To Stretch or Not to Stretch? (tech)

[jdbubb at verizon.net (Dan Bubb)]

Excellent explanation Allyn!

I just have one comment. You reference lubrication and how it can change the clamping force of a bolt. Does it not make sense that torquing to a lower value, and therefore incurring a much lower frictional resistance to turning,  then turning an addition angle of rotation will give a more consistent clamping force irrespective of lubrication?
Just because the tightening spec specifies a torque plus an additional angle of rotation doesn't necessarily mean the bolt is a stretch bolt. It's simply a way to get a more accurate clamping force in a critical application.
As far as the crank bolt IIRC there are no reduced sections or other geometric features typical of stretch bolts. 

From: "Allyn" <amalventano1@tds.net>
Sent: Tuesday, January 02, 2007 1:40 PM
Subject: RE: To Stretch or Not to Stretch? (tech)


>> 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
> 
> 
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