Cylinder-head bolts play an important role in the construction of an engine. Connecting the head to the block is just the beginning. They also help form a vital seal created by the cylinder-head gasket.
This is pivotal in maintaining the correct combustion ratio. The seal is formed by the creation of the correct surface pressure between the head and the block. Apart from containing and focusing the combustion pressure, the head-gasket also seals the lubrication and cooling channels.
The correct bolts are needed if these functions are to be reliably achieved. Modern engines have cylinder-head gaskets which do not require re-torquing.
This can only be accomplished with the correct cylinder-head bolts. How does this work? The modern bolts – which have a reduced diameter in comparison to the older rolled shank bolts – are able to be tightened (torqued) beyond what is known as their elastic limit or yield point and into the ‘plastic region.’
This ensures high and uniform clamping forces. By doing this, the head-gaskets will perform properly without ever having to be re-torqued.
The new bolts – known as stretch bolts – also require a slightly different method of torquing. They are inserted and pre-torqued.
The second step sees the bolts torqued beyond the elastic limit and into the plastic region. This is called the torque-angle method.
The resultant variation in bolt clamping force is approximately 10%. A far more even spread of tension than the 30% variation often seen with the older method (and the older bolts).
It is important to note that these stretch bolts should never be reused. New bolts should always be installed when the old ones have been removed.
Victor Reinz cylinder-head bolts are matched perfectly to our head gasket sets and the torquing guidelines are always supplied.