avoid painting  mating surfaces. Exterior aluminum parts should be primed  first with a thin coat of zinc chromate primer. After the primer  is dry, parts should be painted with engine enamel, that should  be air dried until hard, or baked for  1 ⁄ 2 hour at 82 °C (180 °F).  Aluminum parts from which the paint has not been removed  may be repainted without the use of a priming coat, provided  no bare aluminum is exposed. 10-12 Piston and Piston Pins Cylinder Assembly Reconditioning