LO 9-9 Identify and describe the purpose, construction, and applications of cylinder head gaskets

Cylinder Head Inspection Cylinder heads should also be inspected for cracking. Several techniques are available to the technician: Dye penetrant Magnetic flux Pressure testing An effective method for checking for head gasket and combustion leakage into coolant from a cracked cylinder head is to check for a continuous release of gas bubbles from the cooling system Figure 71. After warming up the engine, a hose is connected to the coolant overflow pipe at the pressure cap. The other end of the hose is then submerged in a bottle of water and the amount of gases continuously released from the surge tank or radiator overflow is observed. In a normal engine, there will be a small amount of gas bubbles released in a steady stream as some coolant continuously vaporizes. However, a large and steady stream of bubbles indicates combustion leakage in the cooling system, which requires further diagnosis. A defective air compressor head gasket or air-operated device connected to the cooling system can be eliminated as a cause by opening the air tank drain valves while the engine is operating. Because system air pressure cannot build, little or no air will enter the cooling system if a defect exists in one of these accessories. Cavitated liners can also leak combustion gases into the cooling system, but they tend to produce hydrostatic locks after the engine is shut down when hot.

FIGURE 09-71: A defective head gasket will allow pressurized combustion gases to pass into the cooling system.

A head gasket leaking coolant into the engine can quickly cause catastrophic engine damage because coolant will easily displace oil lubrication film and prevent separation of the bearing journals and engine bearings. To check for an internal head gasket leak, a scheduled oil sampling can detect coolant in the oil. However, to pinpoint the source of a leak, such as a leaking liner O-ring or head gasket, the engine oil is drained and the oil pan is removed. After pressurizing the cooling system with a radiator pressure tester, one can easily inspect the internal parts of an engine from the crankcase and identify where coolant is leaking.

If a cylinder head is suspected of being cracked or leaking, often the over-pressurization of the cooling system is observed only after an engine has run under heavy load for some time and thermal expansion opens a crack. If this is the case, the cylinder head can be removed and pressure tested in a hot tank with special fixtures that direct pressurized air into the intake and exhaust ports where leak paths are formed. In the tank, the head is sealed by the fixtures to prevent leakage out of the valve ports and then it is carefully inspected for any leaking air bubbles. Cracks are easily detected by observing where the bubbles originate from the head.

Cylinder head cracks often develop between the valve seats after an engine has overheated Figure 72. When the head is removed, the valves are also removed and the seat areas visually inspected. Always use an appropriate lifting sling to remove and reinstall cylinder heads Figure 73. Dye penetrant testing, which is a three-step process that allows dye to penetrate cracks to make them more visible, is a convenient way to identify casting cracks. A magnetic flux technique is another method that uses a powerful magnet and iron filings to identify cracks. In this method, a cylinder head is magnetized by either alternating current (AC) using a specialized piece of equipment or a strong magnet placed on either side of a suspected crack. Iron filings are then blown onto the area using a small squeeze bulb filled with filings. Because cracks will develop a north and south polarity, the filings will line up end to end along a crack, which enables the crack to be easily detected.

FIGURE 09-72: Cracks often develop between valve seats and around an injector hole when the engine has overheated.

FIGURE 09-74: A lifting sling is used to remove and reinstall cylinder heads. The sling is adjustable to keep the head balanced and level.