LO 9-8 Outline maintenance procedures and practices used to service diesel engine cylinder heads
| Site: | CDX Learning Systems |
| Course: | DE Section 2-1: Engine Blocks and Cylinder Heads |
| Book: | LO 9-8 Outline maintenance procedures and practices used to service diesel engine cylinder heads |
| Printed by: | Guest user |
| Date: | Thursday, 5 November 2026, 7:50 PM |
1. Maintenance of Cylinder Heads and Valve Trains
LO 9-08: Outline maintenance procedures and practices used to service diesel engine cylinder heads. |
Maintenance of Cylinder Heads and Valve Trains
Maintenance of cylinder heads is often required when coolant or combustion leaks occur. In addition, valve seats and faces can become worn or damaged, which requires replacement during a cylinder head overhaul and service Figure 60. Continual pounding of the valve faces and seats can take place whenever valves are out of adjustment; this prevents effective performance of the quieting ramps on the camshaft, which slows down valve velocity as they seat. Valve faces become recessed when worn out. Excessive oil consumption and extended idle time can contribute to valve seat and face pitting, which eventually results in valve leakage Figure 61. Very long periods of extended idle can lead to excessive formation of slobber on the back of the valve Figure 62. If the slobber solidifies during a cold shutdown period while the valve is open, the valve and piston will make contact and the valve will bend.
FIGURE 09-60: Worn or damaged valves should be replaced during a cylinder head overhaul and service. |
FIGURE 09-61: The intake and exhaust valve faces have worn away with accumulated distance. Most valves recede into the valve face and seat, which causes valve lash to disappear. |
FIGURE 09-62: An exhaust valve slobbered from excessive engine idle. |
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1.1. Video: Checking Valve Concentricity
LO 9-8 Video: Checking Valve Concentricity |
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1.2. Video: Measuring Valve Recession
LO 9-8 Video: Measuring Valve Recession |
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1.3. Video: Replace Valves
LO 9-8 Video: Replace Valves |
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1.4. Cylinder Head Bolts
LO 9-08: Outline maintenance procedures and practices used to service diesel engine cylinder heads. |
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Cylinder Head Bolts Because of their higher cylinder pressures, consistent bolt clamping force is a critical maintenance issue for diesel engines. The size, number, and grade of bolts are increased in four-stroke diesels in comparison to two-stroke diesels and spark ignition engines. Typically, five to seven bolts are used per cylinder, but as few as four may be used. Cylinder head bolts use specialized rolled threads like those used to retain main and rod bearing caps. Unlike machine-cut threads, which have material removed during fabrication, rolled threads are instead formed using a tool that pushes into and displaces the metal while the bolt is turned. This technique does not reduce the root diameter of a bolt. Because no material is removed and the metal grain of the bolt is uninterrupted by cutting, rolled threads are stronger and less prone to damage Figure 63. Usually, the bolt, thread, and grade are proprietary, which means the bolt diameter, thread pitch, and strength are unique to the manufacturer. Care must be taken to inspect bolts for damage before reuse. Nicks, corrosion, or thread damage can lead to stress risers and breakage. Bolts should also be inspected for necking out or stretching using a go-no-go gauge Figure 64. If the bolt is longer than the gauge permits, the bolt is stretched and should be discarded. |
FIGURE 09-63: Reuse of head bolts is disqualified if any of the conditions shown here are discovered during inspection. |
FIGURE 09-64: A go-no-go gauge is used to check cylinder head bolts for stretching. |
FIGURE 09-65: Bolts with clean, lubricated threads achieve higher clamping force. |
Bolt threads should be cleaned and lightly oiled before installation to ensure maximum and consistent clamping force Figure 65. Lubricating the bolts ensures that more of the twisting force used to tighten a bolt turns the bolt and is not instead lost to friction on the bolt threads. Variations between clamping forces of bolts can lead to premature head gasket failure and head warping. Research has revealed that tightening the head bolts with only a torque wrench to the torque specification results in over 30% variation in clamping force. Because of this, several service techniques have evolved for ensuring consistent gasket clamping forces.
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Torque-to-yield (TTY) bolts are cylinder head bolts that have been tightened past their yield strength or elastic point. When tightened to specification, they stretch to a point just over precisely calibrated yield strength and not to the tensile strength threshold where the bolt will break. When stretched to its yield point, the bolt becomes more elastic, allowing it to accommodate changes in the dimensions of the cylinder head caused by temperature fluctuations between low and high operating temperature. The grade of the metal and root diameter determine its clamping force. All engines with aluminum cylinder heads use TTY bolts, as do some cast iron engines. These bolts must be discarded after a single use. The torque-plus-angle or torque-turn method requires initial bolt tightening with a torque wrench and then additional turning by a specified number of degrees past this point Figure 66. This last step may be measured in degrees of rotation, turns, or a specified number of flats on the bolt. Clamping forces with this method are the most consistent and most widely preferred method. When tightening down a bolt using the torque-plus-angle method, it is helpful to mark a socket with whiteout to gauge the amount of turning. Special markings are embossed into the top of torque-plus-angle bolts designating the specialized tensile strength of the bolt. While the bolt is usually higher than grade-8 tensile strength, the threads are annealed to soften them so that they do not strip as easily. The diagram on top shows a hexagon with a circle with the apices touching the circle at 5 points. The corner due North is A and the corner due South is B. A mark is made on the cylinder head above A. An angle in the center is marked 90 degrees, its sides facing North east and East. The illustration below shows the angle in the center having its sides facing South East and South, showing a clockwise turn of 90 degrees. |
FIGURE 09-66: The torque-turn method requires a bolt to receive an initial preload using a torque wrench. Then the bolt is turned several additional degrees, flats, or turns. |
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1.5. Video: Using a Torque Wrench and a Torque Angle Gauge
LO 9-8 Video: Using a Torque Wrench and a Torque Angle Gauge |
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1.6. Anim: Tensile Force and Bolt Tension
LO 9-8 Animation: Tensile Force and Bolt Tension (FULL SCREEN) |
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1.7. Studs Versus Bolts
LO 9-08: Outline maintenance procedures and practices used to service diesel engine cylinder heads. |
Studs Versus Bolts
Some applications or high-performance cylinder head modifications use studs rather than bolts. There are several advantages to using studs. First, studs are often made to higher tensile strengths than stock bolts, permitting higher clamping forces. Also, a stud installed in the cylinder head is far less likely to break when removed.
Studs also can provide an even more consistent clamping force than bolts. With threads at each end, studs are initially finger-tightened into the block. After the cylinder head and gasket are installed, torque is applied to a nut. Unlike a bolt, which is reacting to two separate twisting forces on the bolt head and threads in the block, the stud will stretch only on the vertical axis when the nut is tightened. Rotational force on the stud is avoided and clamping force is directly transferred from the torque applied to the nut. This provides a more even clamping force on the cylinder head.
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