Vance Jaqua wrote:

One note on the bolt question - the primary reason for
lube is not to reduce torsional stress, but to make
the load versus torque value more repeatible. Most
people think that the torque is related to the thread
"ramp" angle, but his is only a small part - most of
it is friction, so reducing that component makes it
more repeatible. Ideally you would torque just short
of yeild, and this is what is done on high class con
rod installations where you torque to a measured
stretch, staying in the elastic limit. 

The problem
with going to yield is that there is the possibility
of inducing micro cracking in the thread root because
of the stress concentrations. I truly doubt that the
"torquing to yield" really yields most of the bolts,
although it comes close enough that you should not
reuse any of those bolts. Also - never rethread a
highly loaded bolt - the factory rolled thread is an
order of magnitude better under any trace of cyclic
load than a cut thread. 

As mentioned before - a bolt
where the preload tension exceeds the magnitude of the
cyclic load - the stresses in the bolt do not cycle -
damn good thing - otherwise we would have cycling
parts scattered all over the landscape, because the
stress concentration factor at the root of a thread is
an outrageously high number.

I once had a tour of the Rockwell tool works where
they had just completed a multimillion dollar machine
for GM, to torque all the bolts in the then new
aluminum Caddilac engine . This machine monitored the
shape of the torque curve as the bolt was tightened,
and they bragged that this allowed Caddilac to go down
one bolt size throughout. When I asked them what would
happen when Mr Goodwrench repaired one of these
engines with his $29.95 Craftsman torque wrench, they
were strangely silent. But they did assure me that the
machine made a digital record of all the values versus
location and engine serial number for the lawyers. Vance

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