The torque is irrelevant, what you are doing is loading a spring. In
fact, the torque will not change much as you turn the bolts. The bolts
are elastic -- by applying a pre-determined amount of "stretch" to them
you are ensuring that they will always have a correct "holding
together" pressure. This is to keep the minumum pressure on the head
gasket at all temperature ranges. On aluminum heads, the head expands
more than the bolts, for instance, and if you just torqued down
standard bolts, the head gasket would be crushed when the engine was at
operating temperature, then loose cold. The "stretch" bolts act as
springs, keeping adequate pressure on the head all the time.
When they become inelastic and "grow" in length instead of returning to
their initial length when unscrewed, they are toast and have to be
replaced, as they are no longer acting as springs.
Note that the "stretch" bolts always have a reduced diameter section.
Peter
