[Vwdiesel] questions about re-using stretched bolts

[James Hansen]

Well, just examining the correct terminology explains some things.  They are
torque to yield bolts.  They have been stressed to the point of yield.  That
point achieves the desired clamping force between the head and the gasket
and the block faces.  Once that point has been met, it will continue to
stretch further with less force applied until the metal eventually fails.
The hardness is not a factor in something like this, rather it is the
composition of the alloy, the thread pitch and cross-sectional area of the
bolt that determines the amount of force the bolt will apply, so rehardening
and tempering will not yield (pun) the desired outcome in this case.
I'm just guessing, but heating the metal just to the point of normalizing
and holding this heat for a time in a neutral atmosphere furnace *should*
allow the molecular structure to reassemble to the pre-yield configuration,
but then you need to know the exact cooling process used originally to
attain the desired yield strength, and how long the material has to age to
attain the desired strength, etc, etc,..  THen there's microfractures to
consider and a bunch of other things, it's just simpler to pretend that we
don't know any of this stuff, and run out and buy new ones like everyone
else does, 'cause the head bolt rebuild may take longer than rebuilding the
entire engine. Besides, then one breaks when torquing the head on, and we
feel stupid.
Not good.

Excellent usage of the terminology by the way, few people know that
tempering metal means softening.
-James

-----Original Message-----
From: vwdiesel-bounces at vwfans.com [mailto:vwdiesel-bounces at vwfans.com]On
Behalf Of Area31 Research Facility
Sent: Monday, October 10, 2005 11:56 PM
To: Roger Brown
Cc: vwdiesel at vwfans.com
Subject: [Vwdiesel] questions about re-using stretched bolts


Can anyone answer these two questions about bolt metalurgy?

I was wondering how a mechanic might know if the head or  rod bolts he has
have been used and stretched before when using them in an engine
build/re-build.

I'd also like to know if one has known used bolts,  can heat treasting make
them as good as new again to be re-stretched?  I'm thinking of the process
involving heating in an oven to the correct color and then quenching in oil
or air and then reheating to temper.

Thanx for any discussion.

Rob

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