LO 9-8 Outline maintenance procedures and practices used to service diesel engine cylinder heads

LO 9-08: Outline maintenance procedures and practices used to service diesel engine cylinder heads.

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.

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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