Bolt used for fastening engine heads, they ensure a perfect gasket seal
Bolts used for fastening engine cylinder heads, usually in a number of 2x no. cylinders+2. They must exert an axial load allowing a steady contact pressure to be maintained on the head gasket, such as to ensure that cylinders are perfectly sealed and therefore avoiding pressure losses in the combustion chamber and preventing any pressurized burned gases from leaking to coolant conduits.
At the same time, they must not apply excessive loads so as to prevent cylinders from being deformed with consequent malfunctioning of the engine. It follows that their mechanical strength must have a rather narrow tolerance range and their surface coatings must have well-defined and constant friction coefficients.
Usually characterized by a rather large ( >10) slenderness or length to diameter ratio, they must however have very contained rectilinearity values. Typical 10.9 and 12.9 strength ratings, more and more so they are designed with custom strength ratings, even with very narrow Rm ranges.
They work in the plastic range and are usually tightened based on torque + rotation. Depending on the architecture of the engine and its dimensions, they may have different head shapes and in some cases require a washer. In most cases they are phosphated and oiled.