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Stretch bolts?



think of all those vws on the road with head rebuilds. you think the shops
put new head bolts on em??? heh.
Al


----- Original Message -----
From: "Larry" <rocco16v@netzero.com>
To: <scirocco-l@scirocco.org>
Sent: Saturday, May 18, 2002 8:50 PM
Subject: 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
>
>
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