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Stretch bolts?



Read a very interesting article on fastening in the May issue of ASSEMBLY
magazine. (we get some pretty esoteric journals at work....) This article
focused on selecting the best tightening system for a bolted joint.  Here's
what I found:

Guess what.... neither our head bolts, rod bolts, nor pressure plate bolts
are 'stretch bolts'.

Our bolts use a method called "angle-controlled" tightening.  The benefit of
this method (apply low torque value, then apply a prescribed, additional
turn of a precise angle) is that it is not affected by friction....dry
threads, dry head, oiled threads and/or head and/or washers, etc. do not
introduce clamping force variables.  This is probably why VW used this
method; the head may be oiled or not, the bolts may be clean or oiled, etc.
This method does NOT elongate the bolt past its yield point (permanent
deformation or 'stretch')
 In contrast, the strategy of tightening to the yield (stretch) point is
done by monitoring the rate of change of torque over fixed angle increments.
This is by far the most consistant way of torqueing a bolt, but requires
very sophisticated, expensive instrumentation, typically costing in the $15K
range for a tool capable of the torque required by our head bolts.

Bottom line:  despite what we have believed for all these years, VW does not
use "stretch bolts" in our engines.
In fact, I could not find any warnings in my Bentley about re-using those
bolts.
They can be re-used.
Any thoughts? comments?

Larry  sandiego16V