content_image/614022.jpg content_image/614023.jpg
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. 

## Cleaning 

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: 

1. Degreasing to remove dirt and sludge (soft carbon). 

2. The removal of hard carbon deposits by decarbonizing, brushing or scraping, and grit-blasting. 

## Degreasing 

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. 

## Removing Hard Carbon 

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 

<img src='content_image/614022.jpg'>

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 

<img src='content_image/614023.jpg'>

