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. Make sure all the head bolts are in perfect condition with clean, undamaged threads. Wire brush all bolt threads, carefully inspect each one, and replace any that are nicked, deformed or worn. Dirty or deformed hole threads in the engine block can reduce clamping force the same as dirty or damaged threads on the bolts.