To Stretch or Not to Stretch? (tech)

[amalventano1 at tds.net (Allyn)]

> Why did VW switch to stretch bolts?  and if you notice a new 
> 1.8L bolt is slightly shorter than a new 2.0L bot for a 16V.  
> The 2.0L bolt length matches the length of the original hex head one.

The purpose of a stretch bolt is to remove the inconsistencies you get with 'normal' torquing.

Picture two bolts, same size, thread pitch, etc.  Imagine both are torqued to the same spec, but one of those two bolts was
lubricated prior to installation.  The lubed bolt will have a greater clamping force for the same head torque applied, since there
was less friction to work against the tightening torque. 

So, stretch bolts correct this.  The shaft of the bolt is designed to plastically deform at a specific tensile force.  This
effectively gives you a 'buffer' while tightening the bolt, where the tensile stress will actually level off for a bit (at the
necessary tensile value).  This is why you have to torque head bolts to a relatively low value, followed by an additional angle of
rotation (regardless of torque).  The additional angle ensures all bolts are within their plastic deformation range.  The end result
is that your head bolts are all applying clamping forces that are much more consistent than could have been obtained with standard
bolts, relying on bolt head torque alone.

Now, as to why a single crank pulley bolt would need to be a stretch type?  Got me.  Perhaps VW corrected the keyway shear issue by
ensuring the clamping force was better regulated?  Either way - if you DO get the stretch bolt, it had better have instructions for
its installation (torque plus angle).  Without the plus angle spec, you are flying blind.

Side note:  I've never seen a stretch bolt that was as short and thick as a crack end bolt.  It would be very difficult to make such
a bolt.  Post a pic if you buy the stretch bolt, as I'm sure some of us will be curious.

HTH
Al