> And if they're being reused, aren't they just stretching alittle
> more each time??
Not necessarily. The studs (as well as the block and heads) have an
"elastic" range, where they will spring back to their original dimension
after being stretched/compressed. Part of the importance of the proper
torque value is to keep the stretch in the studs within this range. This of
course is different than modern "torque to yield" fasteners (which are
supposed to be replaced every time).
The optimum method of torquing a highly stressed fastener is actually to
measure the stretch. This isn't practical for a Triumph head stud, but is
used in some cases. See more at :
http://www.arp-bolts.com/pages/tech/fastener.html
But more importantly, the major reason for the need to retorque is the head
gasket. It is deliberately compressed beyond it's yield point, and doesn't
finish yielding until the engine has been through a few thermal cycles.
There are also stresses setup as you tighten the head nuts (which is why you
tighten them in stages, to minimize the residual stresses), which relax to
some extent.
BTW, someone earlier asked about the thermal expansion rates between cast
iron and steel. Problem is, these are both alloys of iron and other
elements (yes, even cast iron is both an alloy and a mixture); and the
thermal expansion coefficient is different for the different alloys. I've
no idea what the original alloys were.
Randall
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