speed or excessive manifold pressure operation. The visual  evidence produced by galling appears as if particles from  one contacting surface had welded to the other. Evidence  of any galling is sufficient reason for rejecting the complete  rod assembly. Galling is a distortion in the metal and is  comparable to corrosion in the manner in which it weakens  the metallic structure of the connecting rod. After visually inspecting engine recesses for deposits of metal  particles, it is important to clean all engine parts thoroughly  to facilitate further inspection. Two processes for cleaning  engine parts are: Degreasing can be done by immersing or spraying the part in a  suitable commercial solvent.  [Figure 10-3]  Extreme care must  be used if any water-mixed degreasing solutions containing  caustic compounds or soap are used. Such compounds, in  addition to being potentially corrosive to aluminum and  magnesium, may become impregnated in the pores of the  metal and cause oil foaming when the engine is returned to  service. Therefore, when using water-mixed solutions, it is  imperative that the parts be rinsed thoroughly and completely  in clear boiling water after degreasing. Regardless of the  method and type of solution used, coat or spray all parts with  lubricating oil immediately after cleaning to prevent corrosion. While the degreasing solution removes dirt, grease, and soft  carbon, deposits of hard carbon almost invariably remain  on many interior surfaces. To remove these deposits, they  must first be loosened by immersion in a tank containing a  decarbonizing solution (usually heated). A great variety of  commercial decarbonizing agents are available. Decarbonizers,  like the degreasing solutions previously mentioned, fall  generally into two categories, water-soluble and hydrocarbons.  The same caution concerning the use of water-soluble  degreasers is applicable to water-soluble decarbonizers. CAUTION: When using a decarbonizing solution on  magnesium castings, avoid immersing steel and magnesium  parts in the same decarbonizing tank, as this practice often  results in damage to the magnesium parts from corrosion. Decarbonizing will usually loosen most of the hard carbon  deposits remaining after degreasing. However, the complete  removal of all hard carbon generally requires brushing,  scraping, or grit-blasting. In all of these operations, be careful  to avoid damaging the machined surfaces. In particular, wire  brushes and metal scrapers must never be used on any bearing  or contact surface. Follow the manufacturer's recommendations when grit- blasting parts for the abrasive material being used. Sand,  rice, baked wheat, plastic pellets, glass beads, or crushed  walnut shells are examples of abrasive substances that are  used for grit-blasting parts. A grit-blasting machine is shown  in  Figure 10-4 . All machined surfaces must be masked properly and  adequately, and all openings tightly plugged before blasting.  The one exception to this is the valve seats, which may be  left unprotected when blasting the cylinder head combustion  chamber. It is often advantageous to grit-blast the seats, since  this will cut the glaze which tends to form (particularly on  the exhaust valve seat), thus facilitating subsequent valve  seat reconditioning. Piston ring grooves may be grit-blasted  if necessary; however, extreme caution must be used to  Figure 10-3.  Typical solvent degreasing tank.  Figure 10-4.  Grit-blasting machine.  Removing Hard Carbon Cleaning Degreasing 1.  Degreasing to remove dirt and sludge (soft carbon). 2.  The removal of hard carbon deposits by decarbonizing,  brushing or scraping, and grit-blasting. 10-6