Threaded Connectors. Head Bolt

At the engine there are generally five critical threaded connection areas; these are explained below:
- Head bolt
- Main bearing cap bolt
- Conrod bolt
- Belt pulley bolt
- Flywheel bolt

In addition, the following threaded connections can be problematic. They need not be characterized as critical from the applications technology viewpoint but may be among the major applications in the engine:
- Camshaft bearing cap bolt
- Oil pan fixing bolt, valve cover fixing bolt

Screw connections for subassemblies and flange mounting points are not discussed further at this point, with the exception of threaded connections for magnesium components. High-strength screws upwards of M 6 in size are used in most of these cases, and these are largely either standard designs or close to standard designs.

Head Bolt. The function of the head bolts is to make an operationally reliability connection for the complete system—comprising the cylinder head, head gasket, and engine block— over long-term operations, taking the maximum possible ignition forces into account. The primary goals are uniform, low component loading and tight seals against combustion gases, lubricants, and coolant.

While in the past, head bolts had to be retightened once or even twice to compensate for gasket setting, the cylinder head configuration requiring no retorqing is state of the art today.

This has been made possible by using waisted-shank bolts or waisted-thread bolts with great elasticity, closer tolerances for tensile strength and friction properties, cylinder head gaskets that resist setting (e.g., all-metal gaskets), and a tightening process with low scatter in the values for preload force. Rotation-angle controlled (turn-of-the-nut) tightening to beyond the elastic limit has established itself as the most common torquing process.

Lightweight engineering is promoted more vigorously, and the resulting reduction in component stiffness at the engine block and cylinder head is normally compensated for by reducing the maximum screw strength. The minimum required screw force can be maintained only with a drastic reduction in the tolerances for tensile strength and friction values. When designing the cylinder head bolting constellation, it is necessary to understand the influence of temperature.

It is conceivable that, while the engine is heating up, the head bolts heat up more slowly than the cylinder head and engine block that they join. There may be a considerable rise in the preload force if components such as aluminum, with higher coefficients of thermal expansion, are used for the latter. Considering this aspect, too, the use of waisted-shank bolts or waisted-thread screws (Fig. 7-325) is advantageous since, by virtue of the lower rise of the spring characteristics, the increase in screw loading is significantly less.

Fig. 7-325. Waisted-shaft or waisted-thread screws for head bolts (KAMAX Company)

The expansion properties of steel can be influenced essentially fundamentally only by alloying with nickel.

Consequently, the latest developments provide for head bolts made of austenitic materials whose coefficients of thermal expansion are similar to that of aluminum. An as yet unsolved problem is the high degree of tool wear resulting from this material’s great strength; consequently, economical manufacturing has not yet been implemented.

The constant need to reduce costs is responded to in two areas when optimizing the head bolts:

- Using waisted-thread screws as a compromise between sufficient elasticity and reduced manufacturing costs in comparison to waisted-shank bolts requiring a significantly more complex manufacturing process.

- Replacing the washer in aluminum cylinder heads by integrating its function into the screw head, in the form of a bolt with a flanged head. To avoid seizure during screw assembly, it is necessary to impose narrow limits on the geometry of the contact surface under the head and to select manufacturing technology that adheres to those limits.

This includes surface treatment with extremely low variation in the friction values and excellent adhesion to the substrate material as is found, for example, in the thin-layer phosphating process with quasiamorphous crystal formation.