Stretch bolts?

[jkarlsso@metabolex.com (Jonas K.)]

Found this from the weekend, sorry if it's been beat to death, but I wanted
to comment.

On 5/18/02 5:50 PM, "Larry" <rocco16v@netzero.com> wrote:
> 
> Guess what.... neither our head bolts, rod bolts, nor pressure plate bolts
> are 'stretch bolts'.

That goes against what VW states, very blatantly.

> Our bolts use a method called "angle-controlled" tightening.

<snip>

> 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.

I disagree with the argument that this is evidence that the bolts used by VW
are not actually tightened beyond the yield point.

I would imagine that the factory would (upon deciding to go to a
stretch-fastener) do quite a bit of testing to arrive at a proper procedure.
I could also very easily imagine that upon testing many times with a $15k
tool, that they could simply say "torque to X, then give an additional 1/2
turn", because that's exactly what they find themselves doing over and over
again, with their $15k tool.

> Bottom line:  despite what we have believed for all these years, VW does not
> use "stretch bolts" in our engines.

I think the main difference is that "ASSEMBLY" magazine is written for an
entirely different audience than a VW workshop manual.

So what if the VW procedure seems to more closely resemble a non-deforming
assemly technique as printed in "ASSEMBLY". One would have to use a $15k
tool to actually verify that the bolts did not reach their yeild point.

Or, we could trust that the VW engineers did their homework and actually
have developed a procedure that the lay-mechanic can do, while at the same
time really being stretch-type bolts.

IMHO it would be unwise to dismiss the clear directions from VW based on the
findings in "ASSEMBLY" magazine.

> In fact, I could not find any warnings in my Bentley about re-using those
> bolts.

I _know_ it's in there, I just don't know in which one, or if in all. I
could try to find it for you.

> They can be re-used.

No doubt there are many stories of people re-using the stretch style bolts.
Some very successfully, apparently. I don't think we have a sample set large
enough to difinitively say if there is or is not an increase in premature
failure rate. Play at your own risk.

All I know is that VW states that they are stretch bolts, and that they are
to be used only once. I have no reason to think that they would lie about
that. 

$30 is not worth it for me.
Jonas