Connecting ElementCylinder-head bolts are the connecting elements between engine block, cylinder-head gasket, and cylinder head. The bolt tightening force is an important factor for the mutual sealing of combustion chamber, lubricant and coolant channels, and the ambient air. A task that must be performed perfectly with cold or extremely hot engines, at negative and high positive pressures.Surface PressureWhen tightening (torquing) the cylinder-head bolts, the bolt force generates the so-called surface pressure, which is essential for achieving a perfect seal. Hereby, and in combination with the applied torquing method, the bolts have a strong effect on surface pressure quality.
Stretch BoltsBolts that are tightened beyond their elastic limit (yield point) into the plastic region ensure high and uniform bolt clamping forces. An important prerequisite for reliable sealed joints without retorquing. In this respect, stretch bolts offer decisive advantages over rolled shank bolts. By reducing its shaft diameter, the bolt is able to respond elastically to dynamic sealing gap movements, i.e. adaptation is greatly improved.Torque-angle MethodIn the first step, the bolt is snugged down to the cylinder head by means of a pretorque. With the second step, the so-called torque or tightening angle, the bolt is tightened into the plasticregion, i.e. beyond its elastic range.
TIP FROM THE EXPERTSWith the torque-angle tightening method, the head bolts are elongated permanently. Therefore, cylinder head bolts should only be used once for safety reasons.
