Cylinder-head bolts guarantee a secure and releasable joint. However, this does not mean that the same bolt can be reused for the same application. When reassembling a cylinder head, always make sure to use new head bolts. The result will be a high sealing pressure for a reliable and tight joint.

More detail explanation:
Long head bolts are used to secure the cylinder head to the engine. They are unlikely to wear out themselves, but are usually replaced as a matter of course when replacing the cylinder head or other engine parts

The reason for this is that many head bolts are 'stretch bolts' which stretch to fit when mounted in the engine, and so cannot be reused. Head bolts come in different sizes and are car model specific.

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