INSPECTING, TESTING, AND REPAIRING CYLINDER HEADS
Conditions requiring repair of a cylinder head are similar to those for cylinder liners and can be grouped under cracks, corrosion, distortion, and fouling.
The symptoms of a cracked cylinder head are the same as those of a cracked liner. Cracks in cylinder heads are best located by either visual inspection or magnetic powder inspection. On some types of engines, a defective cylinder can be located by bringing the piston of each cylinder, in turn, to top dead center and applying compressed air. When air is applied to a damaged cylinder, a bubbling sound indicates leakage. When the cylinder head is removed from the engine, it can be checked for cracks by the hydrostatic test used on cylinder liners equipped with integral cooling passages.
Cracks generally occur in cylinder heads on the narrow metal sections between such parts as valves and injectors. The cracks may be caused by adding cold water to a hot engine, by restricted cooling passages, by obstructions in the combustion space, or by improper tightening of studs.
Aboard ship, cracked cylinder heads usually must be replaced. It is possible to repair them by welding, but this process requires special equipment and highly skilled personnel normally found only at repair activities.
Burning and corrosion of the mating surfaces of a cylinder head may be caused by a defective gasket. Although regular planned maintenance ordinarily prevents this type of trouble, burning and corrosion may still take place under certain conditions. When corrosion and burning occur, there may be a loss of power due to combustion gas leakage out of or water leakage into the combustion space. Other symptoms of leakage may be (1) hissing or sizzling in the head where gases or water may be leaking between the cylinder head and the block, (2) bubbles in the cooling water expansion tank sight glass, or (3) overflow of the expansion tank. Gaskets and grommets that seal combustion spaces and water passages must be in good condition; otherwise the fluids will leak and cause corrosion or burning of the area contacted. Improper cooling water treatment may also accelerate the rate of corrosion.
In general, cylinder heads that are burned or corroded by gas or water leakage are so damaged that they must be replaced.
Warpage or distortion of cylinder heads becomes apparent when the mating surfaces of the head and block fail to match properly. If distortion is severe, the head will not lit over the studs. Distortion may be caused by improper welding of cracks or by improper tightening of the cylinder head studs. Occasionally, new heads may be warped because of improper casting or machining processes.
Repair of distorted or damaged cylinder heads is often impracticable. They should be replaced as soon as possible and turned in to the nearest supply activity, which will determine the extent of damage and the method of repair.
If the combustion chambers become fouled, the efficiency of combustion will decrease. Combustion chambers are designed to create the desired turbulence for mixing the fuel and air; any accumulation of carbon deposits in the space will impair both turbulence and combustion by altering the shape and decreasing the volume of the combustion chamber.
Symptoms of fouling in the combustion chambers are smoky exhaust, loss of power, or high compression. Such symptoms may indicate the existence of extensive carbon formation or clogged passages. In some engines, these symptoms indicate that the shutoff valves for the auxiliary combustion chambers are stuck Combustion chambers may also become fouled because of faulty injection equipment, improper assembly procedures, or excessive oil pumping.
Cleaning of fouled combustion spaces generally involves removing the carbon accumulation. The best method is to soak the dirty parts in an approved solvent and then wipe off all traces of carbon. You may use a scraper to remove carbon, but be careful to avoid damaging the surfaces. If oil pumping is the cause of carbon formation, check the wear of the rings, bearings, pistons, and liners. Replace or recondition excessively worn parts. Carbon formation resulting from improperly assembled parts can be avoided by following procedures described in the manufacturer’s technical manual.