There are two types of friction: Static and sliding. Static friction
represents the force needed to get a stationary object moving. Sliding
friction is the force necessary to maintain that movement.
General science and high school physics classes teach that sliding
friction is almost always less than static friction. We pulled weights
using spring balance scales to measure and test the concept.
An engineering reference shows that hard steel against hard steel has as
static friction of 0.78 while sliding is 0.42. That means the force
necessary to start movement is almost twice the force necessary to
maintain the motion. All cylinder heads that I am aware of have steel
washers under the nuts. Thus it makes no difference what material
the head is made off.
Therefor when retorqing, back off the nut using a breaker bar, or beam
torque wrench, just until it breaks loose and no more. You do not want
to loosen it completely. Then torque it back up to the recommended
setting. Do each nut separately and in the same sequence as recommended
by the engine manufacturer. That is, back off number one and retorque
to the recommended setting. Back off number two and retorque to the
recommended setting, etc.
Blake
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