Cylinder-head Bolts

Strong Joints for Perfect Sealing

Connecting Element
Cylinder-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 Pressure
When 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.



When torquing the cylinder head bolts, the bolt force generates a surface pressure, which is essential for achieving a perfect seal.

TIP
Surface pressure describes the force per unit of contact surface area between two components, e.g. between cylinder head and head gasket, or engine block and head gasket. As opposed to the clamping force, surface pressure is not constant across the entire contact area.

Special Bolts for Higher Bolt Forces

Stretch Bolts
Bolts 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 Method
In 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.


Using the torque-angle method, the variations in bolt clamping force lie in a range of ±10%. Using the simple torquing method with several steps, the variations will be within ±30% of the calculated bolt clamping force. The reasons for this lie in the dispersion range of torque values and the overall coefficient of friction, i.e. the friction occurring under the bolt head and in the threads.


Advantage of stretch bolt: As opposed to a rolled shank bolt, it can be tightened beyond its elastic limit (yield point) into the plastic region – a guarantee for high and uniform bolt clamping forces.

TIP
Stretch bolts are designed so that they can be elongated beyond their elastic limit into the plastic region without problems. When a specific tightening torque has been reached, the bolts are turned further through a defined angle, which pre-loads them into the plastic region. Consequently, no retorquing is necessary.
 

TIP FROM THE EXPERTS
With the torque-angle tightening method, the head bolts are elongated permanently. Therefore, cylinder head bolts should only be used once for safety reasons.



Torque-angle method using pretorque and tightening angle.


Practical Information

Cylinder-head Bolts and Cylinder Head Installation