Many later model engines require torque-to-yield (TTY) head bolts that are only good for one-time use and should be purchased every time you remove a cylinder head.  These bolts must be torqued to an exact specification and an exact sequence to ensure a proper seal of the head gasket.  Some engines, especially those with aluminum heads and MLS head gaskets, require more clamping strength because of increased expansion. Torque-to-yield head bolts are designed to achieve maximum clamping strength when they are stretched to their “yield” point.  It’s very important to refer to your owner’s manual for the torque specs on your vehicle.  In some engines, it’s as simple as tightening down each bolt to a specified number of foot/lbs while others are much more intricate.