When a head gasket is installed between the cylinder head and engine block, tightening the head bolts compresses the gasket slightly allowing the soft facing material on the gasket to conform to the small irregularities on the head and block deck surfaces. This allows the gasket to "cold seal" so it won't leak coolant before the engine is started.
The head gasket's ability to achieve a positive cold seal as well as to maintain a long-lasting leak-free seal depends on two things: its own ability to retain torque over time (which depends on the design of the gasket and the materials used in its construction), and the clamping force applied by the head bolts.
Fel-Pro "PermaTorque®" head gaskets are made with top quality materials and designed to remain resilient so they retain torque. That's why retorquing isn't necessary (some competitive head gaskets that are claimed to be "no retorque" can lose as much as 50 to 60% of their original torque after only 100 hours of service!). But even the best head gasket won't hold and maintain a tight seal if the head bolts have not been properly torqued. The amount of torque that's applied to the bolts as well as the order in which the bolts are tightened determine how the clamping force is distributed across the surface of the gasket. If one area of the gasket is under high clamping force while another area is not, it may allow the gasket to leak at the weakly clamped point. So the head bolts must all be tightened in a specified sequence and equally torqued to a specified value to assure the best possible seal.
Another consequence of failing to torque the head bolts properly can be head warpage. Uneven loading created by unevenly tightened head bolts can distort the head. Over a period of time, this may cause the head to take a permanent set. So any head that has not been properly torqued should be checked for flatness prior to installing a new head gasket.
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